Warm hints: The word in this article is about 3000 words and  reading time is about 10 minutes.SummaryFull bridge push-pull bi-directional DC/DC converters are mostly modeled by state space averaging method with the complex modeling process. Taking the isolated form push-pull bi-directional DC/DC converter as the research object, this article adopted the pulse-width modulation switch model method to obtain the equivalent circuit model of this converter. Based on this model, this article constructed a closedloop control system of the converter under different working modes, carried out the design and verification of voltage control loop and realized the constant voltage charge-discharge control strategy. The experimental and simulation results verify the correctness of the conclusion. CoreFull bridge push-pullPurposeTo obtain the equivalent circuit model of the converterEnglish nameBidirectional DC/DC convertersCategoryElectromechanical deviceFunctionConverting a source of direct current (DC) from one voltage level to anotherFeatureAdopting the pulse-width modulation switch model method CatalogsCatalogs1. Foreword2.2 Modeling of bi-directional DC/DC converter4 Epilogue2. Principle and design2.3 Voltage Closed-loop Control and Simulation 2.1 The working principle of circuit3 Experimental result   Introduction1. ForewordBidirectional DC/DC converters have been widely applied in many fields such as electric vehicles and battery energy storage systems. Bidirectional DC/DC converters have different circuit topologies according to their applications.I have seen some examples of using Buck/Boost converter to realize the bi-directional flow of electric energy. Because there is no transformer in the converter, so the electrical isolation of the high and low voltage side can not be realized, and only a small voltage range has been allowed. In order to solve this problem, it is proposed to use isolation transformer in bidirectional full-bridge DC/DC transform circuit, but the converter uses too many switching devices and therefore causes too many power losses. The isolated full bridge push-pull bi-directional DC/DC converter has been widely used due to the advantages of both types of converters of itself.At present, the state space averaging method is used to deduce the small signal model of bidirectional DC/DC converter, which needs a large amount of mathematical derivations and calculations. In order to simplify the modeling process, the full-bridge push-pull bi-directional DC/DC converter is modeled by using PWM-switch modeling method. The transfer function between output voltage and duty cycle of the converter is obtained in two modes: boost mode and buck mode. In addition , the voltage closed loop control system is constructed based on the derived circuit model, and the voltage loop is designed and corrected by the compensation network , realizing the constant voltage control of the full-bridge push-pull bidirectional DC/DC converter.First step in developing feedback control for a dc-dc converter is modeling. Here, we model the buck converter in terms of average behavior. Detail2. Principle and design2.1 The working principle of circuitFigure 1 shows the topology of the main circuit of full bridge push-pull bi-directional DC/DC converter. ^The power switch tubes S1～S4  are arranged in the form of a full-bridge circuit;^the power switch tubes S5～S6 are arranged in the form of a push-pull circuit;^CHV is a parallel capacitor with HVDC busbar;^CLV is a parallel capacitor with LVDC busbar;^L is a  low-voltage-side energy storage filter inductor;^HV is high-voltage-side DC bus;^LV is a low-voltage-side DC bus.Figure 1 Main circuit of bidirectional DC/DC converterFigure 2 and 3 respectively give the schematic diagrams of the PWM driving waveform of the power switch tube working in boost and buck mode, and the schematic diagram of the voltage waveform of the primary and secondary sides of the transformer. The u12 shown in the diagram is the transformer primary-side voltage and the u34 is the transformer sub-side voltage.Figure 2 Waveforms of the  power switch tube working in boost modeFigure 3 Waveforms of the  power switch tube working in buck mode2.2 Modeling of bidirectional DC/DC converterThe basic idea of PWM-switch modeling method: the nonlinear part will be linearized by taking an average value of the voltage and the current of it in one switching period when it is a stable circuit, and therefore the nonlinear circuit will turn into a linear circuit. The push-pull bidirectional DC/DC converter working in the buck mode can be equivalent to the circuit shown in Fig. 4 (the circuit in the dashed box is meant to indicate the nonlinear circuit), which turns on during [0, DTs] and turns off during [DTs, Ts], where Ts is the switching cycle and D is the duty ratio of the full bridge push-pull bi-directional DC/DC converter turn on. In the mathematical expressions presented in this article, all the uppercase variables represent the steady-state values, and all the lowercase variables represent the instantaneous values.Figure 4 Conducting circuit in buck mode operationIn the equivalent circuit shown in figure 4, the voltage and current are averaged during a PWM switching cycle, among which idc is primary instantaneous current and iL is secondary instantaneous current of transformer; iC is instantaneous current of electric capacity CLV; ucp is the instantaneous voltage between nodes c and p; uap is the instantaneous voltage between nodes a and p; n is the transformation ratio.Assuming that the duty cycle is d=D, now add an AC small signal to its value attachment (where small angle brackets denote AC small signals and the other variables following are the same), the complete instantaneous value expression for duty cycle is as shown in formula 3:Then substituting equation 3 into the original yields equation 1 and 2, so we have:Neglecting the product term of AC small signal, and equation 4 and 5 of  AC small signal can be simplified as equation 6 and 7:The mathematical relations of voltage and current working in steady state (following yields equation 8 and 9) derived from original yields equation 1 and 2, along with equation 6 and 7 can be used to obtain equivalent circuit model of bidirectional DC/DC converter in Buck mode (see the two-port circuit shown in the dashed box of figure 5). The model is linear since the constraint equations describing the two-port circuit are all linear equations. It can be learned from the above modeling process that the linear model of the full bridge push-pull bidirectional DC/DC converter can be easily established by using the switching model modeling method, which has the advantages of less calculation and simple derivation. It can be learned from the above modeling process that the linear model of the full bridge push-pull bidirectional DC/DC converter can be easily established by using the PWM-switch modeling method, which has the advantages of less calculation and simple derivation.The equation 10 gives transfer function between the output voltage and duty cycle of low voltage side bus according to laws of KCL and KVL.Figure 5 gives the equivalent circuit of bidirectional DC/DC in Buck mode obtained from equation 10, among which UHV is the operating voltage of high voltage side bus in steady state, uLV is the instantaneous voltage of low voltage side bus, n is the transformer ratio, R is the load resistance of low-voltage-side DC bus, Uap/D can be regarded as controlled voltage source, and IL/n can be regarded as controlled current source.Figure 5 Small signal equivalent circuit in Buck mode operationThe transfer function between duty cycle and output voltage in Boost mode can be obtained by using the same analysis method as Buck mode, which has shown in equation 11. R1 is the load of high voltage side bus.2.3 Voltage Closed-loop Control and SimulationFigure 6 gives the block diagram of the closed loop control system of small signal circuit model of the full bridge push-pull bidirectional DC/DC converter in two different operating modes. Charge and discharge at constant voltage in two modes can be realized by voltage closed-loop control.What we can see from figure 6:Uref is the given voltage of high-voltage-side and low-voltage-side DC bus; Hv(s) is the transfer function of sampling; Gm(s) is the transfer function of PWM; Gcv(s) is the transfer function of PI of voltage control loop; Gvd(s) is the transfer function of voltage versus duty cycle.Figure 6 Block diagram of voltage closed loop controlTo theoretically verify the correctness of mathematical model establishment and the feasibility of voltage control strategy, we use the PSIM 9.0 software tool for simulation and analysis in this article. Figures 7 and 8 have respectively shown the simulated waveforms in different operating modes. In Buck mode operation, the voltage of low voltage bus can be stabilized at 12V, so the constant voltage charging has been realized; and in Boost mode operation, the voltage of high voltage bus can be stabilized at 400V, so the constant voltage discharge has been realized, which theoretically verifies the correctness of mathematical model establishment and the feasibility of voltage control strategy. u12 and u34 are the voltages of the primary and secondary sides of the transformer respectively, uHV and uLV are voltages of high-voltage side and low-voltage side bus.Figure 7 Simulated voltage waveform in boost mode operationFigure 8 Simulated voltage waveform in buck mode operation Analysis3 Experimental resultIn order to verify the correctness of the small-signal circuit model and the feasibility of voltage feedback control, we built an experimental platform of full-bridge push-pull bi-directional DC/DC converter using TMS320F28035 as the core control chip.Table 1 gives the main circuit parameters of the bidirectional DC/DC converter based on the full bridge push-pull circuit structure.Table 1 Basic circuit parameters of converterFigure 9 gives the output voltage waveform of secondary voltage and low-voltage-side DC bus of transformer in buck mode operation when steady input voltage of high-voltage-side DC bus is 400V (provided by programmable DC power supply) and load resistance of low-voltage-side DC bus is 0.1Ω (provided by programmable DC electronic load). Here the output current of low-voltage-side DC bus iL is stable at about 120A, the output voltage uLV is about 12V, and the output power of low-voltage-side bus Po is about 1.5kW which meets the requirement of rated power designed, now we have realized  the constant-voltage charging of low-voltage-side DC bus.Figure 9 The voltage waveform in Buck mode operationFigure 10 gives the output voltage waveform of primary voltage and  high-voltage-side DC bus of transformer in boost mode operation when steady input voltage of low-voltage-side DC bus is 12V (provided by low-voltage DC power supply) and load resistance of high-voltage-side DC bus is 105Ω (provided by high voltage resistance load box). Here the output current of high-voltage-side DC bus iH is stable at about 3.7A, the output voltage of high-voltage-side DC bus uHV is stable at about 400V, and the output power of high-voltage-side bus Po is about 1.5kW which meets the requirement of rated power designed, until now we have realized  constant-voltage discharge of high-voltage-side DC bus.Due to the high level at the output, technical problems in the winding process for power transformers and the whole experimental device completing by manual welding, the rate of heat dissipation of power transformer appears to be somewhat low and the parasitic inductance and inductive coupling is also rocking the boat, and then we see the voltage waveform oscillation at zero crossing of transformer voltage in Boost mode moderation. However, under the condition of steady state parameters, this experimental platform can work normally and stably for a long time according to the design requirements and can meet the needs of practical applications.Figure 10 The voltage waveform in Boost mode operationFigure 11 is a schematic diagram of the hardware platform which is mainly composed of main circuit board, control circuit board and corresponding test equipment.Figure 11 physical photographs of experimental devices4 EpilogueAccording to the characteristics of bidirectional DC/DC converter applied in fields of battery and electric vehicle energy storage management, a bidirectional DC/DC converter based on full-bridge push-pull topology is obtained by comparing with different circuit topologies. Different from the traditional state-space average modeling, the method of PWM-switch modeling is used now to build small signal equivalent circuits in different mode operations, and the voltage loop has also been designed and corrected. The high-power constant voltage charging is realized in Buck mode moderation and the high-power constant voltage discharge is also realized in boost mode operation. Both modes can work normally and stably to meet the performance requirements of practical applications, which appear to be in high value at fields of battery and electric vehicle energy storage management. Book SuggestionSoft-Switching PWM Full-Bridge Converters: Topologies, Control, and DesignJun 23, 2014This book intends to describe systematically the soft-switching techniques for pulse-width modulation (PWM) full-bridge converters, including the topologies, control and design, and it reveals the relationship among the various topologies and PWM strategies previously proposed by other researchers. The book not only presents theoretical analysis, but also gives many detailed design examples of the converters.---by Xinbo RuanPower Electronics: Converters and RegulatorsOct 28, 2016This book is the result of the extensive experience the authors gained through their year-long occupation at the Faculty of Electrical Engineering at the University of Banja Luka. Starting at the fundamental basics of electrical engineering, the book guides the reader into this field and covers all the relevant types of converters and regulators. Understanding is enhanced by the given examples, exercises and solutions. Thus this book can be used as a textbook for students, for self-study or as a reference book for professionals.---by Branko L. Dokić and Branko BlanušaPulse-Width Modulated DC-DC Power ConvertersOct 26, 2015With improved end-of-chapter summaries of key concepts, review questions, problems and answers, biographies and case studies, this is an essential textbook for graduate and senior undergraduate students in electrical engineering. Its superior readability and clarity of explanations also makes it a key reference for practicing engineers and research scientists. Following the success of Pulse-Width Modulated DC-DC Power Converters this second edition has been thoroughly revised and expanded to cover the latest challenges and advances in the field.---by Marian K. Kazimierczuk Relevant information "Modeling and Control of Full Bridge Push-Pull Bi-Directional DC/DC Converter"About the article "Modeling and Control of Full Bridge Push-Pull Bi-Directional DC/DC Converter", If you have better ideas, don't hesitate to  write your thoughts in the following comment area. You also can find more articles about electronic semiconductor through Google search engine, or refer to the following related articles:How to Learn Analog Circuit DesignLook Forward to the Future of Semiconductor

