A half bridge is an electronic circuit topology consisting of two switching elements (usually MOSFETs or IGBTs) stacked in series across a power supply, with the load connected to their midpoint, enabling bidirectional current flow or split-voltage output. What it changes in a real circuit is the ability to take a static, unidirectional DC source and synthesize a controllable AC waveform, or create a highly efficient stepped-down DC voltage without the massive heat penalty of linear regulators. By rapidly toggling the top and bottom switches, you can push and pull current through a load, which is the foundational building block for modern motor control and switched-mode power supplies.
The Core Mechanics: High-Side, Low-Side, and the Shoot-Through Hazard
A half bridge is built from two distinct switches: the high-side (HS) switch connected to your positive supply rail (V+), and the low-side (LS) switch connected to ground (GND). The junction between them is called the switch node (often labeled SW, OUT, or MID), which connects directly to your load.
The fundamental rule of a half bridge is that the HS and LS switches must never be turned on at the same time. If both conduct simultaneously, you create a direct short circuit from V+ to GND. In power electronics, this catastrophic failure mode is called shoot-through, and it will vaporize your MOSFETs in microseconds, often taking your gate driver and power supply down with them.
To prevent shoot-through, gate drivers introduce dead-time—a brief window (typically 100ns to 500ns) where both FETs are explicitly held off during the transition between high and low states.
Half Bridge vs. Full Bridge: Clearing the Confusion
The most common point of confusion for makers is mixing up a half bridge with a full bridge (commonly called an H-bridge). While both are used to drive loads like motors, their capabilities and component counts differ significantly.
| Feature | Half Bridge (2 Switches) | Full Bridge / H-Bridge (4 Switches) |
|---|---|---|
| Output Voltage Swing | Swings between V+ and GND (or +V/2 and -V/2 on split rails) | Swings directly between +V and -V across the load |
| DC Motor Reversal | Cannot reverse a single DC motor (requires split supply or dual half-bridges) | Can easily reverse a single DC motor by toggling diagonal pairs |
| Component Count | 2 FETs, 1 Gate Driver channel | 4 FETs, 2 Gate Driver channels |
| Primary Use Case | BLDC/Stepper phases, Synchronous Buck converters, Class-D audio | Brushed DC motor direction control, full-bridge inverters |
In short: a full bridge is essentially two half bridges placed in parallel, with the load connected horizontally between their midpoints. If you are driving a 3-phase BLDC motor, you don't use an H-bridge; you use three independent half bridges.
Worked Numeric Example: Sizing a 24V BLDC Motor Phase
Let’s design one phase of a half bridge for a 24V, 10A BLDC motor. We need to select the MOSFETs and size the bootstrap components for our gate driver.
1. MOSFET Selection:
Your V+ is 24V. Adding a 20% safety margin for inductive ringing gives 28.8V. We select a 40V or 60V rated FET. A classic bench favorite is the Infineon IRFZ44N (55V Vds, 49A Id, Rds(on) = 17mΩ at 10V Vgs).
2. Conduction Loss Calculation:
Power dissipated as heat is calculated using P = I²R.
P = (10A)² × 0.017Ω = 1.7W per FET.
Since only one FET conducts at a time, the half bridge will dissipate roughly 1.7W continuously. This is easily managed with a modest heatsink or a 2oz copper pour on your PCB, requiring no active cooling.
3. Bootstrap Capacitor Sizing:
If we use an IR2110 gate driver, we need a bootstrap capacitor (Cboot) to power the high-side gate. The IRFZ44N has a total gate charge (Qg) of roughly 72nC. The capacitor must supply this charge without its voltage dropping below the driver's undervoltage lockout (usually ~8V).
Using Q = CV, a standard 1µF ceramic capacitor charged to 12V holds 12µC of charge. This is over 150 times the required 72nC, ensuring the high-side FET gate stays firmly enhanced even during long high-side duty cycles. We pair this with a 1N4148 or UF4007 fast-recovery diode to prevent the cap from discharging back into the 12V supply.
Where You Meet This in Practice
You are likely already using half bridges without realizing it. They dominate modern power electronics in three main areas:
- BLDC and Stepper Motor ESCs: Every Electronic Speed Controller (ESC) for a drone or e-bike uses three half bridges to generate the 3-phase AC waveforms required to spin the rotor. A 60A e-bike controller will have three massive half bridges, often using paralleled FETs to handle the current.
- Synchronous Buck Converters: In older power supplies, a diode was used to provide a path for inductor current when the main switch turned off. Modern synchronous bucks replace that diode with a low-side MOSFET (creating a half bridge). Because a FET's voltage drop (I × Rds(on)) is much lower than a diode's 0.5V forward voltage, efficiency jumps from ~85% to >95%.
- Class-D Audio Amplifiers: These amplifiers use half bridges switching at 400kHz+ to create high-frequency PWM waves. The audio signal is encoded in the pulse width, and an LC low-pass filter on the output recovers the analog audio, delivering massive wattage with minimal heat.
Decision Tree: Which Topology and Driver Should You Pick?
Choosing the right half bridge implementation depends entirely on your load type and voltage. Use this decision path to select your hardware.
| If your application is... | Then you need... | Concrete Part Pick (2026 Standard) |
|---|---|---|
| Driving a brushed DC motor in one direction with dynamic braking (under 40V) | Integrated Half Bridge Driver | TI DRV8871 (Single H-bridge used as half, or dedicated half-bridge ICs like DRV8701) |
| Driving a 3-phase BLDC motor (under 20V, low current like gimbals or small drones) | Integrated 3-Phase Half Bridge (3x HS/LS) | TI DRV8313 or Allegro A4960 |
| Building a custom high-power 48V+ BLDC ESC or Induction Heater | Discrete Half Bridges + External Gate Drivers | FD6288 (3-channel driver) + CSD19536KTT (TI 80V N-FETs) |
| Designing a high-efficiency Synchronous Buck Converter | Integrated Power Stage (DrMOS) | Renesas ISL99380 (Integrates HS, LS, and driver in one package) |
FAQ: Edge Cases and Debugging
Why is my high-side FET getting incredibly hot while the low-side stays cool?
This is the classic symptom of a failing bootstrap circuit. If your bootstrap capacitor is too small, or if your PWM duty cycle is too high (e.g., 99%), the low-side FET never turns on long enough to recharge the capacitor. The high-side gate voltage sags, the FET enters its linear (resistive) region instead of fully saturating, and it burns up. Fix: Ensure your maximum duty cycle leaves at least 1-2µs for the low-side to conduct and recharge Cboot, or use a gate driver with an integrated charge pump.
What happens if I set the dead-time too high?
While too little dead-time causes shoot-through, too much dead-time causes 'body diode conduction.' During the dead-time window, the inductive load forces current through the MOSFET's intrinsic body diode. Body diodes have a high voltage drop (~0.7V to 1V) and slow reverse recovery times. Excessive dead-time will result in severe efficiency losses, voltage spikes, and potential EMI issues. For most hobbyist motor controllers, 200ns to 300ns is the sweet spot.
For deeper technical specifications on gate driver topologies and bootstrap network design, refer to the Texas Instruments Gate Driver Fundamentals and Infineon Gate Driver ICs documentation.
