Electronics · Free Calculator
Heat Sink Thermal Resistance Calculator
Find the heat sink thermal resistance you need to keep a semiconductor below its maximum junction temperature.
θSA in °C/WJunction temperatureThermal padTO 220
W
°C
°C
°C/W
°C/W
Required heat sink θSAmax 11.5 °C/W
Total allowed θJA17 °C/W
Case temperature100 °C
Heat sink temperature97.5 °C
How this result was calculated
- θtotal = (Tj − Ta) ÷ P = (125 − 40) ÷ 5 = 17 °C/W
- θSA = θtotal − θJC − θCS = 11.5 °C/W
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Heat Sink Formula
θSA = (Tj − Ta) ÷ P − θJC − θCS
θJC comes from the device data sheet, θCS from the thermal pad or grease (0.1 to 1 °C/W, more with a mica insulator). Pick a heat sink with a θSA lower than the result.
Worked example
5 W in a TO 220 (θJC 5 °C/W) with a 0.5 °C/W pad, Tj max 125 °C at 40 °C ambient: θSA = 85 ÷ 5 − 5.5 = 11.5 °C/W.
Key insight: design for a junction well below the absolute maximum; every 10 °C cooler roughly doubles semiconductor life.
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Frequently Asked Questions
How do I choose a heat sink?
Calculate the required θSA and pick a heat sink whose rated thermal resistance is lower at your airflow.
What does °C/W mean?
The temperature rise per watt of heat. A 10 °C/W heat sink rises 50 °C above ambient at 5 W.
Does forced air help?
Yes. A fan can reduce a heat sink θSA by half or more.
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