MOSFET Linear & Thermal

Estimate linear-mode dissipation and the heatsink thermal budget.

Formulas

VDS = Vin − Vout; P = VDS × Iload

θtotal = θJC + θCS + θSA; Tj = Tambient + P × θtotal

Pmax = max(0, (Tj,max − Tambient) / θtotal)

θSA,required ≤ (Tj,max − Tambient) / P − θJC − θCS

Temperature margin = Tj,max − Tj; power margin = Pmax − P

Model & assumptions

Steady-state, single series thermal path with constant thermal resistance. It excludes transient heating, hot spots, and parallel heat paths.

This simplified calculation does not verify DC Safe Operating Area (SOA). Check the manufacturer datasheet SOA curve at the actual voltage, current, case temperature, and duration, especially in linear mode.

Example with default inputs

Estimated junction temperature

109 °C

MOSFET voltage drop

7 V

MOSFET power dissipation

14 W

Total thermal resistance

6 °C/W

Maximum thermal power budget

20.8333 W

Required heatsink θSA (maximum)

6.92857 °C/W

Junction temperature margin

41 °C

Thermal power margin

6.83333 W