Heatsink Thermal Sizing
Find the heatsink thermal resistance you need, then check a chosen sink against your junction limit.
Step-by-step
- Enter valid values to begin.
Model: steady-state series thermal-resistance chain, TJ = TA + P(θJC + θCS + θSA), single heat path through the sink, no heat spreading or neighbouring sources. The interface presets are typical values only; the number in the θCS box is the one used. Catalogue θSA values assume natural convection at sea level with vertical fins, so derate for altitude, orientation and enclosure air. Sink-class bands are rules of thumb, not a standard. Verify θJC and TJ,max on the datasheet of the exact device.
Heatsink Sizing: Thermal Resistance as Ohm’s Law
Core Engineering Principles
We treat heat flow exactly like current in a resistor chain. Power is the current, temperature difference is the voltage, and each layer between the silicon and the room is a resistance in °C/W: junction to case (θJC), case to sink through the interface (θCS), and sink to ambient (θSA). They add in series, so the junction sits at TA + P × (θJC + θCS + θSA). Solving for θSA tells you what sink to buy. If it comes out zero or negative, no sink will do and the power, part or ambient must change.
Do not design to the datasheet maximum junction temperature. We subtract a margin, 25 °C in the example, because failure rates climb steeply with junction temperature and because θJC, grease thickness and airflow all vary from unit to unit. The interface matters more than juniors expect: dry contact leaves air gaps, grease is typically 0.2 to 0.5 °C/W, and an insulating pad can cost 1 °C/W or more. Catalogue sink ratings assume sea-level natural convection with the fins vertical. Altitude thins the air and cuts convection, and horizontal fins or a sealed box do worse, so derate the sink. A fan can lower θSA several-fold, which is why a sub-1 °C/W requirement usually means forced air or liquid.
TJ = TA + P × (θJC + θCS + θSA)
Tcase = TA + P × (θCS + θSA) • Tsink = TA + P × θSA
NEC & Standard References
JEDEC JESD51 is the series of standards for measuring and reporting semiconductor thermal resistance, the source of datasheet θJA and θJC figures. IEC 60747 covers discrete semiconductor devices. The datasheet remains the authority for your package. Check the current edition.1. Design TJ = 150 − 25 = 125 °C.
2. Thermal budget = 125 − 40 = 85 K, so total resistance allowed = 85 / 20 = 4.25 °C/W.
3. θSA = 4.25 − 1.5 − 0.5 = 2.25 °C/W, a medium finned sink.
4. Check a 2.0 °C/W sink: TJ = 40 + 20 × (1.5 + 0.5 + 2.0) = 120 °C, which is 5 K under the design limit.
5. At that point Tcase = 40 + 20 × 2.5 = 90 °C and Tsink = 80 °C.
- Datasheet θJA is for a test board. It assumes a standardised board and still air.
- Mounting matters. Uneven torque or a sink that is not flat leaves a thick grease film and can double θCS.
- Neighbours add heat. Adjacent regulators, magnetics or a second device on the same sink raise the local ambient, so add their power.
- Pulsed power is not average power. Short high-power pulses can exceed the junction limit even when the average looks safe so check transient thermal impedance.
- Use the air inside the box. Enclosure air can run 15 to 25 °C above the room, so use that value for TA.