A High-Reliability Bootstrap Circuit for High-Voltage GaN Half-Bridge Gate Driver

Bootstrap circuits are mainly used for supplying the gate overdrive voltage for the high-side GaN power switch in a half-bridge gate driver. A high-reliability bootstrap circuit, including a high-efficiency charge path, bootstrap capacitor voltage sensing technology, and undervoltage lockout(UVLO) circuit for high-side floating supply voltage, is proposed in this article. By removing the external diode and instead using high voltage LDMOS as the charge transistor, the charge efficiency and charge saturation of the bootstrap capacitor can be improved. On the other hand, with directly sensing bootstrap capacitor voltage to control the charge path's turn-on or turn-off, the overcharge issue during deadtime can be eliminated. Moreover, UVLO circuit is adopted to prevent the gate driver from a normal operation at low bootstrap capacitor voltage, which is a great contribution for safety and reliability of the system. The circuit is designed in a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.8 \mu \mathrm{m} 600 \mathrm{V}$</tex> high voltage BCD process. The simulation results show that the bootstrap capacitor can achieve full charge without any overcharge issues during each cycle at 500 kHz switching frequency.