Half Bridge Motor Control: Sizing, Wiring, and Topology Decisions for DC Motors

Half bridge motor control uses two switching elements—typically N-channel MOSFETs or IGBTs—stacked in series across a DC bus, with the motor connected to the midpoint. Unlike a full H-bridge, a half-bridge only permits unidirectional rotation. However, it enables variable speed via PWM and dynamic braking by shorting the motor terminals through the low-side switch. If you need to drive a 12V to 24V brushed DC (BDC) motor up to 50A continuous without reversing it, the definitive default pick is the Infineon IR2110 gate driver paired with two IRFB3207 N-channel MOSFETs.

This topology is the workhorse for unidirectional conveyors, winches, pumps, and cooling fans. Below is the exact engineering framework to select, wire, and troubleshoot a half-bridge drive, terminating in a concrete decision path for your next build.

Motor Type Compatibility Matrix

A half-bridge is not a universal driver. It outputs a single switched DC rail relative to ground. Attempting to drive the wrong motor type will result in stalled rotors or destroyed silicon. Here is how common motor types map to this topology.

Half-Bridge Wiring & Terminal Identification

Driving two N-channel MOSFETs in a half-bridge requires a dedicated gate driver. The high-side MOSFET's source pin is floating at the motor voltage, meaning its gate must be driven 10V–12V above the DC bus voltage to fully enhance the channel. The Infineon IR2110 solves this using a bootstrap circuit.

IR2110 Pinout & Wiring Map

The Bootstrap Mechanism: When the low-side MOSFET turns on, the VS pin is pulled to ground. The VCC rail charges the bootstrap capacitor (connected between VB and VS) through a fast-recovery diode (e.g., UF4007). When the low-side turns off and the high-side turns on, the VS pin rises to the bus voltage. The capacitor acts as a floating battery, keeping the VB pin 12V above VS, ensuring the high-side gate remains fully enhanced. For reliable operation, the PWM duty cycle must never reach 100%; cap it at 95% to allow the bootstrap capacitor time to recharge.

Sizing Rule of Thumb & Worked Load Example

The most common mistake in half bridge motor control is sizing MOSFETs based on the motor's nominal running current. Motors draw stall current the moment they start, and inductive kickback can cause massive transient spikes.

Worked Load Example: 24V DC Winch Motor

Suppose you are driving a 24V brushed DC winch motor. The nameplate says 12A nominal, but you measure the locked-rotor stall current at 75A.

1. Current Target: $75A \times 1.25 = 93.75A$ minimum $I_D$.
2. Voltage Target: $24V \times 1.5 = 36V$ minimum $V_{DS}$.
3. Component Selection: The IRFB3207 N-channel MOSFET is rated for 75V $V_{DS}$ and 170A $I_D$. It easily clears both thresholds.
4. Thermal Check: The IRFB3207 has an $R_{DS(on)}$ of roughly 4.5mΩ. At the 12A nominal running current, conduction loss per FET is $I^2R = (12^2) \times 0.0045 = 0.64W$. This is low enough to run on a modest shared heatsink without forced air, assuming a standard 20kHz PWM switching frequency.

Failure Signatures: Hum, Overheat, and Stall

When a half-bridge fails or operates poorly, the physical symptoms map directly to specific electrical faults. Use this diagnostic framework before replacing blown components.

1. Audible Hum or Whine from the Motor

Cause: The PWM frequency is set below the human hearing threshold (typically < 16kHz), or the gate drive voltage is too low, causing the MOSFETs to linger in the linear (ohmic) region during switching transitions.

Fix: Increase the MCU PWM frequency to 20kHz–25kHz. Verify with an oscilloscope that the gate-to-source voltage ($V_{GS}$) hits a flat 10V–12V plateau, not a sloped 5V logic-level signal. If using a 3.3V MCU, you must use the IR2110's level-shifting architecture; direct GPIO connection will fry the driver or leave the FETs half-on.

2. Catastrophic Overheat (Shoot-Through)

Cause: Both the high-side and low-side MOSFETs are conducting simultaneously for a fraction of a microsecond. This creates a dead short across the DC bus, resulting in massive current spikes that vaporize the silicon die.

Fix: You lack sufficient dead-time. MOSFETs turn off slower than they turn on due to the Miller effect. Insert a dead-time of at least 500ns between the LIN and HIN signals. Hardware fix: Place a 10Ω gate resistor in series with a parallel reverse-biased 1N4148 diode on each gate line. This slows turn-on but allows fast turn-off, naturally generating dead-time.

3. Motor Stalls Under Heavy Load

Cause: Voltage sag on the DC bus. When the motor demands high current, the parasitic inductance of the power supply wires prevents instantaneous current delivery, dropping the bus voltage below the threshold needed to maintain torque.

Fix: Add local bulk energy storage. Solder a low-ESR electrolytic capacitor (e.g., 2200µF, 50V) and a 100nF ceramic bypass capacitor directly across the high-side drain and low-side source pins, keeping the leads as short as physically possible.

The Decision Path: Topology Selection

Do not default to a half-bridge if your mechanical load requires features it cannot provide. Use this decision tree to lock in your drive topology.

Final Verdict

If your application involves a brushed DC motor that only needs to spin in one direction—such as a treadmill belt, a water pump, or a unidirectional winch—the half-bridge is the most efficient, cost-effective topology available. It cuts your silicon count in half compared to an H-bridge while retaining the critical ability to actively brake the load. For 90% of unidirectional DC DIY and industrial prototype loads between 12V and 24V, the IR2110 paired with two IRFB3207 N-channel MOSFETs is the definitive, mathematically sound default. Build your gate drive logic around it, respect the 500ns dead-time, and your drive will outlast the motor itself.