A half bridge motor driver controls current flow in a single direction, acting as a low-side or high-side switch pair. If your application only requires unidirectional speed control for a DC motor, a solenoid, or a heating element, a single half bridge is the most efficient and cost-effective choice. However, if you need bidirectional control (forward and reverse), you must pair two half bridges together to form a full H-bridge.
Selecting the right topology prevents over-engineering your circuit and avoids catastrophic thermal failures. Below, we break down the exact load profiles, wiring schematics, and sizing mathematics required to implement a half bridge motor driver correctly, alongside the failure signatures that indicate your gate drive or flyback protection is inadequate.
Motor Type Comparison & Load Profile Matching
Not every motor can be driven by a simple half bridge. The driver topology must match the motor's commutation method and torque requirements. While brushed DC motors are the native load for a standard half bridge, attempting to drive a stepper or a 3-phase BLDC with a single half bridge will result in immediate failure. Note that stepper and servo motors are fundamentally different: steppers rely on open-loop phase sequencing, while servos require closed-loop feedback and specialized commutation.
| Motor Type | Torque Curve | Control Needs | Driver Demand | Relative Cost |
|---|---|---|---|---|
| Brushed DC | High starting torque, drops linearly with speed | Simple PWM for speed, single polarity | Single Half Bridge (unidirectional) or Full H-Bridge | Low ($) |
| BLDC (Brushless DC) | Flat torque curve across operating range | 3-phase commutation, rotor position sensing (Hall/FOC) | Three Half Bridges (6 MOSFETs total) | Medium ($$) |
| Stepper | High holding torque, drops sharply at high RPM | Open-loop step/direction pulses, microstepping | Dedicated Stepper Driver (H-bridge per phase) | Medium ($$) |
| Servo (RC/Industrial) | Peak torque at zero speed, highly dynamic | Closed-loop PID, PWM position or absolute encoder | Integrated controller + Full H-Bridge | High ($$$) |
| AC Induction | Low starting torque, peaks near synchronous speed | VFD (Variable Frequency Drive), sine wave synthesis | 3-Phase Inverter (IGBT Half Bridges) | High ($$$) |
Wiring, Terminals, and Sizing the Driver
A discrete or integrated half bridge motor driver typically exposes a specific set of logic and power terminals. Understanding these pins is critical for preventing logic-level mismatches and bootstrap capacitor failures.
Terminal Identification
- VCC / VDD: Logic supply voltage (typically 3.3V or 5V). Powers the internal gate drive logic.
- GND: Logic and power ground reference.
- IN / PWM: Logic input. Receives the microcontroller's PWM signal to dictate duty cycle.
- EN (Enable): Active-high or active-low pin to wake the driver from sleep mode, reducing quiescent current.
- HO / OUT (High-Side): Connects to the positive terminal of the load. Requires a bootstrap circuit if using N-channel MOSFETs.
- LO / OUT (Low-Side): Connects to the negative terminal of the load, switching the return path to ground.
- VS / VM: Main motor power supply input (e.g., 12V, 24V, or 48V).
- VB / BOOT: Bootstrap voltage pin. Requires an external capacitor (typically 100nF to 1µF) to generate a gate voltage higher than VS, ensuring the high-side N-channel MOSFET turns on fully.
Sizing Rule of Thumb & Worked Load Example
Never size a motor driver based on the motor's continuous running current. DC motors draw massive current spikes during startup and mechanical stalling. The sizing rule of thumb for a half bridge motor driver is to select a continuous drain current ($I_D$) rating that is 1.5x to 2.0x the motor's stall current.
Worked Example: You are driving a 24V DC linear actuator for a solar tracker.
- Running Current: 5A
- Stall Current: 22A
- Sizing Calculation: 22A × 1.5 = 33A minimum rating.
If you select a generic 20A driver, the internal MOSFETs will experience thermal runaway during a mechanical bind. Instead, you should select a driver like the Infineon BTS7960 (rated for 43A) or a discrete half bridge built with IRF3205 MOSFETs (rated for 75A). Always verify the thermal resistance ($\theta_{JA}$) on the datasheet; if the calculated power dissipation ($I^2R$) exceeds the silicon junction limits, you must attach an extruded aluminum heatsink to the exposed thermal pad.
