Glossary term
H-Bridge
A four-switch power electronic circuit that drives a load with reversible voltage polarity.
Definition
deviceAn H-bridge is a power switching circuit with four controlled switches arranged so a load can be driven with either voltage polarity.
The circuit is named for its schematic shape: the load forms the crossbar of an H, and the switches form the four legs. By closing opposite switch pairs, the bridge applies positive or negative voltage across the load. With pulse-width modulation, it can control average voltage, current, speed, torque, or power. H-bridges are common in DC motor drives, solenoids, speakers, robotics, automotive actuators, power converters, and inverter stages.
An H-bridge allows an electronic controller to reverse the voltage across a load. The classic arrangement has four switches. When the high-side switch on the left and the low-side switch on the right are on, current flows through the load in one direction. When the opposite diagonal pair is on, current flows in the other direction. This makes the circuit useful for reversing a DC motor, driving a bidirectional actuator, or generating an AC waveform from a DC supply.
In real hardware the switches may be MOSFETs, IGBTs, bipolar transistors, relays, or integrated driver devices. Modern H-bridges usually use semiconductor switches because they support fast pulse-width modulation, current limiting, diagnostic feedback, and compact packaging.
Operating states
The main states are forward drive, reverse drive, brake, coast, and recirculation. In forward or reverse drive, one diagonal switch pair applies bus voltage across the load. In brake mode, the load terminals may be shorted through low-side or high-side switches, causing motor back electromotive force to generate braking current. In coast mode, all switches are off and the load current decays through body diodes, freewheel paths, or external clamps.
Pulse-width modulation changes the average voltage by switching rapidly between drive and recirculation states. For a motor, this controls current and torque indirectly. For an inverter, the modulation pattern approximates a sinusoidal output or another required waveform.
Protection and timing
The critical failure mode is shoot-through. If the high-side and low-side switches in the same leg turn on at the same time, the DC bus is shorted. Dead time is therefore inserted between switching commands so one device turns off before the complementary device turns on. Too little dead time risks destructive current. Too much dead time distorts the output waveform and can reduce control accuracy.
Inductive loads require a safe current path when switches change state. Body diodes, synchronous rectification, snubbers, clamps, or flyback paths handle stored magnetic energy. Current sensing is often used for torque control, stall detection, overcurrent protection, and short-circuit detection. Thermal design is also central: conduction loss, switching loss, package resistance, PCB copper area, airflow, and junction temperature determine the safe operating envelope.
Common mistakes
An H-bridge is not just four switches connected around a load. Gate-drive voltage, high-side drive method, dead-time control, layout inductance, reverse recovery, electromagnetic interference, current measurement, braking energy, and supply decoupling determine whether the bridge works reliably. A common error is sizing devices only by steady load current while ignoring stall current, startup current, short-circuit energy, thermal cycling, and transient voltage spikes.