Chip thermal resistance calculation and heat sink selection

At present, electronic products mainly use SMD packaging devices, but many high-power devices and some power modules still use perforated packaging, which is mainly because they can be easily installed on the radiator for heat dissipation. The purpose of heat dissipation calculation for high-power devices and power modules is to select a suitable radiator under certain heat dissipation conditions to ensure that the device or module works safely and reliably.

Radiator Introduction

Small radiators (or heat sinks) are made of aluminum alloy sheets through stamping and surface treatment, while large radiators are made of aluminum alloy extrusion into profiles, which are then machined and surface treated. They are available in various shapes and sizes for different device installations and devices with different power consumption. Radiators are generally standard parts, and profiles can also be provided, which can be cut into a certain length by the user according to requirements to make non-standard radiators. The surface treatment of the radiator includes electrophoretic painting or black oxide polarization treatment, the purpose of which is to improve the heat dissipation efficiency and insulation performance. It can be increased by 10-15% under natural cooling and 3% under ventilation cooling. The electrophoretic painting can withstand a voltage of 500-800V.

Radiator manufacturers give thermal resistance values ​​or related curves for different models of radiators, and give different thermal resistance values ​​under different heat dissipation conditions.

Heat dissipation calculation

Any device has a certain amount of loss when working, and most of the loss turns into heat. Low-power devices have low losses and do not require heat dissipation devices. However, high-power devices have large losses. If heat dissipation measures are not taken, the temperature of the tube core may reach or exceed the allowable junction temperature, and the device will be damaged. Therefore, a heat dissipation device must be added. The most common method is to install the power device on a radiator, use the radiator to dissipate the heat to the surrounding space, and add a cooling fan when necessary to enhance cooling and heat dissipation at a certain wind speed. Flowing cold water cooling plates are also used on power devices of some large equipment, which has a better heat dissipation effect. Heat dissipation calculation is to determine the appropriate heat dissipation measures and heat sinks through calculations under certain working conditions. The power device is installed on the radiator. Its main heat flow direction is from the tube core to the bottom of the device, and the heat is dissipated to the surrounding space through the radiator. If there is no fan cooling at a certain wind speed, this is called natural cooling or natural convection heat dissipation.

There is a certain thermal resistance in the heat transfer process. The thermal resistance from the device chip to the device surface is RJC, the thermal resistance between the device surface and the heat sink is RCS, and the thermal resistance of the heat sink to the surrounding space is RSA. The total thermal resistance RJA=Rjc+Rcs+Rsa. If the maximum power loss of the device is PD, and the allowable junction temperature of the device is TJ and the ambient temperature is TA, the allowable total thermal resistance RJA can be calculated according to the following formula.

R JA≤(TJ-TA)/PD

Then the maximum allowable thermal resistance from the heat sink to the ambient temperature, RSA, is calculated as

R SA≤({T_{J}-T_{A}}\Over{P_{D}})-(R JC+R CS)

In order to leave room for design, TJ is generally set to 125℃. The ambient temperature should also take into account the worst case, and TA is generally set to 40℃ 60℃. The size of R JC is related to the size and packaging structure of the tube core, which can generally be found in the data of the device. The size of R CS is related to the installation technology and the packaging of the device. If the device is installed with thermal grease or thermal pads and then mounted on the heat sink, its typical R CS value is 0.1 0.2℃/W; if the bottom of the device is not insulated and mica insulation is required, its R CS can reach 1℃/W. PD is the actual maximum power loss, which can be calculated according to the working conditions of different devices. In this way, R SA can be calculated, and a suitable heat sink can be selected based on the calculated R SA value.

Calculation Example

A power operational amplifier PA02 (APEX product) is used as a low-frequency power amplifier, and its circuit is shown in Figure 1. The device is an 8-pin TO-3 metal shell package. The device working conditions are as follows: the working voltage VS is 18V; the load impedance RL is 4, the working frequency can reach 5kHz under DC conditions, the ambient temperature is set to 40℃, and natural cooling is used.

According to the PA02 device data, the typical value of the quiescent current IQ is 27mA, and the maximum value is 40mA; the typical value of the device's R JC (from the die to the case) is 2.4℃/W, and the maximum value is 2.6℃/W.

The power consumption of the device is PD:

PD=PDQ+PDOUT

Where PDQ is the power consumption of the internal circuit of the device, and PDOUT is the power consumption of the output power. PDQ=IQ(VS+|-VS|), PDOUT=V^{2}_{S}/4RL, Substituting into the above formula

PD=IQ(VS+|-VS|)+V^{2}_{S}/4RL=37mA(36V)+18V2/4 4=21.6W

The quiescent current is 37mA.

Calculation of heat sink thermal resistance R SA: R SA ≤ ({T_{J}-T_{A}}\Over{P_{D}})-(R_{ JC}+R_{ CS}})

To leave some margin, TJ is set to 125℃, TA is set to 40℃, R JC is taken to be the maximum value (R JC = "2".6℃/W), and R CS is taken to be 0.2℃/W (PA02 is directly installed on the heat sink with thermal grease in the middle). Substituting the above data into the formula, we get

R SA≤{125℃-40℃}\Over{21.6W}-(2.6℃/W+0.2℃/W)≤1.135℃/W

The thermal resistance of HSO4 is 0.95℃/W in natural convection, which can meet the heat dissipation requirements.

Precautions

1. In the calculation, the maximum power consumption value in the device data sheet cannot be used, but it must be calculated according to actual conditions; the maximum junction temperature in the data sheet is generally 150°C, and there is room for 125°C in the design. The ambient temperature cannot be 25°C (the actual temperature of summer and the chassis must be considered).

2. When installing the radiator, you should consider the direction that is conducive to heat dissipation, and open heat dissipation holes at the corresponding positions on the chassis or case (so that cold air enters from the bottom and hot air dissipates from the top).

3. If the device shell 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 need to pass through the radiator, and holes need to be drilled on the radiator. To prevent the pins from colliding with the hole wall, a polytetrafluoroethylene sleeve should be used.

5. In addition, different types of radiators have different thermal resistances under different heat dissipation conditions, which can be used as references during design. That is, in actual applications, the thermal resistances of these radiators can be used for calculations, and radiators composed of profiles with similar structural shapes (cross-sectional area, circumference) can be used as substitutes.

6. In the above calculations, some parameters are set and may differ from the actual values. The substitute models and sizes are not exactly the same. Therefore, simulation tests should be carried out in mass production to confirm whether the radiator selection is appropriate. If necessary, some corrections should be made (such as the length size of the profile or changing the model of the profile, etc.) before mass production can be carried out.