Analysis and Design of Load-Independent Series Resonant Power Amplifier With Constant Current Output and Its Application for WPT System

This article presents an analysis and design of a load-independent (LI) series resonant (SR) power amplifier with constant current (CC) output, along with its application for an MHz wireless power transfer (WPT) system. A novel inverse Class E power amplifier is introduced, which essentially produces a sinusoidal output current even with a low-
$Q$
SR filter. Besides, the proposed amplifier achieves zero-current switching and CC output simultaneously, regardless of the load resistance. The LI operation is obtained for a specific set of component values, whose design conditions are clarified analytically in this article. The experiment was carried out with a WPT system incorporating the proposed amplifier as a transmitter and the Class D rectifier as a receiver. Although the input reactance of the Class D rectifier changed against dc-load variations due to the parasitic capacitances, the proposed amplifier showed consistent CC operation by using the low-
$Q$
SR filter. Also, the proposed WPT system maintained a low total harmonic distortion of the transmission current over the wide load range, even with the low-
$Q$
output filter. The prototype WPT system with the proposed amplifier achieved 88% power-delivery efficiency with 60 W output power at 3.39 MHz transmission frequency. The experimental results showed the effectiveness of the proposed amplifier.