H-Bridge Motor Drives
Reverse and regulate current through an inductive load using two coordinated half-bridges.
An H-bridge places a motor between two movable endpoints. Make the left end high and right end low for one direction; swap them to reverse the applied voltage.
An H-bridge contains two half-bridges with the load between their midpoints. Opposite diagonal devices apply positive or negative bus voltage across the load. This enables bidirectional current and torque control for a brushed DC motor, solenoid or transformer primary.
PWM controls average applied voltage, but the winding current is smoother than the switching voltage because of inductance. When an active pair turns off, current must recirculate. Fast decay, slow decay, brake and coast states use different switch or diode paths and produce different current decay and mechanical behavior.
Never turn on both devices in the same leg. Driver truth tables, dead time and fault handling are safety functions, not optional software polish. A direction command should not instantly reverse a large current; reduce current, enforce a safe transition, then build current in the new direction.
Motor drivers must tolerate startup and stall current, which can be much higher than the normal running current. Check current sensing, overcurrent response, supply pumping during braking, thermal limits and whether the DC-link can absorb returned energy.
Reverse and regulate current through an inductive load using two coordinated half-bridges.
Core ideas
- Diagonal switch pairs set voltage polarity.
- Inductive current recirculates when PWM turns the drive off.
- Brake, coast and decay mode are electrically different states.
Motor voltage states: +Vbus, 0, −VbusElectrical power: P = VIShunt voltage: Vsense = I × RshuntWork through a practical example
1A 12 V motor is driven with 60% PWM in one direction.
2A simple average-voltage estimate gives 0.60 × 12 = 7.2 V, before switch drops and current dynamics.
3When PWM turns off, choose a documented recirculation state rather than opening every path blindly.
4If the motor can stall at 8 A, design switches, current sensing and protection for that condition rather than the lower no-load current.
Changing direction while large current is still flowing, or treating coast and brake as interchangeable because both can produce zero applied voltage.