What is the formula for Heatsink Thermal Resistance & Junction Temperature?

Core formula: θsa,max = (Tj,max − Ta)/Pd − θjc − θcs. The tool returns results instantly using this formula and the selected parameters.

Heatsink Thermal Resistance & Junction Temperature

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From dissipation, ambient temperature and package thermal resistance, find the θsa the heatsink must achieve and back-calculate junction temperature with the heatsink you have.

Key points

Heatsink Thermal Resistance & Junction Temperature: From dissipation, ambient temperature and package thermal resistance, find the θsa the heatsink must achieve and back-calculate junction temperature with the heatsink you have. Formula: θsa,max = (Tj,max − Ta)/Pd − θjc − θcs. Enter parameters on the DD-CAT IDE online tool page to get results instantly, with a calculation-model diagram that updates with the inputs. Heatsink θsa is usually specified at a given airflow (e.g. 1 m/s); in natural convection multiply it by roughly 1.5–2×. θcs is often ignored: a TO-220 with an insulating pad can hit 1.5°C/W — 7.5°C at 5W.

Inputs

θsa,max = (Tj,max − Ta)/Pd − θjc − θcs

Calculation model (updates with parameters)

Allowed total θja,max	17.00 ℃/W	(Tj,max − Ta)/Pd
Package + interface resistance	2.50 ℃/W
Required heatsink θsa,max	14.50 ℃/W
Junction temperature with this heatsink	72.5 ℃	Ta 40℃ + Pd×(2.00+0.50+4.00)
Junction margin	+52.5 ℃	pass
Cooling recommendation	Natural-convection heatsink is enough

Engineering notes

Heatsink θsa is usually specified at a given airflow (e.g. 1 m/s); in natural convection multiply it by roughly 1.5–2×.
θcs is often ignored: a TO-220 with an insulating pad can hit 1.5°C/W — 7.5°C at 5W.

FAQ

What is Heatsink Thermal Resistance & Junction Temperature for?: From dissipation, ambient temperature and package thermal resistance, find the θsa the heatsink must achieve and back-calculate junction temperature with the heatsink you have.
What is the formula for Heatsink Thermal Resistance & Junction Temperature?: Core formula: θsa,max = (Tj,max − Ta)/Pd − θjc − θcs. The tool returns results instantly using this formula and the selected parameters.
What engineering notes should I keep in mind when using Heatsink Thermal Resistance & Junction Temperature?: Heatsink θsa is usually specified at a given airflow (e.g. 1 m/s); in natural convection multiply it by roughly 1.5–2×. θcs is often ignored: a TO-220 with an insulating pad can hit 1.5°C/W — 7.5°C at 5W.