H-Bridge Calculator — losses, dead time and driver sizing

Motor current, dead-time and driver dissipation for an H-bridge stage.

Example: Driving 5 A from 24 V: 1 W of conduction loss, 2% of duty lost to dead time, 11.42 V at the motor.

Inputs

Supply voltage(V)

Motor current(A)

R_ds(on) per switch(Ω)

At your gate drive voltage and expected junction temperature.

PWM frequency(Hz)

Dead time(s)

500n is 500 ns. Must exceed the switch turn-off time plus driver skew.

Duty cycle

Result

Conduction loss

1 W

Per switch: 500 mW
Bridge voltage drop: 200 mV
Duty lost to dead time: 2%
Effective duty: 48%
Voltage at the motor: 11.42 V
Bridge efficiency: 98.3%

Current passes through two switches in series, so conduction loss is 1 W — twice what one switch alone would dissipate. Each device sees 500 mW, which is what the thermal design must handle.
Dead time exists to prevent shoot-through: both switches in a leg conducting at once puts the supply across them with nothing but their own resistance limiting it. That destroys both devices in microseconds, so dead time must exceed the worst-case turn-off delay plus driver propagation skew — not the typical figure.

Dead-time cost against PWM frequency

Frequency	Duty lost	Usable duty at 100% command
1 kHz	0.1%	99.9%
8 kHz	0.8%	99.2%
20 kHz	2%	98%
50 kHz	5%	95%
100 kHz	10%	90%

Formula

P_cond = 2 × I² × R_ds(on)    duty lost = 2 × t_dead × f_sw

Worked example

Driving 5 A from 24 V: 1 W of conduction loss, 2% of duty lost to dead time, 11.42 V at the motor.

P_cond = 2 × I² × R_ds(on)
2 × (5 A)² × 20 mΩ
1 W
The factor of two is the high-side and low-side switch in series.
duty lost = 2 × t_dead × f_sw
2 × 500 ns × 20 kHz
2%
V_motor = effective duty × (V_supply − bridge drop)
48% × (24 V − 200 mV)
11.42 V

Frequently asked questions

Why is H-bridge loss twice a single switch?

Because current flows through two devices in series — one high-side and one low-side — to reach the motor and return. Both dissipate I²R, so the bridge total is 2I²R even though each individual switch only sees half of it.

What is dead time and why do I need it?

A deliberate gap where both switches in a leg are off, inserted so one has fully turned off before the other turns on. Without it, both conduct briefly and put the supply across them with almost nothing limiting the current — shoot-through, which destroys both devices in microseconds.

Why does dead time reduce my top speed?

It is inserted at both transitions of every PWM cycle, so the loss scales with frequency. 500 ns of dead time at 20 kHz costs 2% of duty; at 100 kHz it costs 10%. That is a direct cap on the maximum voltage you can deliver.

What PWM frequency should I use for a motor?

Above 20 kHz if you want it silent — anything lower produces an audible whine at the switching frequency. The trade is more switching loss and a proportionally larger dead-time penalty, so 20 to 25 kHz is the usual compromise.

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