Warm hints: The word in this article is about 3500 words and  reading time is about 15 minutes.   Based on the heat balance principle, the basic theory for the calculation of steady and transient temperature rise of transformer is discussed.   The calculation of transformer temperature rise is mainly used to make sure that the steady-state temperature rises produced by the transformer with rated load in long-term continuous operation will not exceed the limit specified in the standard or technical contract. In addition, the data of overload operation capacity of transformer running under various rated loads are also used to offer support to the measures taken to ensure safe overload operation of electrical systems.   However, from the author's current reading of recent info on transformer principles, design, and calculation theories available, it may be due to the limitation of space or different aspects of emphasis that they are always making an introduction to various practical formulas for transformer temperature rise calculation in a practical way. As to the involved thermal principle of calculating the transformer temperature rise is always a bit of an oversimplification I think, especially the thermal analyses demonstrating how heat energy be absorbed(or dissipate) and the temperature goes up(or goes down) during a heating process. In this article, therefore I would like to make some theoretical discussions to solve this problem and add some extra explanations needed.   Catalog   I. Brief Introduction II. Heating & Cooling Process 2.1 Heating process when the thermal power is constant 2.2 The cooling process in which the heated body is no   longer supplied with heat 2.3 Just as same as the situation of 2.1, but Τ≠0 when   t=0 III. Further Analysis of Some Formula 3.1 Mechanism of temperature-rising process of heated   body 3.2 Properties and applications of Τu in formula 2 3.3 Definition, function and derivation of the thermal   time constant Τ IV. Conclusion FAQ I. Brief Introduction   It is well known that heat is always transferred automatically from a high-temperature object to a low-temperature object. Heat can transfer (or move) in three ways: conduction, convection, and radiation, usually a combination of two or three of them may cause the transmission (also known as heat dissipation).   Before formally describing this section, I would like to quote two conclusions from laboratory research that are fundamental to thermal science:   1) the temperature rise of an object is directly proportional to the heat (heat) supplied by the outside world and inversely proportional to its own mass and Specific Heat Capacity.   2) the heat energy (calorific energy) that a heating object disperses into a cooler object(cooling medium) within a unit of time is directly proportional to the area of its radiating surface, the temperature difference, and the Heat Transfer Coefficient between the heating object and cooling medium.   Specific heat capacity: the ratio of the heat added to (or removed from) an object to the resulting temperature change per unit mass of material.   Heat transfer coefficient: when the temperature difference between the heating object and the cooling object is 1K, the heat from the unit dissipation surface area to the cooler object within a unit of time.   For the convenience of discussion, the heating object is referred to as the heated body in this paper, and the object which gets the heat released from the heated body is called the cooling body (or the cooling medium).   Take a dry transformer, for example, its winding and core are heating bodies, and the cooling body refers to the air around the transformer. For oil-immersed transformers, in addition to the winding and core known as heating bodies, the oil-filled in the transformer can also be called a heating one in relation to ambient air or cooling water. However, relative to the winding and core, oil is also called a cooling body(cooling medium).   Fig. 1 Transformers are found everywhere alternating current is used   II. Heating & Cooling Process   2.1 Heating process when the thermal power is constant Suppose that in the heating process (that is, the heat is still in a transition state, not reaches a stable state yet), the temperature rise by Τ relative to the cooling medium, the temperature rise increases dΤafter a tiny time unit dt.   The heat supplied by the outside during this dt period is a constant thermal power of Pdt(P. For windings in oil-immersed transformers, it refers to load loss; for oil, it refers to total loss). Let the heat supplied by the outside surroundings during this dt period be Pdt(P is the constant thermal power; for windings in oil-immersed transformer, P refers to load losses, and for oil, the total losses). Some of the Pdt is the heat absorbed when the temperature rise of the heated body increases by dΤ, and the other part is dispersed into the cooling medium.   In order to understand the nature of this physical phenomena in the heating process better, suppose the heated body is an isothermic one, therefore the density, specific heat capacity, and any other parameters being the same, including the heat dissipation capacity of each point on the surface.   According to the principle of thermal balance (it comes down to the law of energy conservation), the states of the heating process described above can be represented by the following equations:   Where   P——Constant value of thermal power c——Specific heat capacity of the heated body G——the mass of the heated body F——Heat dissipation surface area of the heated body k——Heat transfer coefficient  Τ——Temperature difference (or temperature rise) between the heated body and cooling body at a certain time   The first item on the right of Equation (1) indicates the heat energy absorbed by the heated body when it increases the temperature of dΤ; The second item indicates that the heat energy is dispersing to the cooling body while the heated body is storing heat.   When the temperature rise of the heating body does not go up, that is, when the temperature rise reaches a stable state, then there is dΤ=0 and Τ=Τu (Τu means the steady-state temperature rise).   At this point, from formula (1) there is:   According to the above analyses we can see: formula (1) is the mathematical expression of the heat balance principle under the transient state; formula (2) is the mathematical expression of the heat balance principle under steady-state.   From Formula (1) and Formula(2) we can get a first-order differential equation of Τ:   Where: From formula (4), the parameters on the right side of the equation are the physical parameters of the object itself, so Τ is a constant and has a dimension of time, so it is called the thermal time constant. In the physical sense, Τ is the ratio of the heat storage capacity of the heated body to the heat dissipation capacity per unit time of the heating system studied, which is an attribute of the heated body. Therefore formula (4) is considered to be the definition of Τ and the formula (4') is another expression for calculation.   To obtain the solution of formula (3) we let t=0 and Τ=0, then this is what we get:   Note that sometimes it is easier to express the formula (5) with temperature θ instead of temperature rise Τ, so it is reworded as follows: Where:   θ——The temperature of heated body at any given moment and there is θ-θа=Τ θа——The temperature of cooling body θu——The steady state temperature of the heated body that reaches a steady state Δθu——θu-θа=Τu   The rising curve in Fig. 1 shows the temperature rise of heated body changes with time t.   In order to visually see how the Τ changes with the temperature rise, Fig. 1  shows two curves with different Τ and same Τu. Fig. 2 Relation between temperature-rise(Τ)of heated body and time(t) As can be seen directly from formula (5), formula (5') and Fig. 1, the process of temperature rising of a heated body is characterized by the fact that it changes fast at the very start, then gradually slows down, and when the time t becomes equals to (4～6)Τ, it remains almost unchanged, at this point it can be assumed that Τ reaches Τu (theoretically t reaches ∞).   2.2 The cooling process in which the heated body is no longer supplied with heat   When the temperature rise of the heated reaches Τu and no heat will be emitted, the temperature rise begins to go down from Τu to zero, which we call the cooling process. At this point, its transient process equation can still be deduced by formula (1), in which you need only to let P be equal to zero.   Therefore the first-order differential equation of its temperature rise is as follows:   The solution is: All the symbols of parameter in the formula above are identical with those in formula (5), except that Τu is the initial value of temperature rise when t=0; when (4～6)Τ later, Τ≈0.   Why would we worry about Formula 6?    This is because the temperature rise of winding got at the end of the current transformer temperature rise test, is still the value of temperature rise calculated through measurements of the resistance value of the winding which will change with the temperature. The resistance value measurement is still carried out in the temperature rise test under a way of supply voltage been removed, thus the formula (6) should be used to calculate.     2.3 Just as same as the situation of 2.1, but Τ≠0 when t=0 When the heating body is supplied with constant heat power and Τ=Τ0 (≠0) when t=0 satisfied, the whole process of deduction of temperature rise calculation, with the exception of the situation of Τ=Τ0 when t = 0, is the same as 2.1, in other words, the formula of temperature rise can still be deduced from formula (5) as follows: Comparing this with the formula (5) we notice that there is a new second item occurs. Considering the physical meaning of the expression is not obvious enough, it is now rewritten (which will not change the result of the calculation) as: These two formulas above show a case that the transformer is suddenly asked to conduct a overload operation running beyond the steady load.   III. Further Analysis of Some Formula   3.1 Mechanism of temperature-rising process of heated body Formula (5) and Fig. 1 describe the rising process of heated body temperature from a mathematical point of view. This process is characterized by a rapid start and then a gradual slow down until it finally stops rising and reaches a stable temperature rise of Τu .   In this section, characteristics of heating and cooling mechanism in this process will be described from a physical point of view. So we divide the t into n small and equal time periods when t=Τu, that is Δt1=Δt2=……=Δtn=Δt (theoretically t=∞, but in practice, it is desirable to suppose that t=(4～6)T, considering the value required of Tu with a higher accuracy). Therefore, the derivative symbol "d" in this article is replaced with a increment sign "Δ".   Please note that the heat energy supplied by surroundings in each time period is equal to P·Δt (P is the constant thermal power).   Now let us take a look in the first time period Δt1. Since the temperature of heated body has already made be equal to the cooling body when t=0, that is Τ=0, according to the second item of formula (1), we can assume the heat energy emitted is also equal to 0 during the period of Δt1 until the end of present stage. Therefore, during the Δt1 period, the final increments of the temperature rise of the heated bodyΔΤ1 is determined by the total external heat energy (P·Δt). So the temperature rise at the end of first time period Δt1 is Τ1=ΔΤ.    According to the same analytical principle, we continue with the second time period Δt2. Since the initial temperature rise at the beginning of Δt2 is the that of the first time period Δt1, it can be included that Τ1=ΔΤ1. The heat emitted during Δt2 is no longer zero, but . At the end of the Δt2, the increment value of temperature rise of heated body ΔΤ2 is determined by , so there we have ΔΤ2<ΔΤ1 (That is, the increment of temperature rise in the second period is smaller than that in the first period).   Finally, at the end of the second time period Δt2, the temperature rise Τ2 is equal to ΔΤ1+ΔΤ2. The rest may be deduced by analogy, at the end of Δtn time period, the temperature rise is , also ΔΤn=0 has been illustrated at the same time.   According to the changes of temperature increment ΔΤn in each of time period above, there always are: ΔΤ1>ΔΤ2…>ΔΤn-1>ΔΤn (ΔΤn=0), therefore demonstrating the trend that all the heat absorbed by heated body in each time period gradually goes down from P·Δt absorbed in the time period of Δt1 to 0 absorbed in the time period of Δtn. The heat energy emitted in each of the corresponding time periods is gradually increased from 0 to (from the first time period Δt1 to the end of the NO.n time period Δtn). That also means the final temperature rise (steady temperature rise) is: After the temperature rise reaches the stable value of Τu, the heated body no longer absorbs external heat energy, which indicates that all the external heat energy has been dispersed to the cooling body.   3.2 Properties and applications of Τu in formula 2 Formula 2 is a theoretical formula derived from the heat balance principle for calculating the steady-state temperature rise of the heated body. It is difficult to calculate the temperature rise directly for complex heated bodies such as transformers.   Therefore, various manufacturers and scientific research institutions have respectively obtained many practical formulas, according to their own practical experience or scientific research results and the characteristics of the heated body structure and three different heat dissipation forms, including conduction, convection, and radiation.   Furthermore, a heated body such as a transformer having a complex structure does not always have the characteristics of the homogeneous isothermal body assumed in formula 2, and the heat-dissipation capability at each point on the surface is not equal, either.   Therefore, the steady-state temperature rise (Τu) calculated from formula 2 (including the relevant practical formula based on formula 2) on the whole indicates the average value of temperature rise at different points in the heat source.   