There is no such thing as a 'half bridge motor.' When makers and engineers search for this term, they are conflating the electromechanical load (the motor) with the silicon drive topology (the half-bridge circuit). A half-bridge is a specific arrangement of two power switches—typically MOSFETs or IGBTs—used to control current flow through a load. Understanding this distinction is the first step to correctly sizing your driver, selecting the right motor for your load profile, and avoiding melted silicon on your workbench.
This guide cuts through the terminology confusion, provides a concrete framework for matching motor types to driver topologies, and gives you exact sizing math and part numbers for your next build.
The Topology Reality: Half-Bridge vs. Full-Bridge
A half-bridge consists of a high-side switch and a low-side switch. The load (motor) is connected between the midpoint (the switch node) and either ground or the supply rail. This topology allows you to pulse-width modulate (PWM) the voltage to control speed, or use the low-side switch for regenerative braking. However, a single half-bridge can only drive current in one direction. If you need to reverse a DC motor, you must use a full-bridge (H-bridge), which essentially places two half-bridges in parallel across the motor, allowing current to flow in either direction.
Motor Type Comparison: Matching Load to Driver
Selecting the right driver starts with understanding the torque curve and control demands of your motor. Treating a stepper and a servo as interchangeable is a fast track to a failed project; their control architectures are fundamentally different. Steppers rely on open-loop phase sequencing (driven by half-bridge arrays), while servos require closed-loop feedback and typically use three-phase BLDC inverters.
| Motor Type | Torque Curve | Control Needs | Cost (10-50W range) | Required Driver Topology |
|---|---|---|---|---|
| Brushed DC | Max torque at stall, drops linearly with speed. | Simple voltage/PWM. Open-loop. | $5 - $15 | Half-Bridge (unidirectional) or H-Bridge (bidirectional) |
| BLDC (Brushless) | Flat torque curve up to base speed, high efficiency. | 3-phase commutation, requires Hall sensors or sensorless back-EMF tracking. | $20 - $60 | 3x Half-Bridges (3-phase inverter) |
| Stepper | High holding torque, drops sharply at high RPM. | Precise pulse/direction stepping, microstepping current regulation. | $15 - $40 | 4x Half-Bridges (Dual H-Bridge with current chopping) |
| AC Induction | Low starting torque, peaks near synchronous speed. | VFD (Variable Frequency Drive) with complex vector control. | $50 - $150+ | 3x Half-Bridges (High voltage IGBT modules) |
Half-Bridge Wiring, Terminals, and Sizing Rules
When wiring a discrete half-bridge or evaluating a half-bridge controller IC, you must correctly identify the terminals. Miswiring the switch node to ground will instantly short your power supply and destroy the high-side MOSFET.
Standard Terminal Identification
- VM / VCC: Main motor power supply input (e.g., 12V or 24V).
- OUT / SW: The switch node. This connects directly to one terminal of the motor. The voltage here swings between GND and VM at the PWM frequency.
- GND / PGND: Power ground. The other motor terminal connects here (often through a current-sense shunt resistor).
- IN / PWM: Logic-level input from your microcontroller (3.3V or 5V).
- VCP / Bootstrap: Charge pump capacitor pin. Essential for driving the gate of the high-side N-channel MOSFET above the VM rail voltage.
The Sizing Rule of Thumb
Never size a motor driver based on the motor's 'running' or 'nominal' current. Motors draw maximum current when they are stalled or starting from a dead stop. If your driver cannot handle the stall current, it will trigger thermal shutdown or suffer catastrophic junction failure.
I_driver_continuous >= I_motor_stall * 1.25If the stall current is not on the datasheet, measure it by locking the rotor and applying nominal voltage, or use:
I_driver_continuous >= I_motor_running * 3.0
Worked Load Example: Automotive Wiper Motor
Let's size a half-bridge for a standard 12V automotive wiper motor used in a DIY robotics chassis.
