# Power Devices: Thermal Design | Heat Sink Calculation

Introduction

The heat sink has a thermal conductor that carries heat away from the device into fins that provide a large surface area for the heat to dissipate throughout the rest of the components, thus cooling both the heat sink and processor. Both a heat sink and a radiator require airflow and, therefore, both have fans built in.

At present, the main failure form of electronic equipment is thermal failure. According to statistics, 55% failure of electronic equipment is caused by temperature exceeding the rated value. With the increase of temperature, the failure rate of electronic equipment increases exponentially. Therefore, thermal design of power devices is most important in the structural design of electronic equipment, which directly determines the success of the products. Good thermal design is the basis for the stable and reliable operation of the equipment.

Electronics Thermal Heatsink Design Tutorial

Main Parameters of Thermal Properties

The thermal stress of the power device can come from the inside of the device or from the outside of the device. If the heat dissipation capacity of the device is limited, the consumption of power will lead to the rise of temperature and junction temperature in the active region of the chip inside the device, reducing the reliability of the device lower and making the device unable to work safely. The main parameters to characterize the thermal capacity of power devices are junction temperature and thermal resistance.

The active region of the device can be the PN junction region of the junction device (such as transistor), the channel region of the field effect device, the diffused resistor or the thin film resistance of the integrated circuit and so on. When the junction temperature Tj is higher than the ambient temperature Ta, the heat through the temperature difference to form a diffusive heat flow, which is emitted from the chip through the tube shell, and the heat emitted increases with the increase of the temperature difference (Tj-Ta). In order to ensure that the device can work properly for a long time, an allowable maximum junction temperature Tj max has been made. Tj max is determined by chip materials, packaging materials and reliability of devices.

The heat dissipation ability of power devices is usually characterized by thermal resistance, called Rt. The larger the thermal resistance is, the worse the heat dissipation ability is. Thermal resistance is also divided into internal thermal resistance and external thermal resistance. Internal thermal resistance is the inherent thermal resistance of the device itself, which is related to the thermal conductivity, thickness and cross-sectional area of the tube core, shell material and processing technology, while the external thermal resistance is related to the form of tube package. Generally speaking, the larger the shell area, the smaller the external thermal resistance. The external thermal resistance of metal shell is obviously lower than that of the plastic.

When the power consumption reaches a certain level, the junction temperature of the device goes up and the reliability of the system decreases. In order to improve the reliability, thermal design of the power device should be carried out.

Thermal Design of Power Device

Thermal design of power device is mainly to prevent thermal failure caused by overheating or alternating temperature. It can be divided into thermal design of internal chip, thermal design of package, thermal design of tube and thermal design in practical use.

For general power devices, only the thermal design of the device's interior, package and tube should be considered. But when the power consumption is high, the appropriate radiator should be installed, through which the heat can be effectively dissipated to ensure the device works normally and reliably within the safe junction temperature.

Heat Dissipation Calculation

The most commonly used heat dissipation method is to install the power device on the radiator, using the radiator to disperse the heat into the surrounding, if necessary, to add the fan to strengthen the heat dissipation with a certain wind speed. Flow cold water cooling plate is also used in some large power devices, which has better heat dissipation effect. Heat dissipation calculation is to determine the appropriate heat dissipation measures and radiators through calculation under certain working conditions.

There is a certain thermal resistance in the heat transfer process. The thermal resistance from the core of the device to the bottom is Rjc, between the bottom and the radiator is Rcs, radiator that spreads heat into the surrounding is Rsa, total resistance is Rja=Rjc+Rcs+Rsa. If the maximum power loss of the device is Pd, and the permitted junction temperature of the device is Tj, ambient temperature is Ta, the reasonable total thermal resistance Rja can be obtained by the following formula.