The calculation of the maximum temperature rise (temperature) of an object at a "hot spot" of concern has so far could only be mainly estimated by experience and the use of temperature rise measurements in certain heaT tests (for example, confirming the difference between the maximum temperature rise and the average value or confirming the multiple values between them to estimate).   3.3 Definition, function and derivation of the thermal time constant Τ Formula 4 is the definition expression of thermal time constant. It is obtained in the derivation of formula 5 from formula 2 and formula 1. Therefore, it is possible to think that equation 4 is obtained under the admission that when t=∞ it has dΤ=0 and Τ=Τu, the latter of which is obtained under the boundary conditions of objective reality. Because the definition of "Τ" and some functions have been described in section 2.1, here i only do some additional analyses to formula 7 which shows applications of transformer temperature rise under a short-time overload operation. Fig. 3 What is a Transformer   The basic theory of transformer and working principle of transformer   Fig. 4 Large power transformers have their core and windings submerged in an oil bath to transfer heat and muffle noise, and also to displace moisture which would otherwise compromise the integrity of the winding insulation. Heat-dissipating "radiator" tubes on the outside of the transformer case provide a convective oil flow path to transfer heat from the transformer's core to ambient air.   First of all, according to the statistics of a large number of dry and oil-immersed power transformers with different capacities, which have been manufactured at home and abroad, the value of Τ is generally not less than 1h (for oil-immersed transformer, although the T of winding is quite low, about 5 min-20 mins, the T of oil is 1h～5h.    Noting that the oil actual temperature rise is generally not lower than the temperature difference between the winding and the oil and that the thermal time constant of the oil should therefore be considered to control the temperature rise of the winding during the overload operation, that is, the temperature difference between winding and ambient air or temperature of cooling water still plays a decisive role.    Although the Τu of the transformer will obviously exceed the temperature rise limit of rated-load operation, as long as the time t has been controlled within T, it can still make the actual temperature rise running in short-time overload not excess the temperature rise limit of short-time overload operation. These permissible limits, according to the standard of the load guide of the oil-immersed power transformer, is related to the operation type within nameplate capacity, namely, normal periodicity, long-term or short-term emergency, and generally higher than that of rated temperature rise.   It can be seen from this that the function of the thermal time constant T is relatively large, so it is necessary to pay close attention to its definition, various affecting factors, and derivation of its formula. However, I have seen some people added another boundary condition except dΤ=0 during the derivation of the expression of Τ: in extreme cases, the heat will not be transferred into the surrounding medium at all, which is also called "adiabatic condition". In this regard, I would like to put forward the following different views for discussion.   (1) In this article, the derivations of formula 4 and formula 5 have only used a "boundary condition" of dΤ=0 (Τ=Τu) when t=∞, so there is no need to add another adiabatic condition for derivation.   It is said that without heat dissipation, the time required to reach a stable temperature is called time constant (Τ), but that will only be true when the second item (dissipated heat) on the right of the equation meets a condition of , now that to reach a steady temperature Τ must be equal to Τu, which is at variance with objective reality of electric accessories including transformers.   Some might say that when people calculate the temperature of a transformer at a short-circuit current, don't they also use the formula obtained under the "adiabatic condition" to calculate the temperature of the transformer? Yes, but the case being considered is only the "short time" one, that is, the formula only applies when short-circuit durations never exceed 10s (actually 2s).    The time is very small compared with the thermal time constant of winding in oil-immersed power transformer (about 5min-20mins) and that of oil (about 1lh～5h). This is still true when compared with the thermal time constant of the windings of dry-type transformers (about or above 30min). In section 3.1 of this article, such a short duration makes it is possible to consider it as an adiabatic transient system. In a broad sense, if the heat energy emitted accounts for only a very small part of the heat supplied by the outside world during a same period, it can be roughly regarded as adiabatic process. At this point, it is precisely because of the recognition of  that it is in line with objective reality.   This proves that adiabatic conditions should not be used as the basis in the derivation of expression of Τ (formula 4) and that of transient temperature rise (formula 5).   (3) From the details of the derivation process in this article, I would like to say that the true expression of temperature rise of the heated body still has not been obtained yet in the end. Think about that, a task for you, my readers.   IV. Conclusion   (1) Starting from the principle of heat balance, this article expounds the model of the heating mechanism in the rising process of temperature rise of the heated body with concise mathematics and physical language.   (2) It is clearly pointed out in this paper that the steady temperature rise of the heated body can be calculated by using formula 2, and formula 5 and formula 7 are the formulas for calculating the temperature rise of the heated body during the transient process.   (3) It is pointed out that to obtain the expression of the transient temperature rise of the thermal time constant Τ and the heated body, the adiabatic condition should not be used for the process of derivation, which is not necessary either.   (4) Due to space constraints, this article has not covered the heat dissipation mechanism of heated body, but you readers can refer to other references about heat loss or transfer in Kynix and other sites.   FAQ   1. How hot is too hot for a power transformer? Transformers designed with high-temperature insulation systems can run safely at temps up to 200°F. But remember, a hot-running transformer is an angry transformer.    2. What is ambient temperature of transformer? The average ambient temperature for a transformer over a 24 hour period should not exceed 30 degrees Celsius. For instance, if the transformer ambient temperature was 40 deg. C for 12 hours, then the transformer must not exceed 20 deg.   3. Should doorbell transformer be hot? Transformers are always going to produce some heat. It's a part of the step-down process. It should only be warm to the touch, however.   4. What is maximum ambient temperature? In general, a safe range is between 60 and 75 degrees Fahrenheit or 15 and 25 degrees Celsius, although the cooler end of that range is better. Ambient temperatures above those ranges make it difficult for a computer's cooling system to keep it at a safe operating temperature.   5. What is the name of oil used in transformer? Mineral oil and Synthetic oil are the majorly used transformer oil. These are the petroleum products, like Naphthenic based transformer oil and Paraffinic based transformer oil. Naphthenic based transformer oils are known for their heat distribution, which is one of the main problems with transformer.   6. What will happen if the regulation of a transformer is poor? If the transformer supplies a very low lagging power factor, large secondary currents will flow resulting in poor voltage regulation due to greater voltage drops in the winding. ... Therefore positive regulation produces a voltage drop in the winding while a negative regulation produces a voltage rise in the winding.   7. What is high transformer temperature? Standard Ratings and Overload Capacity：Dry-type transformers are available in three standard temperature rises: 80C, 115C, or 150C. Liquid-filled transformers come in standard rises of 55C and 65C. These values are based on a maximum ambient temperature of 40C.   8. What is hot spot temperature in transformer? Modern transformers make use of thermally upgraded paper that has been chemically treated to improve the stability of cellulose structure. The rated hot spot temperature for this kind of paper is 110°C and it can be seen that an increase of 7°C will double the aging acceleration factor.   9. How much heat does a 75 kVA transformer give off? According to Cutler-Hammer, a 75-kVA, 150°F-rise, dry-type transformer has an efficiency of 97.2% at 1/4 load and 96.7% at full load. So, figure 3% loss at 75 kVA, which would represent 2,250 W.   10.What happens when transformer is overloaded? The weakening of the system will happen faster if the transformer is frequently overloaded. The net result of small, incremental increases in loading capacity over time is a weakened insulation system. Overloading causes overheating, and eventually thermal degradation that acts thrrough cracks in the insulation.   You May Also Like: Some suggestions about protecting transformers Learn Some Basic Knowledge about Capacitor Voltage Transformer  

In ancient Asia and Europe, "cutting the flesh to cure a boil" has a history of nearly 2500 years, but considering the level of medical treatment and anesthesia at that time, this seems to be more of torture. For irreparable damage to skin tissue, skin grafting is almost the only option considering the body's repulsive reaction. Doctors mainly rely on the removal of the patient's own skin or the skin of others for transplantation repair. Not to mention the unbearable pain, it will also leave new wounds on the patient's skin.In addition, the source of skin grafts for patients with large-area skin injuries is also a problem. The skin after transplantation is very fragile, with sequelae such as weakened sense of touch and decreased immunity.With the efforts of scientists from all over the world, the super-simulation electronic skin model is maturing. If it is put into human trials, this will be a good news for patients. This is a video introducing electronic skin Based on the development of electronic skin in recent years and the shortcomings of current wearable devices, this blog will introduce you to the structure of electronic skin and its future applications in the field of mobile health. Catalog  Ⅰ Introduction to electronic skinⅡ Development of electronic skinⅢ Electronic skin system architecture3.1 Flexible substrate3.2 Flexible battery3.3 Wireless communicationsⅣ Electronic Artificial Skin for   ApplicationⅤ ConclusionFAQ   Ⅰ Introduction to electronic skin Electronic skin, a system that allows robots to produce tactile sensations. It is not only simple in structure, but also can be processed into various shapes, and can even be attached to the surface of the device like clothes, allowing the robot to perceive information such as the location, orientation, and hardness of the object.Basic functions of electronic skin:From obtaining physical stimulation to distributed sensor array;Preprocess the sensor signal;The signal is transmitted wirelessly to higher-level systems (such as smart phones)The electronic skin is equipped with highly sensitive conductive nanomaterials, which can accurately cause slight tremors of the electrical changes of the muscle group. At the same time, the electronic skin is extensible (for example, it supports joint movement), and can even form integrated chemical sensors and biosensors.Therefore, electronic skin enables us to perceive different shapes and textures, temperature changes and different contact pressure levels. And, this is an integrated, scalable sensor network that can provide tactile and thermal signals to the brain, allowing us to operate safely and effectively in the surrounding environment. Human skin with distinctive features is a physical barrier to our interaction with the surrounding environment. Inspired by these features of human skin, researchers are working hard to create a flexible, scalable, and highly sensitive electronic device. Therefore, the development of electronic skin has become a research hotspot, especially in the fields of intelligent robots and electronic medicine.