As we have introduced in the last blog, UC3842 is a fixed frequency current-mode PWM controller. This IC is specially designed for Off-Line and DC to DC converter applications with minimum external components. In the blog today, we'll have a further discussion about the application of UC3842 in the boost conversion circuit.CatalogBoost Conversion Circuit OverviewI. Circuit Model of Boost Converter in DCM Mode1.1 Mathematical Model of DCM Working Mode1.2 Working Conditions of DCM ModeII. DCM Circuit Design Based on UC38422.1 DCM Circuit Design Based on Adder2.2 UC3842 Working PrincipleIII. Simulation and Analysis of DCM Mode CircuitIV. ConclusionV FAQBoost Conversion Circuit OverviewBoost converter s can reduce the output current and the capacitance and volume of the output filter capacitor under a certain output power, and are widely used in switching power supplies and electronic ballasts. Commonly used control methods are voltage feedback control and current feedback control, Current feedback control can force the inductor current to track the reference current signal, which has the advantage of a fast response. When working in continuous current mode (CCM), the Boost converter needs to introduce multiple feedback methods. When working in discontinuous current mode (DCM), the converter automatically shapes the input current, has a natural zero-current turn-on characteristic, requires a small inductance value, simple control, and is suitable for low-power applications. At present, there is much research on the CCM mode of Boost conversion circuit, and many circuit models have been established, and gratifying research results have been obtained; the research on DCM mode is mainly DC/DC circuit, and the research on DCM mode in AC/DC circuit Very little. Based on the requirements of a low-power switching power supply with low cost and high-cost performance, this paper uses the universal UC3842 chip to design a Boost conversion circuit, analyzes the working characteristics and design points of the DCM mode, and simulates the rationality of the designed circuit verification. I. Circuit Model of Boost Converter in DCM Mode1.1 Mathematical Model of DCM Working ModeThe Boost conversion circuit structure is shown in Figure 1(a). In the CCM mode, the switching tube M and the diode VD5 are turned on in a complementary manner; when the inductance is small or the switching cycle is relatively long, before the start of the next cycle, the diode VD5 and M are all turned off, and the Boost conversion circuit works in DCM mode at this time. The corresponding waveform between the inductor current and the pulse width modulator (PWM) output pulse is shown in Figure 1(b).Figure 1 Boost conversion circuit and DCM mode waveform diagram In the K-th switching modulation cycle, the on-off states of the switch tube M and the diode VD5 satisfy the relationship:T is the high-frequency modulation period of PWM; KT is the K-th high-frequency modulation period of PWM; D1T is the rise time of the inductor current in the high-frequency modulation period; D2T is the fall time of the inductor current in the high-frequency modulation period; D3T is the time when the inductor current is zero in the high-frequency modulation period. When the Boost circuit works in DCM mode, as the switch tube M and the diode VD5 turn on and off, the state of the system can use a differential equation:In the formula, KT+D1T+D2T+D3T=(K+1)T; iL(t) is the instantaneous value of the inductor current; υin(t) is the instantaneous value of the input voltage on the grid side; υo(t) is Boost The instantaneous value of the converter output voltage. 1.2 Working Conditions of DCM ModeIt can be seen from Figure 1 that the inductor current of the Boost converter in DCM mode increases linearly from zero. In each modulation cycle, there is:When the circuit output filter capacitor is large, the output voltage ripple can be ignored compared with the output voltage amplitude, and the output voltage υo(t) can be regarded as a constant υo. In a dual-loop control system, the current-loop reference current Iref is determined by the output of the voltage outer loop, and there is:In the formula, υr(t) is the instantaneous value of the reference voltage provided by the voltage loop for the current loop; Rs is the sampling resistance for detecting the current. When the inductor current follows the sinusoidal input voltage waveform, the power factor is close to 1, and the available duty cycle D1 can be expressed as: In the formula, Vref is the peak value of υr(t) in the power frequency period; Vin is the peak value of υin(t) in the power frequency period. The same principle can be obtained, in each modulation period, the expression of the duty cycle D2 is:When the Boost converter works in the critical mode, the duty cycle satisfies the relationship D1+D2=1. From this, it can be deduced that the critical condition for the Boost circuit to enter the CCM mode from DCM is:When L<LCRM, Boost circuit works in DCM mode; When L>LCRM, Boost circuit works in CCM mode. At this point, the following conclusions can be drawn: (1) In the DCM mode, the on-time D1T of the switch tube M is a fixed value, which does not change with the size of the inductor current. (2) When the switch is turned off, the time D2T when the inductor current drops to 0 changes with the output voltage and input voltage. The greater the input voltage, the greater the D2T, the greater the output voltage, and the smaller the D2T. II. DCM Circuit Design Based on UC38422.1 DCM Circuit Design Based on AdderThe DCM-type Boost circuit includes two