Rja ≤(Tj-Ta)/Pd

The thermal resistance of the maximum allowable Rsa is: Rsa ≤(Tj-Ta)/Pd-(Rjc+Rcs)

For design consideration, Tj is generally set to 125℃, Ta=40℃ ~ 60℃ generally used in the case of bad ambient temperature. The size of Rjc depends on the size of the core and the package structure, which can be found from the parameter list. Rcs size depends on the installation technology and device packaging. If the device adopts heat conducting grease or heat transfer pad, installing with the radiator, the typical value of Rcs is 0. 1 ℃/W / ~ 0. 2 ℃/W; If the bottom surface of the device is not insulated and additional mica insulation is required, the Rcs can reach 1 ℃/W. Pd is the maximum power loss calculated according to the working conditions of different devices. In this way, Rsa can be calculated to select an appropriate radiator.

Calculation Example

A power operational amplifier PA02 as low-frequency power amplifier, the device is 8-pin and TO-3 metal shell package. The operating conditions are as follows: the operating voltage Vs is 18 V, the load impedance RL is 4Ω, the ambient temperature is 40 ℃, and the natural cooling is adopted.

According to the data of PA02: the typical value of static current Iq is 27mA, the maximum value is 40mA, and the typical value of Rjc (from tube core to shell) is 2.4 ℃/W, and the maximum value is 2.6 ℃/W. The power consumption of the device is Pd=Pdq+ Pdout(Pdq is the internal power consumption and Pdout is the output power consumption). The calcualtion is as follows

Pdq=Iq(Vs+|-Vs|)  Pdout=Vs2/(4RL)  Iq=37mA

Pd=Iq(Vs+|-Vs|)＋Vs2/(4 RL)

=0.037×(18+18)＋182/(4×4)

=21.6 W

Tj=125℃, Ta=40℃, Rjc=2.6℃/W, Rcs=0.2℃/W(PA02 installed directly on radiator with heat conductive grease in the middle)

Substitute the above data into the formula to get Rsa≤ (125-40)/21.6-(2.6+0.2)≤ 1.135℃/W

The thermal resistance HSO4 in natural convection is 0. 95 ℃/W, which can meet the requirement of heat dissipation.

Radiators are generally standard parts, but also provide customization. The surface of radiator is treated by electrophoretic coating or black oxygen polarization, which aims to improve heat dissipation and insulation performance. In natural cooling can be increased by 10%~15%, in ventilation cooling can be increased by 3%, and electrophoretic coating can withstand pressure 500V~800V. The heat resistance of different types of radiators in different heat dissipation conditions is given by the radiator manufacturers.

The radiator is used to control the temperature of the power device, especially the junction temperature (Tj), making is lower than the safe junction temperature of the power device, so as to improve the reliability of the power device. Conventional radiators tend to be standardized, serialized, universal, and new products develop towards low thermal resistance, multifunction, small volume, light weight, and suitable for automatic production and installation. The internal thermal resistance of various power devices is different and the difference of contact surface and installation torque, it will lead to the thermal-resistance difference between the contracts. The main factor of selecting radiator is the heat resistance Rtf. Under different environmental conditions, the heat dissipation of power devices is also different. Therefore, environmental factors, the matching between radiator and power device, and the volume and quality of the whole electronic equipment should be taken into account in selecting the appropriate radiator.

First of all, according to the performance parameters and environmental parameters of the power device in normal operation, calculate whether the junction temperature of the power device is within the safe condition, determine whether it is necessary to install the radiator, and calculate the corresponding thermal resistance of the radiator if it needs to be installed. The junction temperature of power device is recalculated to determine whether the junction temperature of power device is within the range of safe junction temperature, so as to judge whether the selected radiator meets the requirements. For the radiator that meets the requirements, the optimum design should be carried out according to the actual engineering requirements.

Inclusion

Through the analysis and calculation of the heating principle of the power device, it can guide the design of the heat dissipation mode and the selection of the radiator, ensure the power device to work in the safe temperature range, reduce the quality problem, and improve the reliability of the electronic products. The reliability of electronic equipment is also related to the components, structure, assembly, process, processing quality and so on. In practical engineering applications, feedback data should be obtained through various tests to perfect the design and further improve the reliability of electronic equipment.

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