Required Heatsink

Rθsa = (Tj,max − Ta) / P − Rθjc − Rθcs

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Result
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Formula

Rθsa = (Tj_max - Ta) / P - Rθjc - Rθcs

Description

This formula determines the maximum allowable heatsink-to-ambient thermal resistance to keep the junction temperature below its rated maximum. It is the key design equation for selecting heatsinks. After calculating the required Rθsa, find a heatsink with an equal or lower thermal resistance at the expected airflow conditions. If the result is negative, the device cannot be operated at those conditions even with a perfect heatsink, and either the power must be reduced or forced air cooling added.

Variables

  • Rθsa — Required sink-to-ambient thermal resistance (°C/W)
  • Tj_max — Maximum allowable junction temperature from datasheet (°C)
  • Ta — Maximum expected ambient temperature (°C)
  • P — Power dissipation (W)
  • Rθjc — Junction-to-case thermal resistance (°C/W)
  • Rθcs — Case-to-sink thermal resistance (°C/W)

Practical Notes

When the required Rθsa is very low (below 1°C/W), consider switching to a forced-air heatsink, liquid cooling, or reducing the power dissipation by using a more efficient topology. Common TO-220 heatsinks range from 3-20°C/W in natural convection.

Related Formulas

Junction Temperature Full Thermal Chain Maximum Power (Junction to Case) Thermal Time Constant Forced Convection Heat Transfer Power Derating (Temperature)

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