A 99%‐efficiency GaN converter for 6.78 MHz magnetic resonant wireless power transfer system

The authors developed a high‐efficiency gallium‐nitride (GaN) Class‐E converter for a 6.78 MHz magnetic resonant wireless power transfer system. A negative‐bias gate driver circuit made it possible to use a depletion mode GaN high‐electron‐mobility transistor (HEMT), and simplified the converter circuit. As the depletion mode GaN HEMT with very small gate–source capacitance provided almost ideal zero‐voltage switching, the authors attained a drain efficiency of 98.8% and a total efficiency of 97.7%, including power consumption of a gate driver circuit, at a power output of 33 W. In addition, the authors demonstrated a 6.78 MHz magnetic resonant wireless power transfer system that consisted of the GaN Class‐E converter, a pair of magnetic resonant coils 150 mm in diameter with an air‐gap distance of 40 mm, and a full‐bridge rectifier using Si Schottky barrier diodes. The system achieved a dc–dc efficiency of 82.8% at a power output of 25 W. The efficiencies of coil coupling and the rectifier were estimated to be ∼ 94 and 90%, respectively.