Heat Sink Sizing Calculator Formula
Understand the math behind the heat sink sizing calculator. Each variable explained with a worked example.
Formulas Used
Max Heat Sink θsa
max_theta_sa = (tj_max - t_ambient) / power_w - theta_jc - theta_csTotal Thermal Budget
total_budget = (tj_max - t_ambient) / power_wUsed by JC + CS
used_budget = theta_jc + theta_csVariables
| Variable | Description | Default |
|---|---|---|
power_w | Power Dissipated(W) | 10 |
tj_max | Max Junction Temperature(°C) | 125 |
theta_jc | Junction-to-Case (θjc)(°C/W) | 2 |
theta_cs | Case-to-Sink (θcs)(°C/W) | 0.5 |
t_ambient | Ambient Temperature(°C) | 40 |
How It Works
Heat Sink Sizing
Formula
θsa_max = (Tj_max - T_ambient) / P - θjc - θcs
This gives the maximum thermal resistance the heat sink can have. Select a heat sink rated at or below this value. Forced air cooling reduces effective θsa.
Worked Example
10 W device, Tj_max = 125C, theta_jc = 2, theta_cs = 0.5, ambient = 40C.
- 01Thermal budget = (125 - 40) / 10 = 8.5 C/W
- 02Used = 2 + 0.5 = 2.5 C/W
- 03Max theta_sa = 8.5 - 2.5 = 6.0 C/W
Frequently Asked Questions
What if the result is negative?
The component cannot stay within limits even with a perfect heat sink. Reduce power, lower ambient temperature, or choose a different package.
Does fan cooling help?
Yes. A fan can reduce heat sink theta_sa by 50-75%. Most heat sink datasheets list natural and forced convection ratings.
Thermal paste vs thermal pad?
Paste gives lower theta_cs (0.2-0.5 C/W) than pads (0.5-2 C/W) but is messier. Pads are easier for production.
Ready to run the numbers?
Open Heat Sink Sizing Calculator