The thermal chain
Heat flows from the chip's junction to its case, through the mounting interface to the heatsink, and from the heatsink to the air. Each step has a thermal resistance in °C per watt, and they add up like resistors in series: Tj = Ta + P × (θjc + θcs + θsa). Rearranged, the heatsink you need is θsa = (Tj max − Ta) ÷ P − θjc − θcs. Pick a heatsink with a figure lower than this.
Practical points
- Use a margin. Running 20 °C below the maximum junction temperature greatly improves reliability.
- Allow for the case temperature. Inside an enclosure, the air can be 20 °C above room temperature.
- Insulating washers add thermal resistance, but are needed where the tab is live, as on many regulators and transistors.
- Fans can cut a heatsink's thermal resistance by half or more.
Linear regulators turn their whole voltage drop into heat, so the LM317 calculator shows the power first. For RF power amplifiers, check the transistor's datasheet for its own θjc.
Questions
How do I choose a heatsink?
Work out the thermal resistance you need with the formula, then pick a heatsink rated lower, in °C/W.
How much power can a TO-220 handle without a heatsink?
About 1 to 1.5 W at room temperature, because its junction-to-air resistance is around 60 °C/W.
Does thermal paste matter?
Yes. A dry joint can add 1 to 2 °C/W, which matters at high power.
What does °C/W mean?
How many degrees the temperature rises for each watt of heat flowing through.