Cooling a power device is a thermal version of Ohm's Law: power dissipation is the "current", temperature rise is the "voltage", and thermal resistance (in °C per watt) is the "resistance". Heat flows from the silicon junction, through the device case (θjc), across the case-to-heatsink interface (θcs), and finally from the heatsink to the air (θsa). Add them up, multiply by the power, add ambient, and you have the junction temperature. Sizing a heatsink means choosing a θsa small enough to keep that junction temperature safe.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: semiconductor thermal-management application notes (TI, Infineon).
The thermal-resistance formula
The first term, (Tj,max − Ta) / Pd, is the total allowed junction-to-ambient resistance θja. Subtract the fixed device and interface resistances and what remains is the most the heatsink is allowed to add. Pick a heatsink whose rated θsa is equal to or lower than this. If the required θsa is negative, no heatsink can do it — you must reduce the power or use a lower-θjc device.
Worked example — an LM317 dropping 3.5 W
Scenario: A regulator dissipates 3.5 W, Tj,max 125 °C, ambient 25 °C, θjc 2 °C/W (TO-220), θcs 0.5 °C/W with grease.
So even a modest TO-220 clip-on heatsink (around 20 °C/W) is comfortably enough. With a 20 °C/W heatsink the junction settles at Tj = 25 + 3.5 × (2 + 0.5 + 20) = 104 °C, safely under the 125 °C limit. Get the dissipation figure from the LM317 calculator or power calculator.
Frequently Asked Questions
θsa = (Tj,max − Ta) ÷ Pd − θjc − θcs. For 3.5 W, 125 °C max, 25 °C ambient, θjc 2, θcs 0.5: θsa = 100/3.5 − 2.5 ≈ 26 °C/W. Choose a heatsink rated at or below that.
The total rise per watt from junction to air: θja = θjc + θcs + θsa, and Tj = Ta + Pd × θja. Lower θja means a cooler junction for the same power.
Devices are rated 125–175 °C max, but aim for ~70–80% of that for reliability — under about 110–120 °C for a 150 °C part.
No. If the required θsa exceeds the part's own θja in free air (≈50–60 °C/W for a TO-220), no heatsink is needed. Under ~1 W is usually fine; above that, add one.
θcs covers the imperfect device-to-heatsink contact. Grease or a pad fills the air gaps, giving ≈0.2–0.5 °C/W; an insulating pad adds a little more.