Power MOSFET Thermal Calculator

Conduction losses occur when current flows through the MOSFET in the 'on' state, calculated as I²×RDS(on). Switching losses occur during transitions between on/off states due to simultaneous voltage and current during switching.

The required heat sink thermal resistance is calculated as Rth_hs = (Tj_max - Ta)/P_total - Rth_jc - Rth_ca, where negative values indicate no heat sink is needed for the given conditions.

Gate charge losses are proportional to switching frequency and gate charge, typically resulting in milliwatts. They become significant only at very high switching frequencies or with large gate charges.

If junction temperature exceeds the maximum rating, the MOSFET may fail or have reduced reliability. You need to reduce power losses, improve cooling, or use multiple MOSFETs in parallel.

Duty cycle directly affects conduction losses since the MOSFET only conducts when 'on'. A 50% duty cycle means the MOSFET conducts half the time, reducing average conduction losses.

Junction-to-case thermal resistance (Rth j-c) represents the thermal barrier between the semiconductor junction and the package case. It's typically much lower than case-to-ambient resistance and determines heat transfer efficiency to external cooling.

Longer rise and fall times increase switching losses because the MOSFET spends more time in the linear region where both voltage and current are present. Faster switching reduces losses but may increase EMI.

For parallel MOSFETs, divide the total current by the number of devices and calculate for one MOSFET. Each device will have similar power dissipation if properly matched, so multiply the result by the number of devices for total system losses.