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

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 <inline-formula><tex-math notation="LaTeX"> $E^2$</tex-math></inline-formula> converters combining the soft-switching-based Class <inline-formula> <tex-math notation="LaTeX">$E$</tex-math></inline-formula> power amplifier (PA) and Class <inline-formula> <tex-math notation="LaTeX">$E$</tex-math></inline-formula> 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 <inline-formula> <tex-math notation="LaTeX">$E$</tex-math></inline-formula> 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 <inline-formula> <tex-math notation="LaTeX">$E^2$</tex-math></inline-formula> wireless charging system during the entire battery charging cycle. A <inline-formula><tex-math notation="LaTeX">$LC$</tex-math></inline-formula> matching network is added to improve the loading conditions of the Class <inline-formula><tex-math notation="LaTeX">$E$</tex-math> </inline-formula> 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.

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