This paper presents a bootstrap driver that can operate a normally-on, a depletion-mode, GaN FET power device as a normally-off device. The bootstrap driver based on a diode-clamp circuit which is composed of a capacitor and a diode, shifting the input voltage by a negative amount (-10V) to turn off the power switch with a negative threshold (-8V). However, due to the clamp capacitor discharge by the reverse leakage current of the clamp diode, the depletion-mode GaN FET cannot remain off-state. To address this problem, the proposed bootstrap driver employs a capacitor auto-recharging circuit for compensating the amount of the leakage charge of the clamp capacitor. This capacitor auto-recharging circuit senses the gate voltage of GaN FET and recharges the clamp capacitor if it is partly discharged. The prototype driver is implemented with discrete components, and the bootstrapping and the leakage compensating operations of it are demonstrated with a boost converter including a customized 60mm channel depletion-mode GaN FET. The boost converter delivers 17-W power at 36-V output with 1-MHz switching frequency while the proposed bootstrap driver consumes 15.5mW for leakage-compensation.
