動作原理
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.
計算式
- Minimum dead time (datasheet)
- t_dead,min = t_d(off) + t_f − t_d(on)
- Turn-off delay (gate charge)
- t_off ≈ R_g · (Q_gd + Q_gs) / V_drv
- Recommended dead time
- t_dead = t_dead,min · (1 + margin)
- Dead-time register
- DT = round(t_dead · f_timer / prescaler)
計算例
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.
よくある質問
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.
計算結果は工学的な推定値です。データシート、シミュレーション、実測で設計を確認してください。