An H-bridge is four switches arranged so that a load (typically a DC motor) can be driven in both directions. The name comes from the schematic shape: two "legs" with the motor bridging them horizontally, forming the letter H. By closing diagonal pairs of switches, current flows through the motor in either direction, giving you full bidirectional control.

The Basic Topology: Label the four switches Q1–Q4. Q1 (high-side left) and Q4 (low-side right) form one diagonal; Q2 (high-side right) and Q3 (low-side left) form the other. Close Q1+Q4: motor spins forward. Close Q2+Q3: motor spins reverse. Open all four: motor coasts. Close both low-side switches (Q3+Q4): motor brakes by shorting its terminals.

The cardinal sin is shoot-through — turning on both switches in the same leg simultaneously (e.g., Q1+Q3). This creates a dead short from supply to ground, limited only by MOSFET on-resistance. At 12V with 10mΩ total R_DS(on), that's 1,200A of fault current. The MOSFETs die instantly. To prevent this, you must implement dead-time: a brief interval (typically 100ns–1µs) where both switches in a leg are off before the complementary switch turns on.

Practical implementation: Use N-channel MOSFETs for the low side and either P-channel or N-channel with a gate driver for the high side. N-channel MOSFETs have roughly 2–3× lower R_DS(on) than P-channel at the same price, so all-N-channel designs with a bootstrap gate driver (like the IR2110) are preferred for anything above a few amps. The bootstrap capacitor charges when the low-side switch is on, then provides the above-rail voltage needed to hold the high-side gate open.

Flyback diodes are mandatory. Motors are inductive loads. When a switch opens, the collapsing magnetic field generates a voltage spike that will destroy your MOSFETs. Place a fast-recovery or Schottky diode anti-parallel across each switch. Many power MOSFETs have an adequate body diode built in, but check the datasheet — the body diode's reverse recovery can be slow and lossy.

Sizing example: For a 12V, 5A brushed DC motor, select MOSFETs rated for at least 30V (2.5× supply) and 10A continuous. With R_DS(on) = 20mΩ per FET and two FETs conducting at any time, conduction loss is I²×2R = 25×0.04 = 1W total. That's manageable without a heatsink on a TO-220 package.

Real-world shortcut: For prototyping with small motors (under 2A), use an integrated H-bridge IC like the DRV8871 or TB6612FNG. These include shoot-through protection, gate drivers, and current limiting in a single chip. Save the discrete design for higher currents or when you need PWM frequencies above 100kHz.