A half bridge is a power electronics circuit configuration using two active switching devices (like MOSFETs or IGBTs) and a split DC bus—usually two series capacitors—to control voltage and current direction across a load. In a real circuit, it changes a fixed, unidirectional DC voltage source into a controllable, bidirectional AC or pulsed-DC output, enabling variable speed motor control, regenerative braking, and high-frequency power inversion without mechanical commutators.
The Core Mechanics of a Half Bridge Topology
To understand the topology, look at the two switches: a high-side switch connected to the positive DC rail, and a low-side switch connected to ground. The load connects between the midpoint of these two switches and the midpoint of two equal-value capacitors that split the DC bus voltage.
Think of it like a single-lane drawbridge over a canal. You can open the left side to let water (current) flow one way, or open the right side to let it flow the other, but you cannot route it through a complex multi-directional grid like a full bridge. When the high-side switch turns on, current flows from the positive rail, through the load, and into the capacitor midpoint. When the low-side switch turns on, current flows from the capacitor midpoint, through the load, and down to ground. By alternating these switches using Pulse Width Modulation (PWM), you synthesize an AC waveform or control the average DC voltage delivered to the load.
Half Bridge vs. Full Bridge: Clearing Up the Confusion
Beginners commonly confuse the half bridge with a push-pull converter, or mistakenly assume the word 'half' means it delivers half the power of a full bridge. In reality, 'half' refers to using half the number of active switches (two instead of four).
While a full bridge (H-bridge) can swing the full bus voltage across the load in either direction (+V to -V), a half bridge can only swing half the bus voltage (+V/2 to -V/2) relative to the capacitor midpoint. To get the same power output as a full bridge, a half bridge must operate at a higher bus voltage or draw twice the current.
| Feature | Half Bridge | Full Bridge (H-Bridge) |
|---|---|---|
| Active Switches | 2 | 4 |
| Max Voltage Across Load | V_bus / 2 | V_bus |
| Gate Drive Complexity | Moderate (1 high-side, 1 low-side) | High (2 high-side, 2 low-side) |
| Primary Use Case | High-voltage inverters, Class D audio, 3-phase BLDC building blocks | Low-voltage DC motor reversing, robotics, H-bridge motor drivers |
Where You Meet This in Practice
You will rarely see a discrete half bridge built from raw transistors in consumer appliances today; as of 2026, integrated gate drivers and power modules dominate. However, the topology is everywhere:
- 3-Phase Motor Drives (VFDs): A variable frequency drive for an industrial AC motor is essentially just three half bridges wired in parallel, one for each motor phase.
- Class D Audio Amplifiers: The output stage of a high-efficiency Class D amp uses a half bridge to switch the audio signal at ultrasonic frequencies, which is then filtered by an LC network to drive the speaker.
- LLC Resonant Converters: In high-efficiency switch-mode power supplies (like your PC's ATX PSU), a half bridge generates the high-frequency AC needed to drive the main isolation transformer.
- Induction Cooktops: The high-frequency coil driver relies on a half bridge to generate the 20-50 kHz alternating magnetic field.
Worked Numeric Example: Sizing a 24V Motor Driver
Let's size the MOSFETs for a half bridge driving a 24V DC brushed winch motor.
- Voltage Rating (Vds): The bus maxes out at 28V. Applying a 20% safety margin for inductive kickback spikes, we need a minimum of 33.6V. We select a 40V or 60V rated MOSFET. Let's use a 60V N-channel FET like the CSD17571Q5A for robust transient handling.
- Current & Conduction Loss: At 15A continuous, we want to keep conduction losses low to avoid massive heatsinks. Target an Rds(on) of < 5mΩ at Vgs = 10V. Using P = I²R, the power dissipated per FET is 15² × 0.005 = 1.125W. This is easily manageable with a standard PCB copper pour acting as a heatsink.