Failure Signatures: Hum, Overheat, and Stall Diagnostics
When a half bridge motor driver fails or operates marginally, it exhibits distinct physical and acoustic signatures before catastrophic silicon melting occurs. According to Texas Instruments' motor drive design guidelines, diagnosing these early warnings separates a robust design from a fire hazard.
1. The "Hum" or Whine (Acoustic Noise)
If your motor or the driver board emits an audible high-pitched whine, your PWM frequency is likely set too low (e.g., 500 Hz to 2 kHz). The motor windings act as crude speakers. Fix: Increase the microcontroller's PWM frequency to at least 20 kHz, pushing it above the human hearing range.
Secondary cause: Insufficient gate drive voltage. If the logic VCC is 3.3V but the MOSFET requires a 5V threshold to exit the linear (resistive) region, the MOSFET will act as a variable resistor rather than a closed switch, causing severe ringing and noise.
2. Overheat at Idle or Low Load
If the driver IC is too hot to touch even when the motor is barely loaded, you are likely experiencing shoot-through. Shoot-through occurs when both the high-side and low-side MOSFETs are partially turned on simultaneously during the PWM transition, creating a dead short from VS to GND.
Fix: Introduce "dead time" (a few hundred nanoseconds where both gates are held low) in your microcontroller code or rely on the driver IC's internal hardware dead-time insertion. Ensure your bootstrap capacitor is properly sized; a depleted bootstrap cap causes the high-side gate voltage to sag, trapping the MOSFET in the high-resistance linear region.
3. Stall and Sudden Death (Inductive Kickback)
If the motor mechanically stalls, or if you abruptly cut power to a high-inertia load, the collapsing magnetic field generates a massive reverse voltage spike ($V = L \frac{di}{dt}$). If this back-EMF exceeds the MOSFET's $V_{DS}$ breakdown voltage (often 30V or 40V), the silicon avalanches and shorts out permanently.
Fix: Verify your flyback diodes. For a half bridge, you need fast-recovery or Schottky diodes (like the SS34 or MBR2045) placed in inverse parallel across the load and the supply rails. Standard 1N4007 rectifier diodes are too slow to clamp the nanosecond spike and will result in a destroyed driver.
Half Bridge Motor Driver FAQ
Can I use a single half bridge motor driver to reverse a DC motor?
No. A single half bridge can only switch current in one direction (either connecting the load to VCC, or connecting the load to GND). To reverse a DC motor, you must use a full H-bridge topology, which consists of two half bridges arranged in parallel across the load, allowing you to swap the polarity of the voltage applied to the motor terminals.
Why does my half bridge driver get hot even when the motor is off?
If the motor is entirely off (0% PWM duty cycle) but the driver is hot, check two things. First, measure the quiescent current; some older bipolar drivers draw significant idle current. Second, and more likely, your logic signals are floating. If the microcontroller GPIO pins are left in a high-impedance state during boot-up, the driver's logic inputs can pick up EMI, rapidly toggling the gates and causing severe switching losses. Always use pull-down resistors (10kΩ) on the IN and EN pins.
Do I need a bootstrap capacitor for a high-side half bridge driver?
Yes, if you are using N-channel MOSFETs for the high-side switch. An N-channel MOSFET requires a gate-to-source voltage ($V_{GS}$) of 10V to 12V to turn on fully. When the high-side MOSFET turns on, its source pin rises to the motor supply voltage (VS). To keep the gate 10V above the source, the gate must be driven to VS + 10V. The bootstrap capacitor pumps this extra voltage from the low-side switching cycle. Without it, the high-side MOSFET will only turn on partially, leading to immediate thermal destruction.
What is the difference between a half bridge driver and a relay for motor control?
A mechanical relay is a simple binary switch; it can only turn a motor fully on or fully off. A half bridge motor driver uses solid-state MOSFETs and accepts a PWM (Pulse Width Modulation) signal, allowing you to precisely control the motor's speed and implement soft-start ramps. Furthermore, a half bridge can switch thousands of times per second without mechanical wear, whereas a relay will suffer from contact arcing and pitting if subjected to high-frequency PWM.
