Frequency and Voltage Tuning of Series–Series Compensated Wireless Power Transfer System to Sustain Rated Power Under Various Conditions

In a wireless power transfer (WPT) system, voltage tuning and frequency tuning are two major methods to control the output power. For the design of a WPT system at the rated power, it is crucial to understand the characteristics of a WPT system with a constant-power load (CPL) under voltage tuning and frequency tuning. The model of a series–series compensated WPT system with a CPL is established based on the Thevenin equivalent circuits. The conditions to achieve the maximum coil-to-coil efficiency for the two tuning methods are obtained. For frequency tuning, the tuning zone above the resonant frequency is selected because of its wide tuning range and zero-voltage switching. The maximum output power at different operating frequencies is derived, and the required inverter dc voltage and the tuning range for a target output power can be obtained accordingly. With a strong coupling, the maximum efficiency of frequency tuning can be as high as that of voltage tuning; while with a weak coupling, the efficiency of frequency tuning is much smaller than that of voltage tuning. The model with a CPL is helpful for the design and optimization of a WPT system with the rated power.