Calculate the real loss first
Conduction loss starts with I²R, but RDS(on) must match the actual gate voltage and hot junction. PWM applications also require RMS current from the real waveform and duty cycle.
Switching loss depends on gate-drive speed, Miller plateau, bus voltage, and frequency. Diode reverse recovery and dead time may become major heat sources.
- Use hot RDS(on)
- Separate conduction and switching loss
- Keep margin for tolerance and supply variation
Use thermal resistance for the real structure
RθJA is relevant only when the board resembles the datasheet test fixture. Exposed pads, copper area, via arrays, and nearby heat sources all change the result.
A stronger model separates junction-to-case, case-to-board, and board-to-ambient paths, then calibrates them with a thermal camera or thermocouple.
Rated current is not design current. Allowable junction temperature and the actual cooling path determine continuous capability.
Finish with transients and faults
Inrush, motor stall, and short circuits can be harsher than steady state. Use the safe operating area curve to check pulse width, VDS, and current together.
Verify that the gate cannot float during power-up, power-down, or MCU reset. Add a pull-down and hardware shutdown path where needed.
- Check SOA, not only absolute maximums
- Evaluate repetitive pulses
- Test at worst ambient temperature