Cascoded GaN Half-Bridge with 17 MHz Wide-Band Galvanically Isolated Current Sensor

A cascoded GaN half-bridge with a wide-band galvanically isolated current sensor is proposed. A 650 V depletion-mode GaN field-effect transistor is switched by a low-propagation-delay gate driver in active-mode. The standby and active modes are switched by a 25 V N-ch laterally diffused MOS (LDMOS). The current sensor uses the LDMOS as a shunt resistor, gm-cell-based sense amplifier and a mixer-based isolation amplifier for wider bandwidth. PVT variations of on-resistance of the current-detecting MOSFET are compensated using a reference MOSFET. A digital calibration loop across the isolation is formed to keep the current sensor gain constant within ±1.5 % across the whole temperature range. The wide-band current sensor can measure the power device switching current. In this study, a cascoded GaN half-bridge switching and inductor current sensing using low-side and high-side device current are demonstrated. The proposed techniques show the possibility of implementing a GaN half-bridge module with an isolated current sensor in a package.