Applying normally-off GaN HEMTs for coreless high-frequency wireless chargers

Wide-bandgap (WBG) devices such as Gallium-Nitride (GaN) High Electron Mobility Transistors (HEMTs) have become popular in the power electronics industry as they offer a lower switching loss, higher thermal capability and higher power density than conventional silicon devices. As an attempt of applying WBG devices to the wireless charging technology, this paper adopts two different types of normally-off GaN HEMTs. One adopts the cascode structure provided by Transphorm Inc, operated under 800kHz to charge a battery pack on an electric scooter at 48 V/500W, with the air gap between the transceiver and receiver of ~10cm. The other is enhancement-mode GaN HEMTs provided by GaN Systems Inc, operated at ~6MHz to use one transceiver to charge multiple cell phones @~20W. Both of these chargers have no magnetic cores to reduce the cost and weight. Experimental results show both types of GaN HEMTs significantly increased the charging efficiency over conventional Si devices. Challenges of applying such fast-transition devices are discussed, e.g., common-source inductance and the gate-drive-loop parasitic.