A bootstrap circuit is used for the gate drive of the high-side (upper-leg) devices of a bridge circuit.
Generally, the gate of N-channel MOSFETs or IGBTs is driven at a voltage 10 to 15 V higher than the source voltage or emitter voltage. When a high-side N-channel device turns on, the source (or emitter) voltage becomes equal to that of the high-voltage power supply (VBB). Therefore, for the gate drive of the high-side device, a power supply with very high voltage, which is equal to the sum of VBB and the gate-source (gate-emitter) voltage, is required. The following shows a bootstrap circuit. When the low-side (lower-leg) device turns on, the bootstrap capacitor (C) stores charge, which is used to drive the gate of the high-side device.