MOSFET Gate Drive

How to use this calculator

A MOSFET's gate is capacitively coupled, so driving it requires charging and discharging that capacitance. The gate charge is Qg = CG·VG. If you supply a drive current Ig, the rise time is t_on = Qg/Ig, and the average gate power is P = Ig·VG·f_sw. Enter the gate capacitance, gate voltage, and (optionally) the drive current and switching frequency, and the calculator shows the gate charge, rise time, and average power.

What it means in a real circuit

For CG = 5 nF, VG = 10 V, Ig = 100 mA, f_sw = 100 kHz: Qg = 50 µC, t_on = 0.5 ms, and average gate power = 0.5 W. A higher drive current reduces t_on (faster switching) but increases gate power. Use this to size your gate driver, to check that the driver can supply enough current, and to estimate the thermal budget of the gate drive circuit.

Definitions

Gate capacitance (CG): the total capacitance between gate and source/drain. Gate charge (Qg): the charge required to switch the gate, Qg = CG·VG. Rise time (t_on): the time to charge the gate, t_on = Qg/Ig. Switching frequency (f_sw): how often the gate is switched. Gate power: the power dissipated in the gate driver, P = Ig·VG·f_sw.

Design tips

Use a dedicated gate driver (not a microcontroller pin) for power MOSFETs — it can supply the high peak currents needed for fast switching. Keep the gate loop area small to reduce inductance and ringing. Add a small gate resistor (RG) to damp ringing, but it increases t_on. Check the driver's peak current rating against the MOSFET's Qg and your target t_on.

Common mistakes

Driving a power MOSFET directly from a microcontroller pin (insufficient current, slow switching, high losses). Forgetting that the gate capacitance varies with voltage (CG is not constant). Not accounting for the gate resistor's effect on t_on. Ignoring the gate driver's own power dissipation.

Component-selection / practical advice

Choose a gate driver with peak current ≥ 3–5× the MOSFET's Qg divided by your target t_on. Use a 10–100 Ω gate resistor to damp ringing. Keep the gate loop area small. For high-frequency switching, use a low-inductance layout.

Limitations

The model assumes a constant CG and a constant drive current. In reality, CG varies with voltage, and the drive current is limited by the driver's output resistance and the gate loop inductance. For critical designs, use the MOSFET's datasheet Qg and the driver's specifications, and verify with an oscilloscope.

Example use cases

Sizing a gate driver for a DC-DC converter; estimating the thermal budget of a motor driver; checking that a microcontroller can switch a MOSFET fast enough; designing a gate resistor for a power switch.