Engineering foundation
How to calculate MOSFET conduction loss correctly.
Conduction loss looks simple, but the result is only as good as the RMS current and on-resistance used. Duty cycle, ripple, junction temperature, gate voltage, and current sharing all matter.
Start with RMS current
Pcond = IRMS2 · RDS(on),T
Use the RMS current through that device, not automatically the converter output current. In a two-switch topology, each MOSFET conducts for only part of the cycle. Ripple raises RMS current even when average current is unchanged.
Correct resistance for temperature
RDS(on),T = RDS(on),25 · [1 + αR(TJ − 25 °C)]
A datasheet normalized-resistance curve is preferable to a linear coefficient over a wide range. The loss raises junction temperature, which raises resistance again, so the estimate should be iterated.
Common mistakes
- Using typical rather than maximum RDS(on).
- Ignoring gate-voltage dependence.
- Using average current where RMS current is required.
- Assuming paralleled devices share current perfectly.
- Applying steady-state thermal resistance to a short pulse.