- Nominal Voltage: 12V DC
- Running Current: 4.5A (measured on the bench with no load)
- Stall Current: 18A (measured by locking the output shaft)
Applying the rule: 18A * 1.25 = 22.5A. We need a half-bridge driver rated for at least 22.5A continuous current at our operating temperature. A 15A driver will overheat and fail during the high-inertia startup phase. We select the TI DRV8701 half-bridge controller, which uses external N-channel MOSFETs. By pairing it with two IRFB4110 MOSFETs (rated at 100V, 120A, with an R_DS(on) of just 4.5mΩ), we easily clear the 22.5A thermal threshold with minimal heatsinking required.
Decision Tree: Pick Your Motor and Driver Topology
Stop guessing. Follow this decision path to lock in your exact hardware stack for your next actuator project.
| If your application requires... | Then select this Motor Type... | And this Driver Topology... | Concrete Part Pick (2026) |
|---|---|---|---|
| Unidirectional speed control of a high-torque, high-current load (>10A). | Brushed DC | Half-Bridge (External FETs) | TI DRV8701 + IRFB4110 FETs |
| Bidirectional control (forward/reverse) for a winch or linear actuator (<15A). | Brushed DC | Integrated H-Bridge | TI DRV8871 |
| Precise open-loop positioning, holding torque at zero speed, 3D printer axes. | Bipolar Stepper | Dual H-Bridge (Multi-Half-Bridge) with StealthChop | Trinamic TMC2209 |
| High RPM, high efficiency, dynamic braking for drones or RC vehicles. | BLDC (Outrunner) | 3-Phase Half-Bridge Array (ESC) | BLHeli_32 ESC (30A+ rating) |
Default Recommendation: If you are building a general-purpose DIY rover or robotic arm and need simple, robust, unidirectional drive for heavy loads, default to a Brushed DC motor driven by a TI DRV8701 half-bridge. It provides the best balance of cost, thermal headroom, and simplicity without the complexity of 3-phase commutation.
Failure Signatures: Diagnosing Hum, Overheat, and Stall
When your motor drive fails, the symptoms tell you exactly what went wrong on the bench. Do not just swap parts; read the failure signature.
1. The Audible Hum or Whine
Symptom: The motor emits a high-pitched whine or low hum, and the MOSFETs run warm even at low loads.
Cause: Your PWM frequency is set too low (typically between 50Hz and 4kHz), falling within the audible spectrum and causing the motor windings and ceramic capacitors to vibrate physically (piezoelectric effect). Alternatively, it indicates mechanical cogging in a stepper.
Fix: Increase your microcontroller's PWM timer frequency to 20kHz or higher. This pushes the switching noise above human hearing and smooths the current ripple in the motor inductance.
2. Catastrophic Overheat and Magic Smoke
Symptom: The driver IC or discrete MOSFET gets too hot to touch within seconds, or physically cracks and vents smoke.
Cause: Inductive kickback. A motor is a massive inductor. When the half-bridge switches off, the collapsing magnetic field generates a massive reverse voltage spike (V = L * di/dt). If you omitted the flyback diode (or if the IC's internal body diodes are too slow/undersized), this spike exceeds the MOSFET's V_DS avalanche rating, punching through the silicon junction.
Fix: Always place a fast-recovery Schottky diode (e.g., MBR2045CT) across the motor terminals, with the cathode pointing toward the positive supply. If using an integrated driver, verify the datasheet confirms internal clamp diodes are rated for your specific stall current.
3. Stall and Thermal Foldback
Symptom: The motor stops turning under load, the driver IC flags a fault (nFAULT pin goes low), and the motor refuses to restart until power is cycled.
Cause: The mechanical load exceeded the motor's breakdown torque, causing a stall. The current spiked to the stall limit, triggering the driver's overcurrent protection (OCP) or thermal shutdown (TSD).
Fix: Check your mechanical binding first. If the load is genuinely that high, you have undersized the motor, not the driver. If you must push through high-inertia stalls, implement a soft-start ramp in your firmware to limit the inrush current, or add a physical current-limiting sense resistor to the low-side ground path to gracefully fold back the PWM duty cycle before the thermal threshold is hit.
For deeper reference on bridge architectures and protection circuits, consult the TI Motor Drivers Overview and application notes for the Trinamic TMC2209 Motion Controller. Getting the topology right on paper saves you from burning through expensive prototype boards on the bench.