- Gate Charge (Qg): If we are switching at 20 kHz, we need a low gate charge to minimize switching losses and reduce the burden on the gate driver IC. A Qg of around 15 nC requires a driver capable of sourcing at least 0.3A peak current (I = Qg × f = 15nC × 20kHz).
Bench Scenario Walkthrough: The Bootstrap Capacitor Trap
Theory is clean; the workbench is not. Here is a real-world scenario demonstrating a classic half-bridge failure mode.
The Setup: We are building a single-phase test rig for a 48V BLDC motor using a discrete half bridge and an IR2110 gate driver IC. The low-side FET is tied to ground, and the high-side FET is tied to the 48V rail. We wire up the PWM signals from an ESP32, apply power, and command a 50% duty cycle.
The Numbers: 48V DC bus, 16 kHz PWM frequency, 10A expected phase current.
The Outcome: The low-side FET switches perfectly. The high-side FET never turns on. The motor stutters violently. After a few seconds, the high-side MOSFET gets blistering hot, the silicon cracks, and the 48V bus shorts to ground, tripping the bench power supply's overcurrent protection.
What Went Wrong: We forgot the bootstrap circuit. To turn on an N-channel MOSFET, the gate voltage must be roughly 10V higher than its source pin. For the low-side FET, the source is at ground, so a 10V gate drive is easy. But when the high-side FET turns on, its source pin rises to 48V. To keep it on, the gate needs to be at 58V. The IR2110 uses a 'bootstrap' capacitor and a fast recovery diode to pump charge up to this floating high-side rail. Because we omitted the bootstrap capacitor (typically 1µF to 4.7µF ceramic) and the diode (like a 1N4148 or UF4007), the high-side gate driver starved of voltage, leaving the FET in the linear (high-resistance) region. It dissipated hundreds of watts as heat and failed catastrophically.
Half Bridge Design and Troubleshooting FAQ
Can I use P-channel MOSFETs for the high side to avoid bootstrap circuits?
Yes, but only for low-voltage, low-current applications (under 20V and 5A). P-channel MOSFETs have significantly higher Rds(on) and higher gate capacitance than equivalently sized N-channel FETs. For a 48V or 400V half bridge, high-voltage P-channel FETs are virtually non-existent or prohibitively expensive. You must use N-channel FETs with a bootstrap circuit, an isolated DC-DC gate drive supply, or a dedicated high-side gate driver IC.
How do I calculate the dead time needed to prevent shoot-through?
Dead time is the brief period where both FETs are off during a transition. You calculate it by looking at the MOSFET datasheet's turn-off delay time (td(off)) and fall time (tf), and comparing it to the turn-on delay (td(on)) and rise time (tr). A safe starting point for modern 60V FETs at 20 kHz is 100ns to 200ns. Many modern gate drivers (like the Infineon EiceDriver series) feature built-in adaptive dead-time control or anti-cross-conduction logic to handle this automatically.
Why are my half-bridge capacitors getting hot in an LLC converter?
The split DC bus capacitors in a half bridge carry the full AC ripple current of the load. If you are using standard electrolytic capacitors, their Equivalent Series Resistance (ESR) will cause internal heating at high frequencies (50 kHz+). Switch to low-ESR polymer capacitors or place high-frequency ceramic MLCCs in parallel with your bulk electrolytics to bypass the high-frequency ripple current.
Is a half bridge the same as a totem-pole output?
They are topologically identical (a high-side and low-side switch in series), but the terminology implies different use cases. 'Totem-pole' usually refers to low-power digital logic or signal-level gate driving (mA range), while 'half bridge' implies a power stage designed to drive heavy loads (Amps to kilo-Amps) with specific attention to thermal management, dead time, and inductive kickback protection.
For deeper exploration into integrated motor driver architectures that build upon these discrete concepts, review the Texas Instruments Motor Drivers Overview. Understanding the fundamental half bridge is the critical first step before scaling up to complex 3-phase inverters and high-power resonant converters.
