Current electronic products mainly use chip-packaged devices, but high-power devices and some power modules still have many perforated packages, which are mainly conveniently mounted on the heat sink for heat dissipation. The heat dissipation calculation of high-power devices and power modules is performed to select a suitable heat sink under certain heat dissipation conditions to ensure safe and reliable operation of the device or module.
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Heat calculation
Any device has a certain loss during operation, and most of the losses become heat. Low power devices have low losses and no heat sinks are required. High-power devices have large losses. If heat dissipation is not taken, the temperature of the die can reach or exceed the allowable junction temperature and the device will be damaged. Therefore, it is necessary to add a heat sink. The most common one is to install the power device on the heat sink, use the heat sink to dissipate the heat to the surrounding space, and if necessary, add a cooling fan to enhance the cooling and cooling at a certain wind speed. Flow cooling water cooling plates are also used on the power devices of some large equipments, which have better heat dissipation. The heat dissipation calculation is to determine the appropriate heat dissipation measures and heat sink by calculation under certain working conditions. The power device is mounted on a heat sink. Its main heat flow direction is transmitted from the die to the bottom of the device, which dissipates heat to the surrounding space. If no fan is cooled at a certain wind speed, this is called natural cooling or natural convection cooling.
Heat has a certain thermal resistance during the transfer process. The thermal resistance from the device die to the bottom of the device is R JC , the thermal resistance between the bottom of the device and the heat sink is R CS , and the heat dissipation of the heat sink to the surrounding space is R SA , the total thermal resistance R JA ="R" JC+R CS+R SA. If the maximum power loss of the device is PD, and the junction temperature allowed by the device is known as TJ and the ambient temperature is TA, the allowable total thermal resistance R JA can be found by the following equation.
R JA≤(TJ-TA)/PD
Calculate the maximum allowable heat sink to ambient temperature thermal resistance R SA
R SA≤({T_{J}-T_{A}}over{P_{D}})-(R JC+R CS)
For reasons of design, there is generally a TJ of 125 °C. The ambient temperature should also be considered in a worse case. Generally, TA=40°C and 60°C. The size of the R JC is related to the size of the die package and can be found in the device data sheet. The size of the R CS is related to the mounting technology and the package of the device. If the device is made of thermal grease or thermal pad and then mounted with the heat sink, its R CS is typically 0.1 0.2 °C / W; if the bottom of the device is not insulated, additional mica insulation is required, then the R CS can reach 1 °C / W. PD is the actual maximum power loss, which can be calculated according to the operating conditions of different devices. In this way, the R SA can be calculated and the appropriate heat sink can be selected based on the calculated R SA value.
Radiator introduction
Small heat sinks (or heat sinks) are made of aluminum alloy sheets by stamping process and surface treatment, while large heat sinks are extruded from aluminum alloy to form profiles, which are then machined and surface treated. They are available in a variety of shapes and sizes for different device installations and different power dissipation options. The heat sink is generally a standard part, and can also provide a profile, which is cut into a certain length by the user to make a non-standard heat sink. The surface treatment of the heat sink is electrophoretic paint or black oxygen polarization treatment, the purpose of which is to improve heat dissipation efficiency and insulation performance. It can be increased by 10 15% under natural cooling, 3% by air cooling, and 500 800V by electrophoretic paint.
Radiator manufacturers give thermal resistance values ​​to different types of heat sinks or give relevant curves, and give different thermal resistance values ​​under different heat dissipation conditions.
Calculation example
A power operational amplifier PA02 (APEX company) is used as a low-frequency power amplifier, and its circuit is shown in Figure 1. The device is available in an 8-lead TO-3 metal case. The operating conditions of the device are as follows: the operating voltage VS is 18V; the load impedance RL is 4, the operating frequency can reach 5kHz under DC conditions, and the ambient temperature is set to 40°C, using natural cooling.
Looking at the PA02 device data, the quiescent current IQ is typically 27mA and the maximum is 40mA. The device's R JC (from die to case) is typically 2.4°C/W with a maximum of 2.6°C/W.
The power consumption of the device is PD:
PD=PDQ+PDOUT
Where PDQ is the power dissipation of the internal circuitry of the device and PDOUT is the power dissipation of the output power. PDQ=IQ(VS+|-VS|), PDOUT=V^{2}_{S}/4RL, substituted into the above formula
PD=IQ(VS+|-VS|)+V^{2}_{S}/4RL=37mA(36V)+18V2/4 4=21.6W
The quiescent current in the equation is 37 mA.
Radiator thermal resistance R SA calculation: R SA ≤ ({T_{J}-T_{A}}over{P_{D}})-(R_{ JC}+R_{ CS}})
To leave a margin, TJ is set to 125 ° C, TA is set to 40 ° C, R JC is the maximum value (R JC = "2". 6 ° C / W), R CS is 0.2 ° C / W, (PA02 is directly installed in the heat sink On the device, there is thermal grease in the middle). Substituting the above data into the formula
R SA≤{125°C-40°C}over{21.6W}-(2.6°C/W+0.2°C/W)≤1.135°C/W
HSO4 has a thermal resistance of 0.95 ° C / W during natural convection, which can meet the heat dissipation requirements.
Precautions
1. The maximum power consumption value in the device data cannot be taken in the calculation, but it should be calculated according to the actual conditions; the maximum junction temperature in the data is generally 150 ° C, leaving room for 125 ° C in the design, the ambient temperature is also Can not take 25 ° C (to consider the summer and the actual temperature of the chassis).
2. The installation of the heat sink should consider the direction of heat dissipation, and open the heat dissipation holes in the corresponding position on the chassis or the cabinet (so that cold air enters from the bottom and hot air is discharged from the top).
3. If the outer casing of the device is an electrode, the mounting surface is not insulated (not insulated from the internal circuit). Mica gaskets must be used for insulation during installation to prevent short circuits.
4. The pins of the device are to pass through the heat sink and drill holes in the heat sink. To prevent the pin from colliding with the hole wall, a Teflon sleeve should be placed.
5. In addition, different types of heat sinks have different thermal resistances under different heat dissipation conditions, which can be used for design change. In practical applications, the thermal resistance of these heat sinks can be used to calculate and adopt similar structural shapes ( A heat sink composed of a cross-sectional area and a perimeter is used instead.
6. In the above calculations, some parameters are set, and there may be discrepancies with the actual values. The model size of the substitute is not exactly the same. Therefore, in the mass production, a simulation test should be carried out to confirm whether the radiator selection is appropriate and if necessary. Some corrections (such as the length of the profile or changing the model of the profile) can only be mass-produced.
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