Ⅱ Development of electronic skinThe development of electronic skin technology could be divided into two stages:1）From 1970s to 1990s, the concept of e-skin appeared for the first time and got a preliminary development.2）Since 2000s, more researchers have been involved and have made a significant progress in recent years.In 1974, Clippinger demonstrated the feedback of a discrete sensor for a prosthetic hand. In 1985, General Electric first built a robotic arm sensitive skin which enabled to interact with the environment, placed on a flexible, curved sheet using discrete infrared sensors. In the 1990s, more and more teams began to create large-area, ultra-thin, multi-sensor flexible sheets. Jiang et al. first proposed a bent sensor sheet which obtained by etching thin silicon wafers and then integrating them on flexible polyimide film. In 2000, the organic transistor electronic nose was developed. Later, more achievements were made, such as scalable inverters, flexible active matrix technology, high resolution optical sensors, microstructured pressure sensors and so on. In 2003, the research team at the University of Tokyo in Japan made thin films by using low molecular organic compounds and realized the pressure of electronic skin through the pressure sensors on its surface. In 2010, the University of California, Berkeley, developed a technology to attach nanowire transistors to a sticky substrate, the resulting e-skin therefor could apperceive less than 50 grams of fine pressure and has been subjected to bending 2000 times. A woman scientist of Stanford University Bao Zhenan and her team have developed a highly sensitive flexible plastic film material that mimics human skin and senses subtle pressure. At the same time, the team developed the world's newest stretch solar cells, allowing electronic skin to self-generate electricity. In 2011, a researcher named John A.Rogers introduced an electronic patch for monitoring patient vital signs which described as "electronic skin." This device embedded the sensors in a film and placed the film on a flexible polyester substrate, like a kind of tattoo on the body. Physiological indexes of human health In 2014, electronic skin, developed by a researcher from the Chinese Academy of Sciences, was pasted on the human skin by static electricity, enabling real-time monitoring of physiological indexes of human health such as pulse, heartbeat, body temperature, muscle group vibration and so on, to promptly make a respond with feedback on changes of human health data. Ⅲ Electronic skin system architectureCompared with the current intelligent wearable devices, electronic skin has the characteristics of high sensitivity, ultra-thin, bendability and comfort in guardianship and monitoring the important physiological information of human body. E-skin system is a new type of flexible and extensible sensing system. By making sensors and circuits built on flexible substrates, e-skin systems can obtain unique ductility and more sensible to the various physical, chemical and biological signals. In the field of health care, the emergence of electronic skin will change the imprecise measurement of wearable devices, reduce the number of heavy monitoring equipment in the ward, and enable medical staff obtain the patient's physiological parameters in real time.Figure 1. Architecture of electronic skin systemFigure 2. E-skin structureFigure 3. Relationship between modules of e-skin system in medical applicationsThis system provides flexible circuits on flexible substrate, including microprocessors, Bluetooth and a variety of sensors (temperature, humidity and pressure sensors, blood oxygen, skin impedance and ECG sensors, etc.), which are connected to smart phones via Bluetooth. With the help of big data technology and cloud computing analyzes the data,  giving the diagnosis and treatment in time.Figure 4. Application scene of electronic skin system in mobile healthcareFigure 5. Electronic skin attached to temple to track brain wavesJohn Rogers, a professor of materials at the University of Illinois, Urbana-Champaign, has developed an e-skin called Biostamp, which can track brain waves in real time by sticking a flexible small sensor to a user's temple, able to show your deepest thoughts and feelings and translate them into information.Figure 6. Industrial designers of wearable health-monitoring electronicsIn the above structures, different applications have different requirements for sensors, and microcontrollers are becoming more and more lightweight in the field of electronic skin. So the following three parts of flexible substrate, power management and wireless communication technology will be described in detail.3.1 Flexible substrateOne of the most basic properties of electronic skin is that it has bending property, which can better attach to a large area of surface of human body One of the most basic properties of electronic skin is that it has bending property, which can better attach to a large area of human body surface. To achieve this property, the choice of materials is crucial. Advances in technology have enabled e-skin to be manufactured largely through the development of new materials and new processing methods. At present, polydimethylsiloxane (PDMS) and nanomaterials are commonly used as substrate materials:3.1.1 PDMSPDMS film is one of the most popular flexible substrates, including the advantages of good chemical inertia, being stable in a wide range of temperature, high transparency, variable mechanical properties and good adhesion to silicon wafer. At present, many research groups use PDMS as a flexible substrate. Sigurd Wagner and others used PDMS as a flexible substrate and found that wavy wires built into the film greatly enhanced its extensibility, such as obtaining skin tactile sensor arrays by printing silk screen on the PDMS film. The hypersensitive electronic skin equipment was fabricated by combining homogeneous microcosmic PDMS films with carbon nanocrystalline films.3.1.2 Nanophase materialsNanomaterials are a new type of materials developed in recent years. The current technological trends in the field of new materials are as follows:1. Carbon nanotubes: Compared with the zero-dimensional nanostructures such as carbon black, the one-dimensional carbon nanotubes have higher draw ratio and better electrical conductivity, which are used as conductive filler and then filled with the polymer composite can show lower resistivity and higher electrical conductivity.Figure 7. Schematic of carbon nanotube2. Graphene: Graphene is a hexagonal honeycomb structure consisting of a single layer of carbon atoms. It is the thinnest and strongest superconducting material ever known, which has a superior thermal, mechanical and electrical properties to carbon nanotubes, and with the tunneling effect it obtains a tactile sensor with high sensitivity, having a great application prospect in the field of conductive composite materials. Carbon nanotubes and graphene can be used not only as flexible substrates, but also as various good materials for high sensitivity sensors and flexible batteries. There are teams have so far made achievements in these areas and we are believing that nanomaterials will dominate in the near future.Figure 8. Graphene's atoms arranged in honeycomb pattern3.2 Flexible batteryLightness and softness are two of the most basic characteristics of e-skin. Traditional batteries can no longer meet the requirements of e-skin, but the  foldable and bendable flexible cell has become an indispensable part of e-skin equipment.Table 1. Comparison of current flexible batteryResearch instituteCellPerformanceProLogium Corporation, TaiwanUltra-Thin、Flexible FPC Lithium-Ceramic BatteryCuttable like paper, but cause no fire or explosion under bending, hammering, piercing, and 700-1300 ℃ high temperature gun fireImprint Energy, California, USAFlexible ultrathin zinc polymer battery3D printers in general use can be mass-produced at lower costRice University, USASuper-thin, High -performance flexible lithium free BatteryAfter 10,000 times of charge and discharge, or a thousand bends, it still maintains a capacity of 76%New Jersey Institute of TechnologyFlexible carbon nanotube cellIt can be made into various shapes and sizes, even DIY at homeSamsung Corp.Flexible bendable cellOrganic thin-film solar cellsFraunhofer Institute for Applied Polymer Research, GermanyOrganic thin film solar cellOrganic thin-film solar cellsNorthwestern University,USAFlexible stretchable lithium batteryStretchable, bendable, foldable and rechargeable wirelessly In addition to the flexible batteries mentioned above, a wireless charging technology developed in recent years also provides an alternative to the realization of electronic skin, including kinetic energy (motion, vibration, rotation) thermal energy, piezoelectricity and even radio waves (which can be viewed as wireless energy recovery) can be converted into usable electricity to provide a long-time even permanent energy supply. While it is still hard to really apply it to reality at the moment, it will be a new and innovative breakthrough in the future.3.3 Wireless communicationsIn recent years, in order to meet the requirement of intelligent equipment short-range communication, the automation short-range wireless technology emerges as the times require. At present, among all kinds of short-range wireless communication technologies, several mainstream of it such as Wi-Fi, ZigBeec, NFC, BLEID, UWB and so on become the main means for intelligent devices to communicate with each other at present. In e-skin system, wireless communication is still an indispensable part, and even has a higher requirements in communication performance, application environment and low power consumption.Table 2 Comparison of Wireless Communication TechnologyWireless technologyTransmission range / mMaximum transmission rate /MbpsTransmitting power / MWWi-Fi10-3054<50Zigbee10-100250kbps30NFC<20cm424kbps Bluetooth(BLE)10-1001~10RFID(UHF)<30~100kbps UWB3-10480 Wi-Fi is widely used in smart phones, and its fast transmission speed is an obvious advantage. Bluetooth is a wireless technology with low power consumption, ideal transmission distance and low cost. With the popularity of smart phones and the integration of Bluetooth modules, along with its gradually enhanced storage and computing capabilities, continuous real-time monitoring of the human body becomes possible. At the same time, the smart phone is used in the wearable health monitoring system as the information gateway to transmit the received physiological information, therefore the real-time monitoring of patients' health status has realized together with the emergence of big data technologies and cloud Computing. Bluetooth technology is the first choice for human-body monitoring system to transmit physiological signals in electronic skin. Ⅳ Electronic Artificial Skin for Application Figure 9. Electronic skin to monitor heart rateBiological tissue tends to be curved and soft, and most of the current wearable health monitoring devices are hard, rigid and difficult to achieve a large area of surface attachment. From an application point of view, this is not conducive to obtaining physiological signals from the human body. However because the e-skin has a flexible substrate which can be attached to a large area of tissue surface, and its sensors with high sensitivity can obtain the physiological signals of human body more accurately. Due to the unique properties of e-skin and the development of miniaturization technology, e-skin has great application potential in the fields of health monitoring, prosthesis, robot and so on.Figure 10. Intrinsically stretchable transistorIn the field of health care, electronic skin will have more applications, as shown in figure 11: Blood glucose detection;Speech recognition;Infant temperature monitoring;Intelligent Drug Administration, etc.Figure 11. Applications of electronic skin in health careIt is important for diabetics to be able to know their blood sugar changes all the time. Continuous blood glucose monitoring system can measure the patients’ blood glucose concentration with sensors containing specific enzymes.Speech recognition system(ASR) is an e-skin device attached to the throat of human body. It can monitor the weak pressure changes produced by muscle movement and transform them into speech, helping the deaf and mute to realize their dream of "speaking".Infant monitoring system can monitor the temperature, heart rate and other physical status of the baby in real time, and meanwhile feed back to the intelligent terminal in time.The intelligent drug delivery system can inject drugs regularly and quantitatively by placing them into the e-skin and monitoring the recovery of wounds by intelligent terminal control.With the combination of electronic skin and intelligent equipment, it is only necessary to transmit and analyze the signals obtained by e-skin to the intelligent equipment through wireless communication technology, then it has been able to monitor and provide feedback on the health condition of the human body in real time and in long distance. Electronic skin, as a new type of wearable device, will in the future provide real-time detection of blood pressure, blood sugar, heart rate, body temperature and etc. It is the best choice for real-time diagnosis and evaluation of human health. Ⅴ Conclusion The application prospect of e-skin is very extensive, not only in the field of health care, but also in the fields of consumer electronics, military affairs and even the more sci-fi robot "imitation of human skin", which will bring about revolutionary breakthroughs.With the endless emergence of wearable electronic devices, high sensitivity and miniaturization will become the mainstream trend. The emergence of electronic skin will undoubtedly bring about major technological breakthroughs and innovation opportunities for flexible wearable electronic devices.  FAQ 1. What is electronic skin used for?Flexible circuits inspired by human skin offer options for health monitoring, prosthetics and pressure-sensing robots. 2. What are the advantages of an e-skin?It helps the body to adjust after the transplant. It can make robots more sensitive. The use of tiny electronic wires allows the skin to generate impulses, similar to that of the body's own nervous system. It could lead to advancements in medical equipment. 3. What electronic skin is flexible?Electronic skin refers to flexible, stretchable and self-healing electronics that are able to mimic functionalities of human or animal skin. ... Advances in electronic skin research focuses on designing materials that are stretchy, robust, and flexible. 4. What is the main difference between flexible skin like sensors and the human skin?Like human skin, AISkin also is quite durable; however, while human skin can only stretch about 50 percent, the sensor-based skin can stretch up to 400 per cent of its length without breaking, making the material useful in wearable technology applications. 5. Who invented electronic skin?Researchers from the National University of Singapore have developed an 'electronic skin', capable of recreating a sense of touch thanks to more than 100 small sensors. They hope the technology can be applied to prosthetic limbs, allowing users to feel texture, temperature and pain. 6. Where has electronic skin been developed?National University of Singapore. A team from the National University of Singapore created the skin device, which measures 1 square centimeter. The system contains 100 small sensors that attempt to recreate things like texture, temperature and even pain. The researchers call the device Asynchronous Coded Electronic Skin, or ACES. 7. What can the skin sense?Receptors that let the body sense touch are located in the top layers of the skin - the dermis and epidermis. The skin contains different types of receptors. Together, they allow a person to feel sensations like pressure, pain, and temperature. ... They may sense pain, temperature, pressure, friction, or stretch. 8. What is a skin sensor?Electronic skin sensors, also known as the wearable thin film sensors, can be directly placed on the human body to measure body parameters such as body temperature, heartbeat, sweat composition etc. ... Electronic skin sensors have applications in many areas such as healthcare, sports, robotics and prosthetics, etc. 9. How do you replicate human skin?It was found that the most common materials used to simulate skin are liquid suspensions, gelatinous substances, elastomers, epoxy resins, metals and textiles. Nano- and micro-fillers can be incorporated in the skin models to tune their physical properties. 10. How is electronic skin made?Research into conductive electronic skin has taken two routes: conductive self-healing polymers or embedding conductive inorganic materials in non-conductive polymer networks. ... embedded silver nanoparticles (AgNPs) into a polymer matrix, making the e-skin conductive. 

