MOSFET Switch Calculator (Gate Resistor, Switching Time, Conduction & Switching Loss, Junction Temperature)
Check a MOSFET as a low-side switch or PWM driver: gate charge current and switching time from the gate resistor and driver, conduction loss from Rds(on) and duty cycle, switching loss at your frequency, gate-drive power, and the junction temperature for the package with or without a heatsink.
How it works
I_gate ≈ V_gs ÷ (R_g + R_driver); t_switch ≈ Q_g ÷ (0.6 × I_gate) · P_cond = I² × R_ds(on),hot × D · P_sw ≈ ½ × V_ds × I_d × (t_on + t_off) × f_sw · P_gate = Q_g × V_gs × f_sw · T_j = T_a + (P_cond + P_sw) × R_th(JA)
A MOSFET is a voltage-controlled switch, but its gate is a capacitor that must be charged and discharged every cycle, so how fast it switches is set by the gate charge and the current your driver can push through the gate resistor. Slow edges waste power while the device is half-on — the switching loss — and grow with frequency; while fully on, loss is simply I²R with the hot on-resistance. Add them, multiply by the thermal resistance of the package and heatsink, and you have the junction temperature, which is what decides whether the design lives. The gate resistor is the knob: small for speed and efficiency, larger to calm ringing and limit driver current.
Worked example
A 30 nC FET driven at 10 V through 10 Ω plus a 5 Ω driver: peak gate current 0.67 A, average about 0.4 A, so about 75 ns per edge. Switching 5 A at 24 V with 20 kHz PWM at 60% duty: conduction 0.33 W (22 mΩ hot) + switching 0.18 W = 0.51 W, gate drive 6 mW. In free air a TO-220 (62 °C/W) reaches about 67 °C at 35 °C ambient — fine without a heatsink; at 100 kHz the switching loss would rise to 0.9 W and a clip-on heatsink would be needed.
Frequently asked questions
Can I drive a MOSFET straight from an Arduino pin?
For on/off switching, yes with a logic-level FET (Vgs(th) ≤ 2.5 V, Rds(on) specified at 4.5 V) and a 100–220 Ω gate resistor. For PWM above a few kHz the pin's ~20 mA cannot charge tens of nC quickly, edges stretch to microseconds and the FET heats — use a gate driver IC or a small push-pull driver.
What gate resistor value should I use?
With a proper driver 5–22 Ω for fast, efficient switching; 47–100 Ω to reduce ringing and EMI at the cost of switching loss; 100 Ω–1 kΩ for simple on/off from an MCU. Always add a 10–100 kΩ pull-down so the gate is defined before the driver wakes up.
Why is my MOSFET hot although the current is well within its rating?
Usually under-driven (Vgs too low so Rds(on) is many times the datasheet figure), switching too slowly at PWM frequency, or the headline current rating assumed an infinite heatsink at 25 °C. Measure the gate voltage and the case temperature; hot Rds(on) is 1.5–1.8× the 25 °C value.
Related calculators
Electronics estimates use simplified models and typical component values. Check the datasheet limits (voltage, current, temperature, gate charge, ESR), prototype, and measure — especially for anything connected to mains, batteries or motors.