Heat Sink Calculator — junction temperature and required θSA

Junction temperature from dissipation and thermal resistances, and the θSA you need to stay inside the derating curve.

Example: Dissipating 5 W from 25 °C ambient needs a heatsink of 18 °C/W or better.

Inputs

Power dissipated(W)

Ambient temperature(°C)

Inside an enclosure this is well above room temperature.

Max junction temperature(°C)

125 °C or 150 °C for most silicon. Derate for reliability.

θ_JC (junction to case)(°C/W)

About 1.5 for a TO-220, 0.7 for a TO-247, 5+ for small SMD.

θ_CS (case to sink)(°C/W)

Paste is around 0.5; an insulating pad is 1.0 to 2.0.

θ_SA of your heatsink (optional)(°C/W)

Leave blank to compute what you need. Enter a value to check a specific part.

Result

Required θ_SA

18 °C/W

Total θ budget: 20 °C/W
Fixed by package: 2 °C/W
Temperature budget: 100 °C

Datasheet θ_JC assumes an infinite heatsink and perfect mounting. Real θ_CS depends on flatness, torque and interface material — a dry, unclamped joint can be several times worse than the figure you assumed.
Ambient means the air actually around the part, not the room. Inside a sealed enclosure it can be 20–30 °C above room temperature, which comes straight off your budget.

Junction temperature against heatsink rating

θ_SA	Total θ_JA	Junction temp	Verdict
0.5 °C/W	2.5 °C/W	37.5 °C	comfortable
1 °C/W	3 °C/W	40 °C	comfortable
2 °C/W	4 °C/W	45 °C	comfortable
5 °C/W	7 °C/W	60 °C	comfortable
10 °C/W	12 °C/W	85 °C	comfortable
20 °C/W	22 °C/W	135 °C	over limit
50 °C/W	52 °C/W	285 °C	over limit

Formula

T_j = T_ambient + P × (θ_JC + θ_CS + θ_SA)

Worked example

Dissipating 5 W from 25 °C ambient needs a heatsink of 18 °C/W or better.

ΔT budget = T_j,max − T_ambient
125 °C − 25 °C
100 °C
θ_total allowed = ΔT / P
100 °C / 5 W
20 °C/W
This is Ohm's law with temperature as voltage and power as current.
θ_SA = θ_total − θ_JC − θ_CS
20 − 1.5 − 0.5
18 °C/W

Frequently asked questions

How do I work out what heatsink I need?

Divide the temperature budget by the power to get your total thermal resistance allowance, then subtract the package's θ_JC and the mounting θ_CS. What remains is the θ_SA your heatsink must beat. Lower numbers mean bigger sinks.

What is thermal resistance?

Degrees of temperature rise per watt of heat. It behaves exactly like electrical resistance with temperature as voltage and power as current, and resistances in the path add in series — which is why a bad mounting interface can dominate a good heatsink.

Why does my part still run hot with a big heatsink?

Usually the interface. A dry or poorly clamped joint can add several °C/W, swamping a good sink. Thermal paste, adequate mounting pressure and a flat surface matter more than most people expect. Insulating mica washers are also worse than they look.

What junction temperature should I design for?

Well below the maximum. Silicon lifetime roughly halves for every 10 °C, so a part rated to 150 °C running at 140 °C is technically in spec and will not last. Aiming 30–40 °C below the limit is a reasonable target for anything that has to keep working.

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