MOSFET Gate Drive Headroom

Formula

Description

The gate drive voltage must exceed the threshold voltage by a sufficient margin to fully enhance the MOSFET and achieve its rated Rds(on). A MOSFET specified at Rds(on) = 10 mΩ at Vgs = 10 V may have Rds(on) = 20 mΩ at Vgs = 4.5 V and may not turn on at all if Vgs barely exceeds Vth. The margin between Vgs and Vth determines how deeply the MOSFET is enhanced and directly affects conduction losses. Logic-level MOSFETs (Vth = 1-2 V) can be driven directly from 3.3 V or 5 V logic, while standard MOSFETs (Vth = 2-4 V) require a gate driver with 8-12 V output.

Variables

• V_margin — Gate drive headroom above threshold (V)
• V_gs — Applied gate-to-source voltage (V)
• V_th — Gate threshold voltage (V)

Practical Notes

Vth varies with temperature (typically −3 to −7 mV/°C), so the margin decreases as the MOSFET heats up. Always check Rds(on) at the actual Vgs you can provide, not just the datasheet headline value. For high-side N-channel MOSFETs, a bootstrap or charge pump circuit is needed to generate Vgs above the source voltage. GaN FETs have lower Vth (1-2 V) and require more precise gate drive voltage control.