Heat Sink Calculator
Calculate junction temperature or required heat sink thermal resistance.
How it works
Heat flows from the hot semiconductor junction through the package, thermal interface material, and heat sink into the ambient air. This path has "thermal resistance", measured in °C/W (degrees Celsius per Watt).
Tj (Junction Temp): The internal temperature of the silicon. Usually must be kept below 125°C or 150°C.
Ta (Ambient Temp): The air temperature around the device.
Pd (Power): Power dissipated as heat (Watts).
Rth_jc: Thermal resistance from Junction to Case (from datasheet).
Rth_cs: Thermal resistance from Case to Sink (thermal paste, pad).
Rth_sa: Thermal resistance from Sink to Ambient (the heat sink's rating).
Tj = Ta + Pd × (Rth_jc + Rth_cs + Rth_sa)
Worked example
A voltage regulator dissipates 5W (Pd = 5W). It is in a 25°C room (Ta = 25°C). Its datasheet says Rth_jc is 3 °C/W. You use a thermal pad with Rth_cs = 1 °C/W. You want to keep the junction temp below 80°C (Tj = 80°C).
Maximum total thermal resistance: Rth_tot = (80 - 25) / 5 = 11 °C/W
Maximum heat sink resistance: Rth_sa = 11 - 3 - 1 = 7 °C/W
You need a heat sink rated at 7 °C/W or lower (a lower number means a better heat sink).
Frequently Asked Questions
What if the required Rth_sa is negative?
A negative heat sink thermal resistance means your goal is physically impossible. Even if you attached the component to a perfect, infinitely large piece of copper (0 °C/W), the junction temperature would still exceed your limit due to the internal Rth_jc and Rth_cs.
What is a typical Rth_cs value?
Thermal paste is typically 0.1 to 0.5 °C/W. Silicone thermal pads are often 1 to 3 °C/W. It depends on the contact area and mounting pressure.
Sources
- Thermal Management basics.
Results are for engineering assistance. Verify important calculations and check official datasheets. See the Engineering Disclaimer.