Estimated reading time: 6 minutes

Elementary, Mr. Watson: Heat—The Hidden Villain of Power Electronics

If electricity were a group of college students, then power electronics and the PCB designers who dive into it would insist on driving the car on every road trip because they know the car inside and out—they’re the students with jumper cables in the trunk, a tire pressure gauge in the glove box, and snacks stashed under the seat. While the others argue over playlists and directions, power electronics is busy ensuring the alternator doesn’t fry, the headlights don’t dim, and everyone reaches the destination with fuel still in the tank.

Like that road-trip driver, power electronics is practical, slightly obsessive, and occasionally under-appreciated. Nobody thanks them when things run smoothly, but everyone panics the moment something goes wrong, especially when the faint smell of burnt insulation drifts through the air. But beneath all the drama, nothing magical has changed. Whether you’re tinkering with the simplest of circuits that sips only a few milliamps, or you’re standing in front of a 50-kilovolt power supply—the kind so intimidating that old-timers joke you have to flip it on with a broom handle from across the room—the same fundamental physics is at work: Voltage is still the push, current is still the flow, resistance still pushes back, and Maxwell’s equations still keep the whole universe honest.

What changes is not the rulebook, but the context. The same physics you learned in the first circuits class still rules the road: Ohm’s law, Kirchhoff’s laws, and Maxwell’s equations. These aren’t replaced when you step into power electronics; they’re asked to handle bigger challenges. The difference is in the scale of the laws and the consequences of applying them. 

With power electronics come unavoidable side effects. Heat is chief among them.Whenever large amounts of current are pushed through real-world conductors, resistive losses turn precious electrical energy into unwanted thermal energy. No designer escapes this truth. Heat is the first tax you pay when you scale up power. Unlike the IRS, it doesn’t need forms, deadlines, or receipts. It collects instantly and without mercy. Heat doesn’t ask for permission, it doesn’t negotiate, and it will always hunt down the weakest link in your system like a bloodhound on espresso. Ignore it, and you’ll quickly learn that “thermal runaway” isn’t just an academic phrase; it’s the engineering equivalent of your circuit setting itself on fire to protest poor design decisions.

Ben Franklin said, “Failing to plan is planning to fail.” Of course, he wasn’t talking about PCB thermal design, but he might as well have been. Heat management isn’t something you tack on at the end; it must be baked into the design from the first schematic sketch. Decide to skip that, and your circuit is just a very creative toaster waiting to happen. 

Step 1: Reduce Heat at the Source
The most cost-effective watt to cool is the one you never waste. Every watt lost as heat is a double penalty: you lose valuable energy and create an extra thermal load that needs to be handled another way. That means every wasted watt is also an extra cost, an extra size, and extra noise in your system.

That’s why the first step in thermal management is about efficiency at the circuit level, not fans or heatsinks; . If you can push your converter from 90% to 95% efficiency, the improvement may look small on paper, but in practice it can be dramatic. At 1 kilowatt of output power, that 5% bump means you’re dissipating 50 watts less heat. That’s the difference between a modest aluminum heatsink and an expensive active cooling system.

Think of it as engineering on the front end to avoid engineering on the back end. Every bit of waste you prevent here is one less watt you’ll have to blow away with airflow, one less component to derate, and one less warranty claim waiting for you down the road.

Step 2: Spread the Heat Out
No matter how careful you are, you will always generate some heat. Semiconductors aren’t perfect switches, copper traces aren’t perfect conductors, and magnetic cores don’t behave like ideal transformers. Somewhere in the system, power will be lost as heat, and that heat has to go somewhere. The next step isn’t just to acknowledge it; you must spread it out so that no part gets overcooked while the rest of the board lounges around cool and carefree.

This is where thermal design becomes less math and more of an art form. A good designer thinks in terms of thermal flow as much as electrical flow. Heat, like current, follows the path of least resistance. If you provide big copper pours, thermal vias to connect layers, and wide traces, heat can flow away from hot spots and diffuse across the board. PCB layout in power electronics is about routing heat, not just routing signals.When done well, the board acts like a quiet partner in cooling, conducting, and spreading heat in ways that extend component life and boost reliability. 

Step 3: Get Rid of It
Finally, the heat must be removed from the building. Up to this point, you’ve reduced waste and spread it around, but at some stage, the laws of thermodynamics demand an exit strategy. That’s where active and passive cooling step in to carry the heat away before it cooks your hard work.

Passive cooling is the strong, silent type: chunky aluminum or copper heat sinks that soak up thermal energy and radiate it into the air. But they only work if they’re sized correctly, mounted with good thermal contact, and placed where airflow can reach them. Slapping a heat sink on the wrong side of a board is like putting an umbrella inside your house. It’s there, but not helping much.

Active cooling is what you call in when watts start piling up fast. Fans are the most common, but they’re not just about blowing air randomly. Designers spend hours considering ducting, vent placement, and airflow direction to prevent hot air from circulating within the enclosure like a convection oven. For the most demanding applications, such as industrial drives, data centers, or electric vehicles, liquid cooling takes center stage, removing heat far more efficiently than air can. It’s the engineering version of central air conditioning compared to waving a hand fan. The kicker is that none of this works if you don’t think about airflow design. A beautifully sized heat sink and the best fan money can buy won’t save you if you trap heat inside a sealed metal box. You need vents, ducts, and flow paths that let cool air in and hot air out. Without that, you’re just recirculating misery.

Thermal design is a lot like hosting a crowded party. If you don’t crack a few windows, eventually someone will faint, the vibe will die, and everyone will remember your design for the wrong reasons. In power electronics, effective cooling ensures the system operates smoothly.

John Watson is a professor at Palomar College, San Marcos, California.