Heatsink / thermal calculator
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Thermal chain
Heatsink & junction check
Tj(max) = 125°CJunction temp with no sink
Ta + P·(Rθjc + Rθcs) — optimistic lower bound, ignores case-to-air
Required sink-to-air (Rθsa)
(Tjmax − Ta) ÷ P − Rθjc − Rθcs
Resulting junction temp
Using Rθsa = 6
With the entered sink the junction stays at 65°C — under the 125°C limit.
About the model
Heat flows junction → case → sink → air, each with a thermal resistance in °C/W. Multiply each by the dissipated power and add to ambient to get the temperature at that point. Real designs add derating for sink orientation, airflow and enclosure — and the case-to -sink joint is often the weakest link without thermal grease.
For the device under test: power transistors and voltage regulators carry Rθjc on their datasheets.