Heatsink / thermal calculator
Tj = Ta + P × (Rθjc + Rθcs + Rθsa). Enter the dissipated power, ambient temperature and the thermal chain from the datasheet — the calculator tells you whether a sink is needed, the sink-to-air resistance that holds the junction under its limit, and the resulting junction temperature.
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.
Frequently asked questions
How is junction temperature calculated from thermal resistance?
The calculator uses Tj = Ta + P × (Rθjc + Rθcs + Rθsa): each °C/W resistance in the chain is multiplied by the dissipated power and added to the ambient temperature. With its defaults of 5 W, Ta = 25 °C, Rθjc = 1.5, Rθcs = 0.5 and Rθsa = 6, the junction lands at 65 °C.
How do I know if a power device needs a heatsink?
The tool first computes the junction temperature with no sink at all, Ta + P × (Rθjc + Rθcs). If that optimistic lower bound already exceeds your Tj(max), a sink is required — with the defaults it reaches 35 °C before case-to-air resistance, so the decision depends on your limit and real dissipation.
How do you find the heatsink you need for a target junction temperature?
Leave the sink-to-air resistance blank or use the reported figure: the required Rθsa = (Tjmax − Ta) ÷ P − Rθjc − Rθcs. With the defaults that is (125 − 25) ÷ 5 − 1.5 − 0.5 = 18 °C/W — the sink you must not exceed to hold the junction under 125 °C.
What is a typical case-to-sink thermal resistance Rθcs?
The tool notes that Rθcs for grease plus an insulator is roughly 0.2–1.0 °C/W and defaults to 0.5. It also warns that the case-to-sink joint is often the weakest link without thermal grease, since a dry joint adds resistance and drives the junction hotter.
Steady-state model, natural convection. Real designs derate for airflow, sink orientation and enclosure temperature. ICBOMS provides this tool for reference only.