control loops, namely a voltage loop and a current loop. Its function is to eliminate the grid current spikes, so that the input current becomes a sinusoidal shape and is in phase with the input voltage. For a single switching cycle, the current in each switching cycle is required to be proportional to the input voltage. If for some reason the output voltage increases or the output current increases, the pulse width modulator will change the pulse width of the drive signal, that is, the duty cycle D, so that the average voltage or peak current after the chopping will decrease. So as to achieve the purpose of power factor correction. The DCM circuit schematic diagram based on the adder is shown in Figure 2.Figure 2 DCM circuit schematic diagram based on adder The voltage outer loop uses an adder to replace the multiplier circuit. The feedback voltage on the grid side is used to ensure that the current signal is a sinusoidal signal, and the output feedback voltage is used to ensure that the output voltage is a constant value. The two are synthesized by the adder U2. The output signal is sent to the error amplifier in the UC3842 current loop, compared with a given reference voltage, and the comparison result is sent to the current measurement comparator. The peak current signal L(t) of the inductor in the main circuit is sent to the current measuring comparator at the same time, the comparison result of the two is sent to the R input of the RS latch in the PWM. The clock signal output by the internal oscillating circuit is sent to the S input end of the RS latch in the PWM, which works together to control the opening and closing of the switch tube M1. 2.2 UC3842 Working PrincipleUC3842 is a high-performance single-ended output current-type PWM controller. The current control loop is composed of a PWM latch, a current detection comparator, an error amplifier, and a sawtooth oscillation circuit. Its internal structure is shown in Figure 3.Figure 3 UC3842 working principle diagram UC3842 can generate a drive signal with a fixed frequency and adjustable pulse width. External components RT and CT can be used to set the oscillation frequency and precisely control the duty cycle. The output voltage of UC3842 can be adjusted by controlling the on-off state of the switch tube to achieve the purpose of voltage stabilization. The UC3842 has a good voltage regulation rate, good frequency response characteristics, large stability amplitude, over-current limit, overvoltage protection, and under-voltage lockout function. And it has fewer external pins, small size, is an economical PWM driver control chip. III. Simulation and Analysis of DCM Mode CircuitIn order to verify the correctness of the deduced critical conditions and the designed circuit, the designed circuit was simulated and verified by OrCAD10.5 software. When the power input is power frequency alternating current and its circuit parameters are υin=311.13sin(ωt), υo=385V, RL=1482Q, L=400μH, the Boost converter works in DCM mode, and its output voltage and inductor current waveforms are as follows Shown in Figure 4.Figure 4 Boost converter output voltage and inductor current waveform The waveform of the duty cycle at different times is shown in Figure 5.Figure 5 The size of the duty cycle D1T at different times If the inductance value increases and exceeds the critical value LCRM, the inductor current will change from DCM mode to CCM mode. When the inductance value L=1.2mH, Vo=508V, the waveform is shown in Figure 6.Figure 6 Inductor current waveform when L=1.2mH The waveform of the duty cycle D1T in the DCM stage is shown in Figure 7Figure 7 The duty cycle of the DCWI phase when the inductance exceeds the critical value It can be seen from the simulation results that in the DCM mode, the on-time of the switch is a fixed value. When the inductance L is greater than the critical value, there will be a transition from DCM to CCM. The CCM mode appears near the peak of the power frequency current. IV. ConclusionThis text summarizes the Boost conversion circuit design scheme based on the UC3842 chip. By analyzing the circuit of Boost converter in DCM mode, the circuit model of Boost converter in DCM mode is established, and the duty cycle change rule in this mode and the critical conditions for entering CCM mode from DCM mode are studied. Using the universal PWM modulator UC3842 chip, a Boost conversion circuit based on the principle of addition is designed, and the correctness of the conclusions obtained is verified by simulation software. The circuit simulation results show that the designed DCM circuit can meet the requirement of the inductor current to follow the voltage waveform completely and achieve the purpose of improving the power factor. This research provides design ideas for the development of low-cost low-power switching power supplies.V FAQWhat are two common control methods used by Boost converters?Voltage feedback control and current feedback control. In what mode does the Boost converter need to introduce multiple feedback methods? Continuous current mode What type of circuit is the research on DCM mode?DC/DC circuit What are two control loops in the DCM-type Boost circuit?Voltage loop and a current loop What is the purpose of the DCM-type Boost circuit?Power factor correction What is the current control loop composed of?A sawtooth oscillation circuit What is the UC3842?PWM driver control chip How can the output voltage of UC3842 be adjusted?By controlling the on-off state of the switch tube
kynix On 2022-02-21
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