Calculator KB-35
Heatsink calculator
Heat flows from the die through the case and thermal pad to the heatsink and into the air. There is a temperature drop at each step.
Result
In brief: Calculate the heatsink for a transistor, regulator, or diode: required thermal resistance, junction, case, and heatsink temperatures, and estimated area. Example: Heatsink thermal resistance, no higher than: 6.5 K/W.
Calculation example
| Power | 10 W |
|---|---|
| Ambient temperature | 40 °C |
| Maximum die temperature | 125 °C |
| Die → case | 1.5 K/W |
| Case → heatsink | 0.5 K/W |
| Heatsink thermal resistance, no higher than | 6.5 K/W |
|---|---|
| Case temperature | 110 °C |
| Heatsink temperature | 105 °C |
| Estimated surface area | 154 cm² |
How it is calculated
S ≈ 1000 / Rsa cm²
A thermal path works like an electrical circuit: power instead of current, temperature instead of voltage, and thermal resistances add up. The total temperature rise must not exceed the difference between the die temperature limit and the ambient temperature.
What to look for
- A fan reduces the heatsink's thermal resistance several times over.
- An insulating pad significantly worsens heat transfer. Don't use one unless necessary, but keep in mind that the case may be live.
Frequently asked questions
How do I calculate a heatsink for a transistor?
Rheatsink = (Tj − Ta) / P − Rjc − Rcs. For 10 W, a 125 °C limit, 40 °C ambient, and thermal resistances of 1.5 and 0.5 K/W, you need a heatsink rated at 6.5 K/W or better.
How large should the heatsink be?
For a plate without forced airflow, roughly 1000 / Rheatsink cm²: at 6.5 K/W, about 154 cm². This is a rough estimate; for a more accurate figure, check the heatsink's thermal resistance in its specifications.
Do I need thermal paste and an insulating pad?
Thermal paste reduces contact resistance to 0.2–0.5 K/W. An insulating pad worsens heat transfer and is only needed if the transistor case is live.
