What Is an H-Bridge? Circuit Theory, Motor Control, and IC Selection

An H-bridge is a circuit of four solid-state switches (usually MOSFETs or BJTs) arranged in an "H" shape that allows you to reverse the voltage polarity across a load, enabling bidirectional control of DC motors. What it changes in a real circuit: It transforms a fixed-polarity, unidirectional DC power supply into a reversible, four-quadrant drive capable of forward, reverse, braking, and coasting. What people commonly confuse it with: Beginners often confuse an H-bridge with a half-bridge (which only switches one side of the load, typically used for stepper motors or multi-phase AC) or a motor shield (which is a complete plug-and-play PCB containing an H-bridge IC, logic level shifters, and screw terminals).

Think of it like a double-pole, double-throw (DPDT) relay wired as a polarity-reversing switch, but executed silently with solid-state silicon at 20 kHz PWM frequencies. By closing the top-left and bottom-right switches, current flows one way; by closing the top-right and bottom-left, current flows the opposite way.

The Core Mechanics: Polarity Reversal and Shoot-Through

To understand the topology, label the four switches Q1 (top-left), Q2 (top-right), Q3 (bottom-left), and Q4 (bottom-right). The load (your motor) sits in the middle crossbar of the "H".

• Forward: Q1 and Q4 close. Current flows from VCC through Q1, across the motor, and down through Q4 to GND.
• Reverse: Q2 and Q3 close. Current flows from VCC through Q2, across the motor in the opposite direction, and down through Q3 to GND.
• Brake: Q1 and Q3 close (or Q2 and Q4). The motor terminals are shorted together, causing the back-EMF to rapidly dissipate the kinetic energy as heat in the windings, stopping the motor fast.
• Coast: All four switches open. The motor spins down naturally due to friction.

Worked Numeric Example: Sizing for a 12V Linear Actuator

Let's size an H-bridge for a 12V nominal DC linear actuator with a measured stall current of 2.5A. We will evaluate a modern MOSFET-based driver like the TI DRV8871.

When the actuator binds and hits stall current, the H-bridge must survive the thermal load. The internal resistance of the MOSFETs ($R_{DS(on)}$) acts as a series resistor.

1. Voltage Drop: $V_{drop} = I \times R = 2.5A \times 0.30\Omega = 0.75V$. The motor actually sees 11.25V at stall.
2. Power Dissipation: $P = I^2 \times R = (2.5)^2 \times 0.30 = 6.25 \times 0.30 = 1.875W$. The IC must dissipate nearly 2 watts of heat.
3. Thermal Rise: The DRV8871 in an HTSSOP package with an exposed thermal pad on a standard 2-layer PCB has a junction-to-ambient thermal resistance ($\theta_{JA}$) of roughly 40°C/W.
4. Junction Temperature: $\Delta T = 1.875W \times 40°C/W = 75°C$. Add a 25°C room ambient, and the silicon junction sits at 100°C.

While 100°C is below the typical 150°C absolute maximum rating, it is dangerously hot for an enclosed 3D-printed robotics chassis. The bench fix: Pour a solid copper ground plane directly under the IC's thermal pad and use multiple thermal vias to pull heat to the bottom layer, dropping the effective $\theta_{JA}$ to ~25°C/W and keeping the junction at a safer 72°C.

Where You Meet This in Practice

You will rarely build an H-bridge from discrete transistors unless you are pushing hundreds of amps. In 95% of maker and commercial applications, you will use an integrated H-bridge driver IC. Here is where they show up on the bench:

• Differential Drive Robots: Two independent H-bridges driving left and right wheels (like a Roomba) to execute tank turns.
• Linear Actuators and Winches: Extending and retracting 12V/24V hydraulic or screw-driven actuators for automated hatches or camera sliders.
• Peltier (TEC) Temperature Controllers: Reversing polarity across a Thermoelectric Cooler switches it from heating to cooling mode without moving parts.
• Active Braking in Servos: Shorting the motor windings to hold a robotic arm joint rigidly in place against gravity.

Decision Tree: Selecting Your H-Bridge Driver IC

Do not default to the ancient L298N module you found in a starter kit; its bipolar junction transistor (BJT) design drops 2V to 3V across the bridge, wasting massive amounts of power as heat. Use this decision matrix to pick a modern MOSFET-based driver.

Critical Protection: Dead-Time and Flyback Routing

When you turn off an inductive load like a DC motor, the collapsing magnetic field generates a massive reverse voltage spike (back-EMF). If this spike exceeds the breakdown voltage of your MOSFETs, the IC will fail catastrophically—often shorting VCC to the motor pin and frying your microcontroller.

Internal Body Diodes vs. External Schottky:
Every MOSFET has an intrinsic "body diode" that naturally routes this flyback energy back to the VCC rail. For small motors (under 2A) and low PWM frequencies (under 5 kHz), the body diodes in a modern IC like the DRV8871 are sufficient. However, if you are driving a highly inductive load (like a large 24V winch motor) at 20 kHz PWM, the reverse recovery time of the silicon body diodes is too slow. You must solder external Schottky diodes (like the SS34) across the motor terminals or from the motor pins to VCC/GND to clamp the spike before it rings.

Decoupling Capacitors:
Always place a bulk electrolytic capacitor (e.g., 100µF to 470µF, rated for 1.5x your max voltage) directly across the VCC and GND pins of the H-bridge. This acts as a local energy reservoir. When the motor demands a sudden spike of stall current, the capacitor supplies it locally, preventing the voltage rail from sagging and browning out your ESP32 or Arduino.