Heatsinks

Thermal resistance and heatsink calculations

Learn about thermal resistance, the K/W unit, and the junction-to-case-to-heatsink relationship. Calculate the required heatsink resistance with an illustrated example.

What does thermal resistance represent?

Thermal resistance is the ratio of the temperature difference across a specific heat flow path to the power passing through that path: Rθ=ΔT/P. Its unit can be given as °C/W or K/W; these two units are numerically equivalent when dealing with temperature differences. Lower resistance means a smaller temperature difference for the same power. However, the values being compared must be measured under the same boundary conditions.

Identify the points in the heat path

Tj is the temperature of the semiconductor junction, Tc is the defined case point, Ts is the heatsink base, and Ta is the ambient air. RθJC represents the junction-to-case, RθCS the case-to-heatsink, and RθSA the heatsink-to-ambient path. This chain is an approximate model where heat primarily exits from the selected case surface; it does not apply unconditionally to every package.

Step-by-step calculation example

Assumption: 15 W loss, 45 °C local ambient, 105 °C junction target, 1.2 °C/W junction-to-case, and 0.3 °C/W interface resistance. The total budget is (105−45)/15=4 °C/W. The remaining budget for the heatsink is 4−1.2−0.3=2.5 °C/W. For a candidate providing 2 °C/W under actual mounting conditions, the approximate Tj is calculated as 45+15×(1.2+0.3+2)=97.5 °C. The numbers in the example are for educational purposes; they are not product measurements.

Results change when the environment changes

In the same example, when the ambient temperature is 55 °C, the temperature estimate becomes 107.5 °C, exceeding the 105 °C target. Therefore, a sample that works successfully only at room temperature is not sufficient evidence. Manufacturing tolerances, contamination, and variations in airflow also justify the need for design margins.

Why should RθJA and RθJC not be confused?

The junction-to-ambient value RθJA in a datasheet includes the effect of the test board and environment used. Adding heatsink resistance on top of this value creates an incorrect model in most cases. Different thermal values may exist for the top and bottom of the case; use the definition corresponding to the actual cooling surface in your calculation. Do not substitute characterization parameters like ΨJT directly for series thermal resistance.

Pulsed loads and shared heatsinks

For short power pulses, steady-state resistance alone does not provide the peak junction temperature; the manufacturer's transient thermal impedance curves and duty cycle are required. If there is more than one component on the same heatsink, the heatsink heats up with the total power, and each component's own case path is evaluated separately. The location of heat sources can also change local base temperatures.

Frequently asked questions

Does 2 °C/W mean the temperature of the heatsink?

No. It expresses the temperature difference generated for every watt under specified conditions.

What does it mean if the calculation result is negative?

The case/interface path may be consuming the entire budget with the chosen target and power loss. The power loss, package, interface, or cooling approach must be reconsidered.