Half Bridge Schematic Explained: Theory, Design, and Gate Driver Selection

A half bridge schematic is a power switching topology that uses two active semiconductor switches and a split DC bus (usually two series capacitors) to alternate a load's connection between the positive and negative supply rails, generating an AC or bipolar output from a single DC source. In a real circuit, this topology changes a unipolar DC voltage into a bipolar square wave, enabling motor reversal and transformer coupling without the cost, board space, and control complexity of a four-switch full bridge. Beginners frequently confuse the half bridge with a push-pull converter (which relies on a center-tapped transformer rather than a split capacitor bus) or mistakenly assume it can output the full bus voltage to the load; in reality, the maximum peak output is strictly limited to ±V_bus/2.

The Core Half Bridge Schematic and How It Switches

The classic half bridge consists of two switches (typically N-channel MOSFETs or IGBTs) arranged in a totem-pole configuration, with the load connected between the midpoint of the switches and the midpoint of two series DC-link capacitors. The top switch is the "high-side" device, and the bottom is the "low-side" device.

When the high-side switch conducts, current flows from the positive rail, through the load, and into the capacitor midpoint, pulling the output to +V_bus/2. When the low-side switch conducts, current flows from the capacitor midpoint, through the load, and down to ground, pulling the output to -V_bus/2. The split capacitors act as a virtual center tap, providing the return path for the AC current.

Numeric Design Example: 48V DC to AC Inverter Stage

Let’s design the power stage for a 48V nominal battery inverter driving a 15A continuous AC load at a 20kHz switching frequency. We are using a 16S LiFePO4 battery bank.

• Maximum Bus Voltage (V_{bus_max}): 16 cells × 3.6V/cell = 57.6V
• Output Voltage Swing: ±28.8V peak
• MOSFET V_DS Rating: Parasitic trace inductance will cause voltage ringing during switching. A standard 25% derating margin is required: 57.6V × 1.25 = 72V. Therefore, a standard 55V or 60V MOSFET will fail. You must select an 80V or 100V rated N-channel MOSFET (e.g., the Infineon IRFB4110, rated at 100V / 120A).

DC-Link Capacitor Sizing:
The capacitors must supply the high-frequency AC ripple current. For a 15A load, the RMS ripple current through each capacitor is roughly half the load current, or 7.5A. Standard electrolytic capacitors will overheat and vent at 20kHz. You must use a parallel bank of low-ESR film capacitors or high-frequency aluminum polymer capacitors. A practical bench choice is four 220µF, 100V Panasonic OS-CON polymer capacitors in parallel per leg, yielding an ESR of roughly 6mΩ and keeping ripple heating negligible.

Where You Meet This in Practice

You will rarely see a standalone half bridge used just to make a simple square wave. Its real value emerges in systems requiring high-frequency AC generation or bidirectional current steering:

• Brushless DC (BLDC) Motor Drives: A 3-phase BLDC inverter is essentially three half bridges wired in parallel. Each phase winding is driven between the half-bridge midpoint and the virtual neutral point of the motor.
• LLC Resonant Converters: In modern ATX and server power supplies, a half bridge drives the primary side of a high-frequency isolation transformer. The ±V_bus/2 square wave is fed into a resonant tank (inductor and capacitor) to achieve zero-voltage switching (ZVS), pushing efficiency past 96%.
• Class D Audio Amplifiers: The audio signal is converted to a high-frequency PWM stream. A half bridge switches the PWM signal to the speaker, and the speaker's natural inductance filters it back into an analog audio waveform.

Topology Decision Matrix: Half Bridge vs. Full Bridge vs. Push-Pull

Choosing the right topology dictates your component count, voltage stress, and control complexity. Use this decision tree to lock in your architecture.

Gate Driver Selection: Bootstrapping and Concrete Part Picks

The hardest part of building a half bridge schematic is driving the high-side N-channel MOSFET. To turn on an N-channel device, the gate voltage must be 10V to 15V higher than its source pin. But when the high-side MOSFET turns on, its source pin rises to V_bus. This means your gate driver must output V_bus + 12V.

You have two choices: an expensive isolated DC-DC power supply, or a bootstrap circuit. A bootstrap circuit uses a diode and a capacitor to "steal" charge from the low-side conduction phase and float it up to power the high-side driver IC.

Concrete Part Pick: For any half bridge operating between 24V and 100V, the definitive gate driver is the Texas Instruments UCC27211. Unlike the legacy IR2110 (which requires an external, lossy bootstrap diode and struggles with slow reverse recovery), the UCC27211 integrates the bootstrap diode internally. It supports a 120V absolute maximum bus, delivers 4A peak source/sink current to rapidly charge large MOSFET gate capacitances, and features built-in under-voltage lockout (UVLO) to prevent your MOSFETs from entering the linear region and melting during battery brownouts.

Bootstrap Capacitor Rule of Thumb: The bootstrap capacitor (C_boot) must hold enough charge to turn on the high-side MOSFET without drooping below the UVLO threshold. The minimum value is C_boot ≥ 10 × C_iss (where C_iss is the MOSFET input capacitance). For an IRFB4110 (C_iss ≈ 14nF), the math says 140nF. However, to account for gate driver quiescent current and leakage at low PWM duty cycles, always use a 1µF, 50V X7R ceramic capacitor placed within 2mm of the driver IC's V_B and V_S pins.