How Bootstrap Circuits Drive High-Side MOSFET Gates

This article explains how a bootstrap diode and capacitor network generates a floating gate drive voltage above the high-side source rail in switching electronics. By examining the charging phase, the voltage-boosting phase, and the critical component limitations, you will understand how this simple circuit powers high-side transistors without requiring an expensive, isolated power supply.

The High-Side Driving Challenge

In half-bridge and full-bridge switching topologies, N-channel MOSFETs or IGBTs are preferred for both the low-side and high-side switches due to their low on-resistance and high efficiency. To turn on an N-channel MOSFET, the gate voltage must be driven higher than the source voltage by a specific threshold (typically 10V to 15V).

For the low-side switch, the source is connected to ground, making it easy to drive the gate using a standard ground-referenced power supply (\(V_{CC}\)). However, the source of the high-side switch is connected to the switching node (the connection point between the two transistors). When the high-side switch is fully turned on, its source voltage rises to match the high-voltage DC bus (\(V_{BUS}\)). Therefore, to keep the high-side switch on, the gate driver must generate a voltage that is 10V to 15V higher than the main high-voltage rail.

The Bootstrap Network Components

A bootstrap circuit solves this problem using two primary passive components connected to a gate driver IC:

  1. Bootstrap Capacitor (\(C_{BOOT}\)): Connects between the switching node (high-side source) and the floating power supply pin (\(V_B\)) of the high-side gate driver.
  2. Bootstrap Diode (\(D_{BOOT}\)): Connects between the low-voltage gate drive supply (\(V_{CC}\)) and the positive terminal of the bootstrap capacitor.

Phase 1: Charging the Bootstrap Capacitor

The bootstrap process relies on the alternating switching states of the bridge. The charging phase occurs when the high-side switch is off and the low-side switch is turned on.

  1. When the low-side switch conducts, it pulls the switching node down to ground (0V).
  2. Because the negative terminal of the bootstrap capacitor is connected to the switching node, it is also pulled to ground.
  3. Current flows from the low-voltage \(V_{CC}\) supply (e.g., 12V), through the forward-biased bootstrap diode, and into the bootstrap capacitor.
  4. The capacitor rapidly charges up to approximately \(V_{CC}\) (minus the forward voltage drop of the diode).

Phase 2: Generating the Floating Voltage

The voltage-boosting phase occurs when the low-side switch turns off and the high-side switch is commanded to turn on.

  1. The high-side gate driver connects the charged bootstrap capacitor directly between the high-side transistor's gate and source.
  2. As the high-side switch begins to turn on, the voltage at the switching node rapidly rises from 0V toward the high-voltage rail (\(V_{BUS}\)).
  3. Because the voltage across a capacitor cannot change instantaneously, the bootstrap capacitor maintains its 12V charge. As the negative terminal of the capacitor rises to \(V_{BUS}\), the positive terminal is pushed up to \(V_{BUS} + 12\text{V}\).
  4. The voltage at the positive terminal of the capacitor is now higher than the \(V_{CC}\) supply. This reverse-biases the bootstrap diode, which automatically blocks the high voltage from flowing backward into the low-voltage \(V_{CC}\) supply.
  5. The high-side gate driver is now powered by a "floating" 12V supply (the capacitor) referenced directly to the high-side source rail, keeping the transistor fully enhanced and conducting.

Operational Limitations

While efficient and cost-effective, the bootstrap network has one major limitation: it cannot maintain a 100% duty cycle for the high-side switch.

While the high-side switch is on, the bootstrap capacitor slowly discharges due to gate charge requirements and internal leakage currents within the driver IC. To prevent the capacitor voltage from dropping too low (which would cause the MOSFET to enter a resistive, high-heat state or trigger an under-voltage lockout), the high-side switch must periodically turn off, and the low-side switch must turn on to refresh the charge in the capacitor.

For optimal performance, engineers use ultra-fast recovery or Schottky diodes for \(D_{BOOT}\) to prevent charge from leaking back into the \(V_{CC}\) supply during the rapid transition from low to high states, and low-ESR ceramic capacitors for \(C_{BOOT}\) to deliver fast, high-current pulses to the MOSFET gate.