Heatsink sizing calculator
Work backwards from your thermal budget to the heatsink-to-ambient resistance a part must meet — the number to compare against heatsink datasheets.
Device & thermal budget
Result
Total allowed thermal resistance
(Tj − Ta) / P
Required heatsink resistance (Rθ_sa)
Rθ_total − Rθ_jc − Rθ_cs
Design target (80% of requirement)
Pick a heatsink rated at or below this
About the thermal chain
Heat flows junction → case → sink → ambient, and each link has a thermal resistance in °C/W. Summing them and equating to (Tj − Ta)/P gives the heatsink resistance you must not exceed. Heatsink datasheets rate Rθ_sa in free air (natural convection) or with a fan (forced airflow — usually a 3–5× improvement). The 80% design target leaves margin for mounting, dust and part-to-part variation; forced-air and 2-layer-board packages derate differently, so treat the number as a starting point for selection.
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Frequently asked questions
How do I choose a heatsink for a power device?
The thermal chain is junction → case → sink → ambient: Rθ_sink = (Tj − Ta)/P − Rθ_jc − Rθ_cs. For a 10 W MOSFET in a TO-220 at 50 °C ambient with Tj = 125 °C, Rθ_jc ≈ 3 and Rθ_cs ≈ 0.5, the required sink is 7.5 − 3.5 = 4 °C/W, so a 3.2 °C/W design target.
What is Rθ_jc and Rθ_cs in the thermal path?
Rθ_jc is junction-to-case resistance, listed in the device datasheet (about 3 °C/W for TO-220, 1.5 for TO-247). Rθ_cs is case-to-sink, set by the interface: about 0.5 °C/W with thermal grease and roughly 0.3 °C/W with a good thermal pad. Sum them with the heatsink resistance and compare to (Tj − Ta)/P.
How much does forced air improve heatsink performance?
A heatsink rated at 4 °C/W in free air typically improves to 1–2 °C/W with a fan, because the Rθ_sa figure is the natural-convection rating. Heatsink datasheets quote both values; forced airflow also lets you use a smaller fin profile for the same budget.
ICBOMS provides this tool for reference only.