Maxim Integrated is a company specializing in semiconductor interface, digital signal processor, analog signal chain, communication IC, power supply and battery management, etc. In the past twenty years, Maxim has developed ICs with high reliability, working in the extended temperature range for industrial applications. Now, they also offer products with current and voltage protection, reducing the devices' demand for space and power consumption with excellent performance indicators. CatalogsI MAX7409 / MAX741O / MAX7413 / MAX7414II MAX847 / MAX769III MAXl674 / MAXl675 / MAXl676IV MAX3875V MAX2105VI MAX3690VII MAX4539 / MAX4540VIII MAX7400 / MAX7403IX MAXl710X MAX668 / MAX669XI MAX254BXIII MAX2663 / MAX2671 / MAX2673Intro Integrated circuits (ICs) are a keystone of modern electronics, in this article we will disguss some kinds of ICs in Maxim Integrated. They are the heart and brains of most circuits. They are the ubiquitous little black "chips" you find on just about every circuit board. Unless you’re some kind of crazy, analog electronics wizard, you're likely to have at least one IC in every electronics project you build, so it's important to understand them, inside and out.An IC is a collection of electronic components ——resistors, transistors, capacitors, etc. ——all stuffed into a tiny chip, and connected together to achieve a common goal. They come in all sorts of flavors: single-circuit logic gates, op amps, 555 timers, voltage regulators, motor controllers, microcontrollers, microprocessors, FPGAs…the list just goes on-and-on.Embedded computing is based on microcontroller design and provides fixed function operation control. Embedded computing originated from industrial control applications, and has been widely used in consumer electronics, medical treatment, and even communications. Maxim provides real-time clock, security authentication, interface IC and sensor solutions for these markets,which makes it an important supplier in the industry.  What is IC chip and how does an IC work I Space and power saving 5th order filter IC---MAX7409/MAX741O/MAX7413/MAX7414New 5th order Bessel, Butterworth Switched-Capacitor Low Pass Filters series products, uMAX-8 pins and DIP package. MAXIM's proprietary uMAX package, which is 80% smaller than the 8-pins DIP package, makes it the industry's smallest 5-order switched capacitor filter. MAX7409/MAX7410 works at +5V, MAX7413/MAX7414 works at +3V and both devices draw only 1.2mA of supply current. Low price, small size and low power consumption make this kind of filter extremely suitable for price sensitive portable devices requiring DAX post-filtering or anti-aliasing applications.The MAX7409/MAX7413 Bessel filters have the characteristics of low overshoot, fast establishment and linear phase response, and the MAX7410/MAX7414 Butterworth filters have the flatest band-pass response. The four kinds of chips are all fixed frequency response, and the design tasks of filter is simplified to selecting clock frequency.The angular frequency can be tuned from 1Hz to 15 kHz by the clock at a rate of 100 times the clock/rotation angle. Two modes of clock operation, one is a self-contained clock of an external capacitor, another is an external clock which could strictly control the cut-off frequency. They have a very low output misalignment (±4 mV) and can be further regulated via an offset adjustment pin. Figure 1. Typical application circuit of MAX7409/MAX741O/MAX7413/MAX7414 II Power management ICs for communication equipment---MAX847/MAX769The MAX847/MAX769 DC-DC converter produced by MAXIM Company of USA has the characteristics of low voltage operation, high conversion efficiency and synchronous rectification. It is suitable for the low power digital radio communication system with 1 to 3 batteries, such as two-way paging, GPS receiver and so on, ensuring the cut-in voltage is as low as 0.87V and the quiescent current is 37 μA (the outage current is 2 μA).The built-in synchronous rectifier eliminates the external Schottky diode, meanwhile the conversion efficiency is increased to 90% , and for MAX847, an output current exceeding 50mA may be provided when power is supplied with a single battery. When two batteries are supplied, the MAX769 with function of boost/buck conversion could provide an output current over 90mA. Both chips can digitally adjust the output voltage through a serial interface compatible with SPI, which ranges from 1.8 V to 4.9 V whit an adjustable interval of 100 mV. The no-load current is only 13 μA.The MAX769 is similar to the MAX847 except that it contains a buck/boost DC-DC converter (for 2-cell or 3-cell inputs) rather than a boost-only converter (for 1-cell inputs). Both MAX847 and MAX769 include a multichannel ADC for battery monitoring. Three low noise Linear Regulator outputs for various uses (3V analog, 2.85V logic, and 1V receiver.  Both chips are 28-pin QSOP packaged. Figure 2. Typical application circuit of MAX847 Figure 3. Typical application circuit of MAX769 III Compact and efficient DC-DC converter IC with very low power supply current---MAXl674/MAXl675/MAXl676The MAX1674/MAX1675/MAX1676 DC-DC converter chip can provide up to 94% conversion efficiency. They available in small 8 or 10-pin uMAX package, and the static current is only 16 μA. The built-in synchronous rectifier not only improves efficiency but also eliminates the using of  external Schottky diodes, resulting in smaller dimensions and lower costs.The MAX1674 has a current-limiting of 1A, and the MAX1675 has a current-limiting of 0.5A, allowing the use of very small inductors. The MAX1676 has an optional current-limiting and minimizes the EMI due to an elimination of inductive oscillations. All chips have built-in N channel MOSFET with 0.3Ω and with a pin-selectable output voltage of 3.3V or 5V. The output voltage can also be adjusted in the range of 2 V to 5.5 V using the divider resistance. The input voltage ranges from 0.7 V to VOUT and the cut-in voltage can be as low as 1.1 V. Other features include: efficiency up to 94% when the output current is 200mA, built-in low voltage detection and 0.1 μA shutdown mode. IV Clock recovery and data retiming IC---MAX3875MAX3875 is a compact, low power clock recovery and data retiming chip for 2.488GbpsSDH／SONET systems. The fully integrated PLL can extract the synchronous clock from the serial NRZ input data which is retimed by the recovery clock. The clock and data output of the chip are compatible with the differential PECL and the additional 2.488Gbps serial input is used for loopback test of the system. It can also provide unlock monitoring signal of a TTL level.MAX3875 can be used as regenerator or terminal receiver in 0C-48／STM-16 transmission system. Insert jitter characteristics are higher than any SONET/SDH specification. The single supply is from 3.3V to 5V and when power is 3.3V, the power loss is less than 400mW in the full temperature range from -40 ℃ to 85 ℃.MAX3875 is available in 32-pin TQFP package.Figure 4. Typical application circuit of MAX3875 V Digital DBS direct-conversion tuner IC---MAX2105It is designed for the application of Direct Broadcast Satellite (DBS) TV set-top box. Compared with the structure based on intermediate frequency, the cost of the system is greatly reduced because of the use of direct frequency conversion structure. MAX2105 is supplied by a single power supply of 5V and the input signal frequency ranges from 950MHz to 2150MHz, tuned directly from L band to baseband by broadband I/Q down converter.The MAX2105 internal circuit includes a low noise amplifier with AGC, two down conversion mixers, an oscillating buffer with a 90° orthogonal generator, a prescaler and a baseband amplifier. The range of AGC gain adjustment is 41 dB, and the minimum power of input signal is -60 dB. Since the range of AGC gain adjustment is reduced, MAX2105 can use high gain external LNAs to obtain better noise coefficient, and can also provide a automatic baseband drift correction.MAX2105 is available in 28-pin SO package.Figure 5. Typical application circuit of MAX2105 VI SDH/SONET 8:1 serializer with clock synthesis and TTL inputs---MAX3690The MAX3690 serializer is powered by 3.3V only with a 200mW power consumption. Ideal for converting 8-bit-wide, 77Mbps parallel data to 622Mbps serial data in SDH / SONET system. Other applications include Add/Drop Multiplexers, Digital Cross-Connects.The clock and data input of MAX3690 is TTL logic level, serial data output is 3.3V PECL logic level. A fully integrated PLL synthesizes an internal 622Mbps serial clock from a low-speed crystal reference clock (77.76MHz, 51.84MHz, or 38.88MHz). An unlocked output of a TLL level can be used to indicate whether the PLL is working properly.MSX3690 is available in 32-pin TQFP packages.Figure 6. Functional diagram of MAX3690 VII Single 8-channel and dual-4 channel multiplexer ICs with precision resistance networks---MAX4539/MAX4540The single-8 channel MAX4539 and the dual-4 channel MAX4540 are a kind of multiway switch with calibration function (calibrating multiplexer), which is suitable for system self-testing and precision type MAX4540. Their built-in precision resistive partial voltage networks can provide accurate reference voltage of V+／2、5／8(V+ -V-)、15VREF/4096 and 4081VREF/4096 (Where VREF is external reference voltage).The MAX4539/MAX4540 have enable inputs and address latching. When power supply working at 5V or ±5V, the digital input has a 0.8V/2.4V logic threshold that guarantees compatibility with TTL/CMOS.The MAX4539/MAX4540 are available in small 20-pin DlP, S0, and SSOP packages, both of which can operate in a single supply of +2.7V to 12V or a duplicate supply of ±2.7V to ±6V. The on resistance (maximum 100 Ω) of the same device matches within 12Ω, and Each switch can handle Rail-to-Rail analog signals.The off leakage current is 1nA at TA = +25°C and 10nA at TA = +85°C.Functional diagram of MAX4539Figure 8. Functional diagram of MAX4540 VIII 8th-Order, lowpass, elliptic, switched-capacitor filters---MAX7400／MAX7403MAX7400/MAX7403 is a newly developed 8th-order, lowpass, elliptic, switched-capacitor filters by MAXIM Company of USA. The cut-off frequency ranges from 1Hz to 10kHz, and only draw 2mA of supply current. With a single power supply of 5 V, it is ideal for low power anti-aliasing and post-filtering of D/A converters. They feature a shutdown mode that reduces the supply current to 0.2μA.MAX7400 devices provide a sharp roll-off with a 1.5 transition ratio and 80 dB of stop-band rejection, while the MAX7403 devices provide a sharper roll-off of 1.2 transition ratio and 58 dB of stop-band rejection. For the both filters, the low output offset of ±4 mV can be adjusted via an offset adjustment pin.The internal switching operation of the filter can be controlled by an internal clock of an external capacitor or an external control to obtain a more accurate angular frequency. The fixed frequency response greatly simplifies the design, which only needs to set the clock frequency according to the desired angular frequency. MAX7400 and MAX7403 filters are available in 8-pin SOIC and plastic DIP packages.Figure 9. Typical application circuit of MAX7400/MAX7403 IX High-speed, digitally adjusted step-down controllers fornotebook CPUs---MAXl710The MAX1710/MAX1711 step-down controllers are intended for core CPU DC-DC converters in notebook computers. They feature a triple-threat combination of ultra-fast transient response, high DC accuracy, and high efficiency needed for leading-edge CPU core power supplies. Maxim's proprietary Quick-PWMTM quick-response, constant-on-time PWM control scheme handles wide input/output voltage ratios with ease and provides 100ns "instant-on" response to load transients while maintaining a relatively constant switching frequency.High DC precision is ensured by a 2-wire remote-sensing scheme that compensates for voltage drops in both the ground bus and supply rail. An on-board, digital-to-analog converter (DAC) sets the output voltage in compliance with Mobile Pentium Ⅱ CPU specifications. The MAX1710 achieves high efficiency at a reduced cost by eliminating the current-sense resistor found in traditional current-mode PWMs. Efficiency is further enhanced by an ability to drive very large synchronous-rectifier MOSFETs.Single-stage buck conversion allows these devices to directly step down high-voltage batteries for the highest possible efficiency. Alternatively, 2-stage conversion (stepping down the +5V system supply instead of the battery) at a higher switching frequency allows the minimum possible physical size. MAXl710 is available in Small 24-Pin QSOP Package.Figure 10. Typical application circuit of MAX7400/MAX1710 X Boosting DC-DC controllers with power levels of 20W---MAX668/MAX669The MAX668/MAX669 constant-frequency, pulse-width modulating (PWM), current-mode DC-DC controllers are designed for a wide range of DC-DC conversion applications including step-up, SEPIC, flyback, and isolated-output configurations. Power levels of 20W or more can be controlled with conversion efficiencies of over 90%. The 1.8V to 28V input voltage range supports a wide range of battery and AC-powered inputs. An advanced BiCMOS design features low operating current (220µA), adjustable operating frequency (100kHz to 500kHz), soft-start, and a SYNC input allowing the MAX668/MAX669 oscillator to be locked to an external clock.DC-DC conversion efficiency is optimized with a low 100mV current-sense voltage as well as with Maxim's proprietary Idle ModeTM control scheme. The controller operates in PWM mode at medium and heavy loads for lowest noise and optimum efficiency, then pulses only as needed (with reduced inductor current) to reduce operating current and maximize efficiency under light loads. A logic-level shutdown input is also included, reducing supply current to 3.5µA.The MAX669, optimized for low input voltages with a guaranteed start-up voltage of 1.8V, requires boot-strapped operation (IC powered from boosted output). It supports output voltages up to 28V. The MAX668 operates with inputs as low as 3V and can be connected in either a bootstrapped or non-bootstrapped (IC powered from input supply or other source) configuration. When not bootstrapped, it has no restriction on output voltage. Both ICs are available in an extremely compact 10-pin µMAX packages.Figure 11. Typical application circuit of MAX669Figure 12. Functional diagram of MAX668/MAX669 XI Low cost and small size RS-232 transceiver---MAX254BMAX254B is a complete electrically isolated RS-232 interface developed by MAXIM Company in USA. It is suitable for the system with demanding cost and size. It is mainly aimed at equipment in which noise, high transient voltage and highland potential damage or communication interference maybe occur.  