Back to Basics: Thermal Calculation for Power Circuits

When designing circuits for reasonably high-power DC applications you will run across the case of handling thermals. It’s not just about picking the right IC for your DC power conversion; thermal management plays a key role. Here’s a primer on designing with thermals in mind.

For linear power regulators, the math is straightforward. Subtract the output voltage from the input voltage, multiply by the output current, and voilà, you have the power dissipation. This lost power needs to be dissipated in the IC package. So get the datasheet of the IC, find the thermals section, and hunt for Junction-to-ambient thermal resistance. Say it’s 45°C/W; this means for every watt lost, the IC temperature spikes by 45°C above ambient. So if it’s 25°C ambient, the IC hits 70°C.

For a DC-DC converter, it’s a bit more trickier. Here Efficiency = Pout/Pin and Power Loss = Pout – Pin. Now in a DC-DC converter datasheet, you need to find your efficiency value. You will get this mostly from a graph in the datasheet that shows the efficiency of the converter for a particular load and particular output current. Put that value in and you get the power lost.

As a designer, it’s your task to manage this dissipated heat somehow. It can be via a large heatsink(which has a smaller thermal resistance) or can even be via PCB ground planes. There is a TI app note (AN2020 worth reading) that tells you what size of PCBs you want to have to dissipate heat properly without heatsinks. Check images for the rule-of-thumb calculations. Don’t overlook thermal vias and multi-layer PCB stacks as other ways to remove this heat.

In summary, always keep thermals in mind else the moment you power on the circuit the first time, you will see the magic smoke. Speaking from experience. ????

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