2 Basic Operation of Bootstrap Circuit

A bootstrap circuit is used in half-bridge configurations to supply bias to the high-side FET. Figure 2-1 shows the charging path of a bootstrap circuit in a simplified half-bridge configuration using UCC27710, TI's 620V half-bridge driver with interlock. When the low-side FET is on (high-side FET is off), the HS pin and the switch node are pulled to ground; the VDD bias supply, through the bypass capacitor, charges the bootstrap capacitor through the bootstrap diode and resistor.

Figure 2-1 Bootstrap Charging Path

When the low-side FET is turned off and the high-side is on, the HS pin of the gate driver and the switch node are pulled to the high voltage bus HV; the bootstrap capacitor discharges some of the stored voltage (accumulated during the charging sequence) to the high-side FET through the HO and HS pins of the gate driver as shown in Figure 2-2.

Figure 2-2 Bootstrap Capacitor Discharging Path