XII High-performance laser driver---MAX3867The MAX3867 is a complete, single +3.3V laser driver for SDH/SONET applications up to 2.5Gbps. The device accepts differential PECL data and clock inputs and provides bias and modulation currents for driving a laser. The synchronizing input latch can be bypassed if a clock signal is not available.An automatic power control (APC) feedback loop is incorporated to maintain a constant average optical power over temperature and lifetime. The wide modulation current range of 5mA to 60mA and bias current of 1mA to 100mA are easy to program, making this product ideal for use in various SDH/SONET applications.The MAX3867 also provides enable control, a programmable slow-start circuit to set the laser turn-on delay, and a failure-monitor output to indicate when the APC loop is unable to maintain the average optical power. The MAX3867 is available in a small 48-pin TQFP package as well as dice.Figure 13. Functional diagram of MAX3867 XIII High linearity upconverters---MAX2663／MAX2671／MAX2673The MAX2663/MAX2671/MAX2673 miniature, low-cost, low-noise upconverters are designed for low-voltage operation and are ideal for use in portable consumer equipment. Signals at the IF input port are mixed with signals at the local oscillator (LO) port using a double-balanced mixer. These upconverters operate with IF input frequencies between 40MHz and 500MHz, and upconvert to output frequencies as high as 2.5GHz.These upconverters offer a wide range of supply currents and output intercept levels to optimize system performance. Supply current is essentially constant over the specified supply voltage range. Additionally, when the devices are in a typical configuration with VSHDN-bar=0, a shutdown mode reduces the supply current to less than 1μA.The MAX2663/MAX2671 family of upconverters are offered in the space-saving 6-pin SOT23 package. For applications requiring balanced IF ports, choose the MAX2673 upconverters in the 8-pin μMAX package.Figure 14. Typical application circuit of MAX2663／MAX2671／MAX2673   

Warm hints: The word in this article is about 2500 words and reading time is about 12 minutes.   This paper mainly introduces that how to use a solid state relay to drive a thermostat.    As we all known,relay is an electrical control device, an electrical appliance that makes the predetermined step change in the electrical output circuit when the input (excitation) changes reach the required requirements. Catalog   I. Solid State Relay Basics 1.1    What is solid state relay 1.2    Solid state relay working principle 1.3    Solid state relay appliances II. Thermostat Basics 2.1    What is thermostat 2.2     Types of thermostat 2.3    Features of thermostat 2.4    Applications of thermostat III. Drive Thermostat by Using Solid State Relay 3.1    Power thermostat 3.2    Case of driving thermostat FAQ   I. Solid State Relay Basics   1.1 What is Solid State Relay   A solid-state relay(SSR) is a contactless switch consisting of microelectronic circuits, discrete electronic devices and power electronic power devices. The isolation between the control end and the load side is realized by isolation devices. The input of the solid-state relay is controlled by a tiny control signal to directly drive the large current load.   SSR takes advantage of the switching characteristics of electronic components, such as switch triode, bidirectional thyristor and other semiconductor devices, to achieve the purpose of connecting and disconnecting the circuits without contact and sparkless, and therefore is also called "contactless switch".    A solid-state relay is a four-terminal active device, of which two terminals are input control terminal, and the other ends are output controlled ends. It has both amplification and isolation function. It is suitable for driving high power switching actuator, which is more reliable than electromagnetic relay and has no contact, long life, fast speed and interference to the outside. Because of its small size, it has been widely used.   1.2  Solid State Relay Working Principle   SSR can be divided into two types: AC type and DC type according to the use occasions.    They can switch loads on AC or DC power supply, and they can not be mixed. The following is an example of the AC type SSR as an example of its working principle. The following diagram is a block diagram of its working principle. The components in the diagram constitute the main body of the AC SSR. From the whole, SSR has only two inputs (A and B) and two output terminals (C and D), and is a four-terminal device.   Working principle block diagram of solid state relay   When a certain control signal is added to the A and B, the "switch" and "break" between the two ends of C and D can be controlled and the function of "switch" can be realized. The function of the coupling circuit is to provide a channel between the input/output terminal of the control signal input from the A and B ends, but it disconnects the input and output terminals of the SSR in the electrical circuit.    In order to prevent the effect of the output end on the input end, the coupling circuit used the "optical coupler", which is sensitive, responsive, and high in the input/output insulation (voltage resistance) level; because the input terminal load is a light-emitting diode, this makes the input end of the SSR easily matched with the input signal level. When used, it can be directly connected to the output interface of the computer, that is, the logical level control of "1" and "0".    The function of the trigger circuit is to generate the required trigger signal, drive the switch circuit 4, but because the switch circuit does not add the special control circuit, it will produce the radio frequency interference and pollute the power grid such as the high order harmonic or the peak, so the zero-crossing control circuit is set up. The "zero crossings" means that when the control signal is added and the AC voltage is over zero, the SSR is a passing state, and when the control signal is broken, the SSR is to wait for the junction point (zero potential) of the positive half of the alternating current and the negative half of the half-week (zero potential), and the SSR is broken.    This design can prevent high-order harmonic interference and pollution to the power grid. The absorption circuit is designed to prevent the shock and interference (or even misoperation) of the peak, surge (voltage) transmitted from the power supply to the bidirectional thyristor in the switch device, usually using an "R-C" series absorption circuit or a nonlinear resistor (varistor).   1.3  Solid State Relay Appliances   The special solid-state relay can have the function of short circuit protection, overload protection and overheating protection. With the combined logic curing package, the intelligent module can be realized by the user. It is directly used in the control system.   Solid-state relay has been widely used in computer peripheral interface equipment, thermostat system, temperature regulating, electric furnace heating control, motor control, CNC machine, remote control system, industrial automation device, signal light, light adjustment, scintillator, lighting stage lighting control system, instruments, medical instruments, duplicator, automatic laundry. Machine, automatic fire protection, security system, and power capacitor switching switch as power factor compensation for the power grid, and so on, in addition to the chemical, coal mine, explosion-proof, anti-corrosion, corrosion prevention and so on. The logical curing encapsulation can realize the intelligent modules that users need and is directly used in the control system.     II. Thermostat Basics   2.1  What is Thermostat   The thermostat is a device that directly or indirectly controls one or more hot and cold Yuanlai to maintain the desired temperature. In order to achieve this function, a thermostat must have a sensitive element and a converter. The sensitive element can measure the change of temperature and produce the function required for the converter. The converter converts the function from the sensing element to the proper control of the device that changes the temperature. Thermostat 2.2 Types of Thermostat The types of the thermostat are generally the following: （1）Insert thermostat is installed on the pipe and sensitive element is inserted into the pipeline. （2）Immerse sensitive elements immersed in liquid in pipes or containers to control liquids. （3）Surface sensitive elements installed on the surface of pipes or similar surfaces.   2.3  Features of Thermostat This thermostat pressure gauge setting range (5~35 C) This thermostat measurement accuracy: plus or minus 1 DEG C The thermostat. Size: 86 x 86 (mm) Power supply: AC220V thermostat. This thermostat using ultra-thin design, electrical interface It has a large LCD screen with an LCD thermostat (backlight green, Lan Beiguang) You can display the thermostat in international language (Chinese + English) The thermostat has the function of automatic and manual. This thermostat for refrigeration heating and ventilation three working modes This thermostat high low-speed automatic selection Thermostat timing shutdown function. This thermostat control fan coil end of the fan, water valve, air valve You can also set the password on the thermostat setting temperature and wind speed according to the requirements of users.   Features of thermostat 2.4  Applications of Thermostat   The most common use of the thermostat is to control the room temperature.   Typical uses include: control the gas valve; control the fuel furnace regulator; control the electric heating regulator; control the refrigeration compressor; control the gate regulator.   A room temperature regulator can be used to provide a variety of control functions, such as heating control, heating - cooling control, day and night control (at night at lower temperatures), multistage control, primary or multistage heating, primary or multistage cooling, or multistage heating and cooling control.       III. Drive Thermostat by Using Solid State Relay   3.1  Power Thermostat   There are two kinds of power supply for the thermostat: battery and 24VAC power.    The thermostat needs battery power to run without interruption. It is very important that these batteries consume as low energy as possible, but even if you minimize the power consumption, the users are still inconvenient because the battery needs to be replaced from time to time. In order to reduce the replacement frequency, you can use a 24 VAC power supply. When the C line in the system is not available, the bridge rectifier shown in Figure 1 can convert the AC (AC) voltage to a DC (DC) voltage by the load. Single thermostat signal relay connection with HVAC load   3.2  Case of Driving Thermostat When the HVAC load (compressor, fan, gas valve, etc.) is turned off, the contact of the signal relay is broken. When the contacts are open, the terminals of the rectifier bridge see the voltage of the HVAC transformer is 24VAC, and convert the AC power to DC power, as mentioned earlier. The resulting DC voltage is used to drive the thermostat or subcircuit.   During the HVAC load conduction, the contacts of the signal relay are closed. When the contact is closed, the voltage across the bridge terminal is reduced to zero. This eliminates the need to use 24VAC as a power supply, so the thermostat battery power must be controlled. The range of current required for operating electromechanical relays ranges from tens to hundreds of Ma, which can have a significant impact on battery life.   If there is a way to drive a relay without using a thermostat battery, what will happen? Battery life will increase and replacement frequency will be further reduced. One way is to turn on the relay and charge the control system briefly during the HVAC load conduction (signal relay contact closure).  