Power Electronics
A buck is one-directional: current flows toward the load and the output is always positive. To drive a DC motor forward AND backward, accelerating AND braking, you need four-quadrant operation: ±voltage × ±current. The answer is the bridge family. In the half bridge, the source is split into ±Vdc/2 (two series capacitors or sources) and one leg of two switches produces an output referenced to the midpoint: Vo = (Vdc/2)·(2D−1). At D=50% the output is zero; above it positive, below it negative — you reverse the direction just by shifting the duty.
The full bridge adds a second leg and connects the load between the two legs: Vo = Vdc·(2D−1) — twice the voltage swing from the same source. PWM can be generated two ways: bipolar (diagonal pairs switch together; the output jumps between +Vdc and −Vdc) and unipolar (legs switch independently; a three-level +Vdc/0/−Vdc waveform). Unipolar doubles the effective ripple frequency and halves the dv/dt — a big win for EMI and filter size.
Bridge legs carry two practical dangers: both switches conducting at once (shoot-through — a dead short across the source) and interrupting inductive load current. The first is solved with dead time: one switch never turns on before the other has fully turned off. The second is handled by the MOSFETs' body diodes: when the switches open, motor current freewheels back into the source through them (that is exactly what regenerative braking is). In our ready-made circuit, D=75% gives +6 V w.r.t. the midpoint, D=25% gives −6 V.
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