A Real-Time Calibrated Adaptive-ZVS Class-D Power Amplifier for 6.78 MHz Wireless Power Transfer System

This brief presents a class-D power amplifier (PA) with an adaptive zero-voltage switching (A-ZVS) technique for 6.78 MHz resonant wireless power transfer (R-WPT) system. In R-WPT operation, the loading impedance of a PA can be varied by the process tolerance of the LC resonant components or WPT environments, such as the resonant topology, coupling coefficient, and loading condition of the receiver. In particular, the commonly used series-parallel resonant topology suffers from resonant point deviation of primary LC tank, resulting in hard switching of the PA. Consequently, efficiency degradation and electromagnetic interference can occur under multi-MHz operation. The proposed A-ZVS feedback loop calibrates the equivalent resonant capacitance in real-time to achieve ZVS by adjusting the loading impedance to be slightly inductive. The proposed PA was fully integrated except for one switched capacitor used as the tuning element and fabricated in a TSMC 0.18
$\mu \text{m}$
BCD process. The measurement results demonstrated robust ZVS operation with a peak system efficiency of 52.7 % and an enhanced maximum transmitting power of 107 %.