MOSFET power calculator
Conduction loss from Rds(on) and drain current, the voltage lost across the part, and the junction temperature that decides whether it needs a heatsink.
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Conduction loss
200 mW
P = Id² × Rds(on) × duty
Voltage across the MOSFET
100 mV
Lost from the load, not just wasted as heat.
Temperature rise
12 °C
Junction temperature
47 °C
Most parts are rated to 150 °C. Design for well under 125.
This is conduction loss only. Switching loss matters above roughly 20 kHz of PWM and needs the gate charge and the driver current to estimate — at the frequencies a hobby motor driver runs at, it is small. Always add a flyback diode across an inductive load.
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