Heat Sink & Junction Temperature Calculator

Works out how hot a semiconductor runs and what it takes to keep it cool. Heat leaves the silicon through a chain of thermal resistances — junction to case (θJC), case to heat sink through the interface material (θCS) and heat sink to air (θSA) — and each one raises the temperature by the power times its value in °C/W, exactly as current through a resistor raises the voltage. The junction therefore sits at the ambient temperature plus the power times the sum of the chain.

The tool works that chain three ways. Given the heat sink, it returns the junction, case and heat-sink temperatures and the margin to the part's maximum junction temperature. Given the maximum junction temperature, it returns the largest θSA a heat sink may have, which is the number to look for in a heat-sink catalogue — the lower, the bigger the sink. Given all the resistances, it returns the most power the part can dissipate. Without a heat sink the whole chain collapses into the single θJA figure from the datasheet.

Power can be entered directly or worked out for a linear regulator, where everything not delivered to the load becomes heat: (Vin − Vout) × load current, plus the quiescent current drawn from Vin. A 7805 dropping 12 V to 5 V at 0.5 A with 5 mA of quiescent current dissipates about 3.56 W. In a TO-220 package with no heat sink (θJA around 50 °C/W) at 40 °C ambient the junction would try to reach about 218 °C, far past its limit; with θJC = 5, a 0.5 °C/W pad of thermal compound and a 10 °C/W heat sink it settles near 95 °C. That is why a switching regulator is usually the better answer for large voltage drops.

The figures are steady-state: they describe the temperature after the part has warmed up, not a short burst, where the thermal capacitance of the case and heat sink absorbs heat for a while. θJA in datasheets is measured on a standard test board, so a small board, an enclosed box or still air can make it much worse. θSA of a heat sink also depends on airflow and on its orientation, and interface resistance depends on the material, the pressure and the flatness of the surfaces. Leave a margin of 10–20 °C below the rated maximum, which is typically 125 °C or 150 °C for silicon; °C/W and K/W are the same unit. Everything runs in your browser.