Why MOSFET temperature needs solving, not just calculating
A MOSFET's on-resistance rises as it heats up, typically 1.5–2× from 25 °C to 125 °C. Higher resistance means more conduction loss, which means more heat. This calculator iterates that loop until it settles, and tells you when it doesn't, which is thermal runaway.
- Rds(on) at temperature: scaled exponentially from the datasheet's 25 °C and 125 °C values
- Conduction loss:
I²·Rds(on)(Tj)·D - Switching loss:
½·V·I·(t_r+t_f)·f_sw + ½·C_oss·V²·f_sw - Junction temperature:
Tj = Ta + P·(RθJC + RθCS + RθSA)
Worked example: 48 V motor drive
A 4.5 mΩ MOSFET carrying 30 A at 50% duty, switching 48 V at 20 kHz, heats until its on-resistance reaches about 5.4 mΩ. Conduction loss is about 2.45 W and switching loss about 0.86 W, for 3.55 W in total. On a 1.5 °C/W heatsink with an insulating pad at 50 °C ambient, the junction settles at about 61 °C.
Good to know
- Design to a junction limit well below the datasheet maximum, commonly 125 °C for a 175 °C part.
- Pro works backward from your Tj limit to give the maximum ambient and the heatsink you need, handles paralleled devices, and plots where runaway begins.