Where a switching MOSFET loses power
A MOSFET used as a PWM switch dissipates power in two ways:
- Conduction loss while it is on: the drain current flows through its on-resistance Rds(on).
- Switching loss during each transition, when voltage and current overlap for a few nanoseconds.
The heat raises the junction temperature, which must stay below the rating (usually 150 or 175 °C) with margin.
The formulas
- Conduction: P_cond = I² × Rds(on) × k_hot × duty cycle
- Switching: P_sw ≈ ½ × V × I × (t_rise + t_fall) × f
- Junction temperature: T_j = T_ambient + P_total × R_θ
Rds(on) increases with temperature, typically by 1.5 to 1.7 times at 100 °C, so the calculator applies a multiplier.
Example
Switching 10 A at 12 V with a 10 mΩ MOSFET, 50% duty cycle, 20 kHz and 100 ns of combined rise and fall time:
| Item | Value |
|---|---|
| Conduction loss (hot) | 0.75 W |
| Switching loss | 0.12 W |
| Total | 0.87 W |
| Junction temperature, TO-220 without heat sink (62 °C/W) | about 79 °C |
At 200 kHz the switching loss would grow tenfold to 1.2 W and dominate.
Reducing losses
- Choose a lower Rds(on) part for high currents and a lower gate charge part for high frequencies.
- Drive the gate hard: a proper gate driver with 1 to 2 A of peak current shortens transitions. A microcontroller pin driving a large MOSFET directly switches slowly.
- Use a logic-level MOSFET specified at your gate voltage (for example Rds(on) at Vgs = 4.5 V for 5 V logic).
- Add a heat sink or copper area: the needed thermal resistance is shown in the results.
Thermal resistance
Total thermal resistance from junction to air is the sum of junction-to-case, case-to-sink (with thermal paste or a pad) and sink-to-air. A TO-220 alone in still air is about 62 °C/W; on a medium heat sink it can drop below 10 °C/W.
Limits
This is a first estimate. It ignores gate drive loss, body diode conduction, reverse recovery and ringing, which matter in fast, high-voltage designs. Check the datasheet’s safe operating area and measure the case temperature on the prototype.