How it works
In a half bridge or H-bridge, the high-side and low-side switches must never conduct at the same time. Because a MOSFET or IGBT takes longer to turn off than to turn on, the gate driver inserts a short dead time between switching one device off and the other on. Too little dead time causes shoot-through current; too much increases body-diode conduction loss and output distortion.
The datasheet method uses the switching times from the device datasheet: the minimum dead time is the turn-off delay plus fall time minus the turn-on delay. The gate charge method estimates the same times from the gate charge, the threshold voltage, the driver voltage and the gate resistance, which is useful when your gate resistor differs from the datasheet test conditions.
A safety margin is then added for temperature, component tolerances and driver propagation-delay mismatch. Finally, the MCU register tab converts the chosen dead time into the timer register values for your PWM peripheral and shows the rounding error.
Design procedure
- 1Read t_d(off), t_f and t_d(on) from the datasheet at the gate resistance you will actually use, or scale them if your gate resistor is different.
- 2Compute the minimum dead time, t_d(off) + t_f − t_d(on), for the switch that turns off.
- 3Add the propagation-delay mismatch between the high-side and low-side driver channels, which can be tens of nanoseconds for isolated drivers.
- 4Apply a margin for temperature and part-to-part variation, then convert the result to timer counts and round up, not down.
- 5Verify on the bench: measure both gate signals and the switch-node voltage, and check that there is no shoot-through current at the hottest operating point.
Formulas
- Minimum dead time (datasheet)
- t_dead,min = t_d(off) + t_f − t_d(on)
- Turn-off time (gate charge, approx.)
- t_off ≈ R_g · Q_gd / V_th
- Recommended dead time
- t_dead = t_dead,min · (1 + margin)
- Dead-time register
- DT = round(t_dead · f_timer / prescaler)
Worked example
A MOSFET with t_d(off) = 60 ns, t_f = 20 ns and t_d(on) = 15 ns needs at least 65 ns of dead time. With a 20% margin the recommended value is 78 ns, which on a 170 MHz timer is a register value of about 13.
Common mistakes
- Using datasheet times measured with a different gate resistance or drive voltage.
- Rounding the timer register down, which silently shortens the dead time.
- Forgetting the driver and isolator delay mismatch, which is often larger than the MOSFET switching times.
- Adding a very large dead time to be safe; body-diode conduction then increases loss and output distortion, especially in motor drives.
Frequently asked questions
Why not just set a long dead time to be safe?
During dead time the load current flows through the body diode, which has a higher voltage drop and reverse-recovery loss. Long dead times also distort the output voltage in motor drives and inverters.
Does dead time change with temperature?
Yes. Switching delays usually grow with temperature and with gate-driver supply drop, which is why a margin is added on top of the datasheet value.
Do GaN devices need dead time?
Yes, but much shorter, often a few nanoseconds to tens of nanoseconds, because GaN has no body diode reverse recovery and reverse conduction loss is high.
Results are engineering estimates. Verify your design with datasheets, simulation and measurement.