Battery Charging Profile-Based Parameter Design of a 6.78-MHz Class $E^2$ Wireless Charging System

Abstract

Wireless power transfer (WPT) working at several megahertz (MHz), 6.78 or 13.56 MHz, is widely considered to be a promising candidate for charging electronic devices. The so-called Class $E^2$ converters combining the soft-switching-based Class $E$ power amplifier (PA) and Class $E$ rectifier are known to be suitable for high-frequency applications with improved efficiency. However, the charging of batteries usually need to follow a specific profile, in which battery voltage and charging current vary over time. The input reactance of the Class $E$ rectifier also becomes obvious at megahertz. This nonneglectable and varying reactance significantly lowers system efficiency and complicates parameter design. In this paper, a systematic design approach is developed that minimizes the energy loss of a 6.78-MHz Class $E^2$ wireless charging system during the entire battery charging cycle. A $LC$ matching network is added to improve the loading conditions of the Class $E$ PA and coupling coils, and provides new degrees of freedom in the parameter design. Average power loss is defined based on analytically derived system efficiency and a discretized battery charging profile. It serves as an objective function that is minimized through the proposed battery charging profile-based parameter design. In final experiments, the proposed design achieves a 24.5% reduction of the average power loss when comparing with that through the conventional design.