Compared with the turn off time of the power relay, the time required during charging is very short, which can stimulate the power relay and its corresponding load. Unfortunately, electromechanical (signal) relays are not likely to achieve this goal due to their switching speed limits. The time taken by the contact to the desired location is in milliseconds and will interrupt the HVAC load.   Fortunately, a device can achieve the appropriate switching speed: solid-state relay (SSR). SSR is a semiconductor repeater based on a thyristor or power transistor to perform on / off control.   This recharge method requires a dual MOSFET SSR because it can turn off MOSFET based SSR when necessary. Besides, body diodes of each MOSFET can assist in 24VAC rectification. A full-wave rectifier bridge is built with two diode MOSFET diodes, as shown below.   A power supply for SSR in a HVAC system The following figure shows the rectified waveform corresponding to the color coded diode in the above figure. The voltage ripple of the final waveform can be eliminated by connecting a suitable capacitor to the output of the rectifier bridge. Then, you can reduce the DC voltage of the control system to the desired voltage. Full wave rectifying waveform The use of SSR enables the HVAC system to fully supply the thermostat and reduce the power utilization rate of the battery. When SSR closes, the HV1 and HV2 pipelines will see the full 24VAC voltage and provide a constant 33VDC voltage at the output of the rectifier bridge. When SSR is connected, it may still be circulated through a short-time on/off state to recharge the power supply capacitor. This design can greatly reduce the energy requirements of the thermostat battery and reduce the battery replacement frequency.   FAQ   1. What is solid state relay and how it works? A solid state relay (SSR) is an electronic switching device that switches on or off when an external voltage (AC or DC) is applied across its control terminals. It serves the same function as an electromechanical relay, but has no moving parts and therefore results in a longer operational lifetime.   2. What is the difference between a relay and a solid state relay? The main difference between solid state relays and general relays is that there is no movable contacts in solid state relay (SSR). In general, solid state relays are quite similar to the mechanical relays that have movable contacts. ... SSR provide high-speed, high-frequency switching operations.   3. How fast is a solid state relay? The SSR output is activated immediately after applying control voltage. Consequently, this relay can turn on anywhere along the AC sinusoidal voltage curve. Response times can typically be as low as 1 ms. The SSR is particularly suitable in application where a fast response time is desired, such as solenoids or coils.   4. Do solid state relays get hot? All solid state relays develop heat as a result of a forward voltage drop through the junction of the output device. Beyond a point, heat will cause a lowering (or derating) of the load current that can be handled by the SSR. ... Loads greater than 4 Amps will require heat sinks.   5. What causes solid state relay failure? What are the main causes and solutions of the Solid-state Relays (SSR)'s failures? If an inrush current exceeds the rated making current of the SSR due to the high inrush current of loads such as motors and lamps, SSR output elements are damaged. Consider using an SSR with a higher capacity.   6. Can a solid state relay switch DC? Solid state relays can be designed to switch both AC or DC currents by using an SCR, TRIAC, or switching transistor output instead of the usual mechanical normally-open (NO) contacts.   7. How do you test a solid state relay with a multimeter? Using Multimeter:  1. Set the multimeter in continuity test mode. 2. Place the probes of the multimeter on the coil terminals. 3. If the multimeter beeps (or show any sign of continuity), the coil is electrically closed (good). 4. If the multimeter does not beep, the coil is open & damaged. The relay needs to be replaced.   8. How reliable are solid state relays? Solid-state relays are the preferred choice for system reliability because they have no moving parts or contacts. Over time, the plating on the contacts inside EMRs can erode. This erosion can cause the contacts to weld shut; therefore they no longer open/close properly, and the relay has to be replaced.   9. Is a solid state relay a transistor? Solid-State Relay: A sort of hybrid between a conventional relay and a transistor, these relays switch a load using an LED activated by the control circuitry. The LED activates a light-activated MOSFET that controls the load.   10. How do I know if my solid state relay is bad? Solid-state relays should be checked with an ohmmeter across the normally open (N.O.) terminals when control power is off. The relays should be open, switched to OL, and closed (0.2 , the internal resistance of the ohmmeter) when control power is applied.   11. How do I choose a solid state relay? When selecting a Solid State Relay, consider: Current rating, as a general rule consider using the relay at no more than 70% of its rated current. Electrical environment,. i(In harsh electrical environments, consider a relay with an line voltage rating above the application line voltage.)   12. Do solid state relays need a diode? 2 Answers. The control side of solid state relays is usually just a LED, sometimes two LEDs back to back, and sometimes with integrated resistor. ... If the relay is on the same board as whatever is driving it, then no inductive kickback diode is needed. It's no different than driving any other on-board LED.   13. Do solid state relays leak voltage? Solid State relays have leakage. If you want to repeatedly switch something on / off, use them. But when you want the SSR to be fully off, say after pressing an off switch, a mechanical relay should be across the load to take it off the SSR. ... The SSR control is attached to the atmega328 through a 200ohm resistor.   Relevant information about "How  to Drive Thermostat by Using Solid State Relay " About the article "How  to Drive Thermostat by Using Solid State Relay", If you have better ideas, don't hesitate to write your thoughts in the following comment area. You also can find more articles about electronic semiconductor through Google search engine, or refer to the following related articles.   Making a Arduino Variable Timer Relay Comprehensive Introduction of the Time Delay Relays

Warm hints: The word in this article is about 3000 words and reading time is about 15 minutes   This article is mainly talking about how to design better electromagnetic compatibility if LCD. Electromagnetic compatibility (EMC) is an inevitable issue in the design of LCDs. If the EMC design is not good, it will cause water ripples and strobe flash problems during the broadcast of the TV. EMC design is actually optimized for the electromagnetic interference generated in the product to meet the EMC standards of countries or regions. It is defined as the ability of a device or system to function properly in its electromagnetic environment and does not constitute unacceptable electromagnetic interference (EMI) to anything in the environment.       Catalog I. Brief Introduction II. Power Module EMC Design III. Main Drive Board EMC Design IV. Tuner Board EMC Design V. Whole Machine EMC Design FAQ   I. Brief Introduction Electromagnetic interference is generally divided into conducted interference and radiation interference. Conducted interference refers to the coupling interference of signals on one electrical network to another electrical network through conductive media. Radiated interference means that the interference source couples (disturbs) its signal to another electrical network through space.   The LCD structure mainly includes a liquid crystal display module, a power supply module, a drive module (mainly including the main drive board and a tuner board), and a key button module. General liquid crystal display modules have been tested by EMC before production. Here mainly introduces the design of the power module, drive module, button module, and the whole machine should pay attention to the electromagnetic interference problem.   EMI (ElectroMagnetic Interference) & EMC (Electromegetic Compatibility)   II. Power Module EMC Design The two main functions of the power supply section are to realize the backlight for driving the LCD screen and to provide DC power for other modules (including the drive module and the button module).   The design of the power module directly affects the entire system. If the design is not good, it will cause large water ripples in the TV. In severe cases, the TV will not be used. At the same time, it will seriously affect the normal use of other nearby equipment.   The power supply of LCDs is based on switching power supplies. The causes of electromagnetic interference problems caused by switching power supplies are complex. When designing the switching power supply, it is necessary to prevent the switching power supply from causing interference to the power grid and nearby electronic equipment. It is also necessary to strengthen the adaptability of the switching power supply itself to the electromagnetic interference environment.   To solve the EMC problem of switching power supplies, the following main measures should be taken into consideration during design:   Soft-switching technology: Inrush current and peak voltage are generated when the switching device is turned on/off. This is the main reason for electromagnetic interference and switching loss in the switch. Soft switching technology is an important method to reduce the loss of switching devices and improve the EMC characteristics of switching devices. This technology is mainly to switch the switching tube in the switching power supply at zero voltage and zero current to effectively suppress electromagnetic interference.   Modulation frequency control: Electromagnetic interference is changed according to the switching frequency, and the interference energy is concentrated on the discrete switching frequency point, resulting in large interference intensity. By distributing the energy modulation of the switching signal over a wide frequency band, a series of discrete sidebands are generated. This spreads out the interference spectrum, and the interference energy is distributed on the discrete frequency band, thereby reducing the electromagnetic interference intensity at the switching frequency point.   Component layout and routing: The components associated with the power input signal and output signal are placed near the corresponding ports to avoid interference due to the coupling path. Put components that are related to each other together to avoid interference caused by long traces.   Also, try to avoid parallel routing of signal lines. If unavoidable, try to increase the line spacing. Or add a ground wire in the middle to reduce the interference between each other.   III. Main Drive Board EMC Design The main driver board of the LCD mainly includes an analog signal portion, a high-speed digital circuit portion, and a noise source DC-DC power supply portion.   Component layout and routing: In the layout, the three parts of the analog signal part, the high-speed digital circuit part, and the noise source DC-DC power supply part should be reasonably separated so that the signal coupling between them is the minimum. In terms of device placement, the principle of associating the devices with each other is as close as possible, so that a good anti-noise effect can be obtained.   DC-DC Power Supply Part and Ground: On printed circuit boards, the power line and ground are the most important. Let analog and digital circuits have their own power and ground paths, respectively. The main means of overcoming electromagnetic interference is grounding.   On the driving board of the LCD, the ground of the power supply section (DC-DC) is mainly separated from other grounds such as the decoding and main chip processing, so as to reduce interference of the power supply on the image display and the television sound.   If there are analog ground and the digital ground when designing the circuit, they should be separated when the printed board is laid. To reduce mutual interference. In the layout of double-layer boards and multilayer PCBs, one layer of copper foil is generally used as a dedicated ground plane. The purpose of this is that this ground serves as a shield.   Integrated chip: In the same integrated chip, the ground is also separated from the analog ground and the digital ground. For example, the AD9883 analog-to-digital conversion chip of the AD company, which is often used as the main driver board of the LCD, can be floor-separated between the ground and digital sections of the analog section of the chip during the PCB design. Finally, connect the two points by a relatively short wire. Or connect the two places with a 1nF bypass capacitor.   Crystal oscillator: The clock circuit in the digital circuit is one of the main electromagnetic interference sources in current electronic products and is the main content of EMC design. Crystal is a strong source of radiation. The internal circuit of the crystal generates a large RF current, so that the ground lead of the crystal cannot sufficiently draw a relatively large Ldi/dt current to the ground plane with little loss, and as a result, the metal housing becomes a monopole antenna. The periphery of the crystal is a radiation field.   