How High-Side and Low-Side Switches Work in a Half-Bridge

This article explains how high-side and low-side switches alternate conduction in a half-bridge inverter stage within motor-drive electronics. A half-bridge converts direct current (DC) power into modulated alternating current (AC) waveforms to drive electric motors efficiently. By alternating the on and off states of these two switches using Pulse Width Modulation (PWM), the circuit controls the direction and magnitude of the current flowing into the motor windings. We will cover the core switching states, the necessity of dead time to prevent catastrophic short circuits, the role of freewheeling diodes in handling inductive loads, and the floating gate drive requirements for the high-side switch.

The Basic Half-Bridge Topology

A half-bridge inverter stage consists of two power switches—typically MOSFETs or IGBTs—connected in series across a high-voltage DC bus.

  • The High-Side Switch connects the positive DC supply rail to the output phase node.
  • The Low-Side Switch connects the output phase node to the negative DC return rail (ground).

The motor winding is connected directly to the output phase node between the two switches. By alternating which switch conducts, the phase node is rapidly toggled between the positive voltage rail and ground.

Alternating Conduction via PWM

Motor speed and torque are controlled by a microcontroller or digital signal processor (DSP) that generates Pulse Width Modulation (PWM) signals. These signals dictate the exact timing of the alternation between the switches.

  1. High-Side Conduction: The gate driver turns the high-side switch ON while keeping the low-side switch OFF. The phase node is pulled up to the DC bus voltage, and current flows from the positive power supply, through the high-side switch, and into the motor winding.
  2. Low-Side Conduction: The gate driver turns the high-side switch OFF and turns the low-side switch ON. The phase node is pulled down to ground. Depending on the motor's operating state, current is either drawn from the motor winding to ground or circulates through the low-side path.

By varying the duty cycle (the percentage of time the high-side switch is on versus the low-side switch), the half-bridge synthesizes a variable average voltage, which shapes the sinusoidal current required by the motor.

Dead Time: Preventing Shoot-Through

Power semiconductors do not turn on and off instantaneously; it takes nanoseconds or microseconds for the internal charge to dissipate. If the high-side and low-side switches conduct simultaneously, even for a fraction of a microsecond, they create a direct short circuit across the DC bus. This event is called "shoot-through" and quickly destroys the switches due to massive current spikes.

To prevent shoot-through, motor-drive controllers insert dead time (also known as blanking time) into the switching cycle. During a transition:

  1. The conducting switch is commanded to turn OFF.
  2. Both switches are held in the OFF state for a brief, predetermined dead time.
  3. The opposing switch is commanded to turn ON.

Freewheeling Diodes and Inductive Currents

Motor windings are large inductors. By nature, inductors resist instantaneous changes in current. When a switch turns off and the circuit enters the dead-time phase, the current flowing through the motor winding cannot simply stop. It must find an immediate path to continue flowing, or it will generate destructive voltage spikes.

This path is provided by anti-parallel freewheeling diodes (or the intrinsic body diodes in MOSFETs) situated across both the high-side and low-side switches.

  • If current is flowing into the motor when the high-side switch turns off, the voltage at the phase node drops below ground until the low-side diode becomes forward-biased and conducts the current during the dead time.
  • Once the dead time expires, the low-side switch turns on, taking over the current from its parallel diode to reduce conduction losses (a technique known as synchronous rectification).

Driving the High-Side Switch

While the low-side switch is referenced to ground—making it easy to turn on by applying a standard gate voltage—the high-side switch is referenced to the switching phase node. Because the voltage at this node rapidly jumps between ground and the high-voltage DC bus, the gate driver must apply a voltage higher than the DC bus to keep the high-side switch conducting.

This is typically achieved using a bootstrap circuit. While the low-side switch is conducting and the phase node is at ground, a bootstrap capacitor charges up via an auxiliary low-voltage supply. When the low-side switch turns off and the high-side switch needs to turn on, the capacitor acts as a floating power supply. It rides on top of the rising phase node voltage, providing the necessary gate-to-source voltage to keep the high-side switch fully conducting until the next alternation cycle begins.