Heat Sink Thermal Resistance

Find the maximum heat-sink thermal resistance needed to keep a chip or power transistor below its junction temperature limit, free online.

Power dissipated (W)

Maximum junction temperature (°C)

Ambient temperature (°C)

Junction-to-case θjc (°C/W)

Case-to-sink θcs, thermal pad (°C/W)

Design margin (%)

About this calculator

The thermal chain is T_j = T_a + P·(θ_jc + θ_cs + θ_sa). The heat sink to ambient resistance must therefore satisfy θ_sa ≤ (T_j − T_a) ÷ P − θ_jc − θ_cs. Lower θ_sa means a larger sink or more airflow. Leave a 20% temperature margin for reliability.