Therefore, the crystal oscillator circuit is far away from the interface circuit, such as serial port, address line, and data line. In order to avoid the interface circuit bringing the harmonic signal of the crystal out of the printed circuit board to cause electromagnetic interference. Two legs of the crystal oscillator must be added with an RC filter circuit. At the same time, be sure to connect the metal shell of the crystal to the ground on the printed board. In addition, the crystal is placed as close as possible to the chip pins. The ground is used to isolate the clock area, placing a local ground plane and connecting it to the ground through multiple vias.   Capacitance decoupling: Capacitance decoupling is used to reduce electromagnetic interference. Capacitor decoupling can be divided into three types: overall, partial, and inter-board.   The overall decoupling capacitor operates at low frequencies, providing a stable voltage and current for the entire board. It should be placed close to the printed circuit board power cord and ground. The typical decoupling capacitor value is 0.1μF. The typical value of the distributed inductance of this capacitor is 5μH. The 0.1μF decoupling capacitor has 5μH distributed inductance. Its parallel resonant frequency is about 7MHz. That is to say, it has a better decoupling effect for noise below 10MHz, and it has almost no effect on noise above tens of MHz. So for noise above 20MHz, use a 0.01μF capacitor decoupling.   The local decoupling capacitor makes the supply voltage obtained by the integrated circuit more stable; in addition, the high-frequency noise of the device is bypassed.   The decoupling capacitance between boards refers to the capacitance between the power plane and the ground plane and mainly solves the high-frequency transient current generated in the power supply. A 10~100uF electrolytic capacitor is connected across the input of the power supply. If the position of the printed circuit board is allowed, the anti-interference effect of the electrolytic capacitor with 100uF or more will be better. The lead of the decoupling capacitor can not be too long, generally close to the integrated circuit power supply, the connection should be rougher.   Bead filtering: Bead filtering is applied to all signal inputs (such as YPBPR and VGA) on the motherboard. Magnetic beads are designed to suppress high-frequency noise and spike interference on signal lines and power lines, and also have the ability to absorb electrostatic pulses. It acts as a high-frequency resistor, which attenuates high frequencies. The device allows the DC signal to pass and filter out the AC signal.   When selecting beads, you must pay attention to the following factors:   1. What is the unwanted signal frequency range? 2. Who is the noise source; 3, how much noise attenuation; 4. What is the environmental condition (temperature, DC voltage, structural strength); 5. What is the circuit and load impedance? 6. Is there room to place beads on the PCB board?   The first three can be judged by observing the impedance frequency curve provided by the manufacturer. The three curves in the impedance curve are very important, namely the resistance R, inductive reactance X, and total reactance Z. As shown in Figure 1:     Figure 1: Impedance curve and equivalent circuit topology that reflect the bead resistance, inductive reactance, and total inductance   The total impedance is described by the following formula (1):   Z=(R + 2πFL)   From this curve, beads are selected that have the maximum impedance in the frequency range where attenuation of the noise is desired, and where the attenuation of the signal is as small as possible at low and DC.   Chip beads can affect the impedance characteristics under excessive DC voltage. In addition, if the operating temperature rises too high or the external magnetic field is too large, the impedance of the beads will be adversely affected.   Whether using chip beads or chip inductors is also mainly in applications. Chip inductors are needed in the resonant circuit. When it is necessary to eliminate unwanted electromagnetic interference noise, the use of chip beads is the best choice.   IV. Tuner Board EMC Design The tuner board mainly includes a tuner section and an audio processing section.   When conducting the circuit design of the tuner board part and the layout of the PCB board, it is necessary to pay special attention to the electromagnetic interference problem. The following points must be considered:   （1）First, separate the land of the TUNER section (ie the simulated ground) from the land of the other sections. （2） Be sure to connect the metal shell of TUNER to the ground. The connection points can better eliminate electromagnetic interference. The tuner TUNER inherently has a high-frequency circuit, so it must be shielded. （3）When selecting the interface terminals (such as AV terminal, S-VIDEO terminal, etc.), try to use terminals with good conductivity and strong anti-electromagnetic interference, and also connect the ground of the interface terminal with the earth completely. At the same time also added magnetic beads filter. （4）The signal line should be as short and straight as possible. If it cannot be avoided, fly line transitions can be used. Signal lines should not form a ring. Because the ring is equivalent to the number of turns of the coil, the radiation effect of the ring wiring is the strongest. （5）Try to reduce dead copper in large areas. The solution is to connect them to the ground. If a large area of dead copper forms the antenna, electromagnetic interference will be introduced. （6）Do not run under quartz crystals and under noise-sensitive devices.   The audio processing section should pay special attention to the layout of the printed circuit board, first of all, avoid high-speed signal lines and audio and video lines together. For example, if you connect the clock line SCL and the data line SDA in the I2C bus to the traces of the audio line. Since the clock line SCL and the data line SDA in the I2C bus are constantly changing, they interfere with the sound. Obviously, for example, when you use a TV remote control to switch to a TV channel, you can hear a regular "click, click" sound from the speaker. This may be because of the above issues that were overlooked in the PCB layout.     V. Whole Machine EMC Design   The assembly drawing in the whole machine (taking one of the models as an example) is shown in Figure 2: Figure 2: In-machine assembly drawing of a model that reflects various EMI concerns   The connection line numbered 5 in the figure above is the screen line of the digital panel connection screen. Because the screen line is mainly on the screen data. It will cause a great disturbance to the system. The best way to reduce interference is to use twisted pairs and shielded wires. If it is a TTL screen, the screen line needs to be shielded or a magnetic ring outside the connection line. If on the LVDS screen, you need to use twisted pair, plus a magnetic ring. In order to reduce the screen line to the entire system of electromagnetic interference. With shielded twisted pair, the signal current can flow on the two inner conductors, and the noise current flows in the shield layer, thus eliminating the coupling of the common impedance, and any interference will induce the two conductors at the same time so that the noise cancels.   A magnetic ring is also required on the connection between the power supply and the main control board (referenced 4). The main reason is that the power cord will generate relatively large electromagnetic interference to the motherboard.   A magnetic ring should also be added to the connection between the keypad and the motherboard (referenced 9). The main reason is that there is a constant data change (remote control receiver head) on the keypad which causes electromagnetic interference to the system. Plus magnetic rings can effectively shield electromagnetic interference.   A magnetic ring is added to the audio cable (labeled 10) connected to the speaker to reduce the electromagnetic interference from the audio output to the system. If there is a cable (label 6, 7, 8) between the motherboard and the tuner board, you need to add a magnetic ring on the cable. To reduce the electromagnetic interference between cables.   The magnetic ring added above can be added according to the specific situation and can be determined by repeated experiments.   Use of shields: In general, shields are required for liquid crystal display modules, main control boards (including digital boards and tuner boards), and power supplies.   The main frequency of the main chip is the main cause of electromagnetic interference. Frequency harmonics of the main frequency are most likely to produce electromagnetic interference. In the experiment conducted by EMC, the frequency harmonics of the main frequency had large electromagnetic interference. The main chip must be shielded during design. The main shielding measures include a metal shield on the digital board. Adding a shield is the most effective way to resist electromagnetic interference. However, because of the heat dissipation problem of the driver board and the entire system, it is required that the holes on the shield cover be used to dissipate heat. However, its maximum size must be less than 1/100 of the shortest wavelength of noise.   The shield on the tuner board is mainly shielded from the TUNER section.   The shielding of the power supply section is particularly important. If the shielding of the power supply section is not good, it will cause large interference. This will not lead to conduction. And because the heat of the power supply is very severe, the shield must pay attention to the problem of heat dissipation.   Usually, shields have openings and seams that can cause electromagnetic leaks. As a result, the shielding effect is not good. Solve electromagnetic leaks at joints by using electromagnetic seal gaskets at the joints. The electromagnetic leakage of the opening in the shield is related to the size of the opening, the characteristics of the radiation source, and the distance from the radiation source to the opening. The requirement for shielding is met by designing the size of the opening and the distance of the radiation source to the opening. FAQ   1. What can cause electromagnetic interference? Electromagnetic interference (EMI) is a disturbance caused by an electromagnetic field which impedes the proper performance of an electrical device. EMI can come from man-made or natural sources such as the sun or the Earth's magnetic fields.   2. How do you stop electromagnetic interference? The simplest way to reduce magnetically induced interference is to use twisted pair wires. This applies both for shielded and unshielded cables and for interference caused by shield currents or from other sources. Twisting the wires forces them close together, reducing the loop area and therefore the induced voltage.   3. How do you make electromagnetic interference? Plug both devices into a wall outlet in the same house or building. Since the wall outlets in most houses are tied to the same ground, the ground is a common source of conducted interference, especially from the low frequency hum of an electric motor. Turn on both devices at the same time.   4. Is electromagnetic interference bad for you? There is no doubt that short-term exposure to very high levels of electromagnetic fields can be harmful to health. ... Despite extensive research, to date there is no evidence to conclude that exposure to low level electromagnetic fields is harmful to human health.   5. What are three types of interference? Electromagnetic interference (EMI) Co-channel interference (CCI), also known as crosstalk. Adjacent-channel interference (ACI) Intersymbol interference (ISI)   6. What material can block electromagnetic fields? Typical materials used for electromagnetic shielding include sheet metal, metal screen, and metal foam. Common sheet metals for shielding include copper, brass, nickel, silver, steel, and tin.   7. What blocks electromagnetic interference? Carbons. Carbon materials (e.g., coke, graphite, graphene, carbon fiber, carbon nanofiber and carbon nanotube) are not only conductive electrically, they are good absorbers of electromagnetic radiation over a wide frequency range.   8. Can humans cause electromagnetic interference? The human body functions as an antenna in the low-frequency band used by HBC. Owing to this antenna function, electromagnetic waves radiating from electronic devices or wireless services cause electromagnetic interference (EMI) in HBC devices.   9. What are two sources of electromagnetic interference that can affect data transmission? Electromagnetic interference can be categorized as follows: Narrowband EMI or RFI interference typically emanates from intended transmissions, such as radio and TV stations or mobile phones. Broadband EMI or RFI interference is unintentional radiation from sources such as electric power transmission lines.   10. What is the EMC? Electromagnetic Compatibility, also known as EMC, is the interaction of electrical and electronic equipment with its electromagnetic environment, and with other equipment. All electronic devices have the potential to emit electromagnetic fields. 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