Heatsink Thermal Resistance Calculator
Find the heatsink thermal resistance you need from the power dissipated and temperature limits, and check whether you need one at all.
Example: a 7805 regulator (TO-220) dropping 12 V to 5 V at 0.3 A dissipates about 2 W.
Formula and cautions
θsa = (Tj − Ta)/(P) − θjc − θcs. Heat flows from the die to the case, through the heatsink and into the air, and the sum of those resistances must stay within budget. Pick a heatsink with a lower (better) thermal resistance than the result, and keep your junction temperature target below the datasheet maximum.
Worked example: a heatsink for a 7805 regulator
- Loss: a linear regulator dropping 12 V to 5 V at 0.5 A dissipates (12 − 5) × 0.5 = 3.5 W
- No heatsink: Tj = 40 + 3.5 × 50 = 215 °C, above the 125 °C limit, so a heatsink is needed
- Allowed total (125 °C − 40 °C)/(3.5 W) = 24.3 °C/W, minus θjc 5 and interface 0.5: the heatsink must be 18.8 °C/W or less
Loss without a heatsink, by package
| Package | θja (typical) | Max loss (25 °C) | Max loss (40 °C) |
|---|---|---|---|
| PackageTO-92 | θja (typical)200 °C/W | Max loss (25 °C)0.50 W | Max loss (40 °C)0.42 W |
| PackageSOT-23 | θja (typical)250 °C/W | Max loss (25 °C)0.40 W | Max loss (40 °C)0.34 W |
| PackageTO-220 | θja (typical)50 °C/W | Max loss (25 °C)2.00 W | Max loss (40 °C)1.70 W |
| PackageDPAK (TO-252) | θja (typical)80 °C/W | Max loss (25 °C)1.25 W | Max loss (40 °C)1.06 W |
Loss each package can take without a heatsink before Tj reaches 125 °C. θja depends heavily on the package and board, so treat these as rough typical values and check the datasheet.
Using it in practice
Step by step
- Work out the power the part dissipates; for a linear regulator it is the input-output difference times the current.
- Enter that power, the maximum junction temperature you allow, the ambient temperature and the junction-to-case and case-to-sink thermal resistances.
- Read the heatsink thermal resistance you need and choose a heatsink rated at or below it.
Common mistakes
- Designing to the absolute maximum junction temperature. Running at 150 °C shortens life. Design for 100 to 125 °C to leave margin.
- Using room temperature as ambient. Inside a closed box the air is often 15 to 25 °C warmer than the room. Use the temperature the heatsink will actually see.
- Skipping the interface. A dry metal-to-metal joint or an insulating pad can add 0.5 to 2 °C per watt. Thermal paste or a good pad is part of the design, not a finishing touch.
Practical tips
Heatsink ratings assume free air with fins vertical; mounted flat or crowded by other parts they do worse, and a small fan can improve them several times over. Check the result by measuring the case temperature after half an hour at full load: the junction is hotter than the case by roughly the power times the junction-to-case resistance. If no reasonable heatsink is enough, reduce the dissipation instead, for example with a switching regulator.
Worked example: does a 7805 need a heatsink at 0.3 A?
Dropping 12 V to 5 V at 0.3 A dissipates (12 − 5) × 0.3 = 2.1 W. A TO-220 in free air has a junction-to-ambient resistance of roughly 50 °C/W, so at 25 °C ambient the junction reaches 25 + 2.1 × 50 = 130 °C, just over a 125 °C design limit. It would probably survive (many regulators shut down near 150 °C), but it would sit hot enough to trip thermal shutdown on a warm day.
With θjc = 5 °C/W and paste at 0.5 °C/W, the budget is (125 − 25) ÷ 2.1 = 47.6 °C/W in total, leaving 42 °C/W for the heatsink. A small clip-on TO-220 heatsink rated around 20 to 30 °C/W is plenty.
Cutting the power instead
The cheapest heatsink is less heat. Running the same 7805 from 9 V instead of 12 V gives (9 − 5) × 0.3 = 1.2 W, and 25 + 1.2 × 50 = 85 °C with no heatsink at all. A series resistor ahead of the regulator also moves part of the drop into a part that tolerates heat better, as long as the input stays above the regulator's dropout at full current. Above a watt or two of loss, a switching module usually beats any heatsink on size and cost; the DC-DC converter calculator shows the numbers.
Frequently asked questions
What does °C/W mean for a heatsink?
It is how many degrees the heatsink warms above the air for each watt it sheds. A 10 °C/W heatsink carrying 2 W runs 20 °C above ambient. Lower numbers mean better cooling.
Does thermal paste matter?
Yes. The interface between the part and the heatsink (θcs) can add around 0.5 to 1 °C/W with paste and much more with a dry joint or a thick pad. On high-power parts it can decide whether a design works.