Abstract
This paper presents a coordinated voltage-frequency control (CVFC) method for inductive battery charging systems that ensures full-range output power control at high efficiencies over large variations in coupling conditions. The method automatically switches between sub-resonant frequency control (SRFC) and voltage control at the resonant frequency (VC-ω0) based on changes in the coupling conditions and power flow, without depending on estimation of the coupling coefficient. The design basis for the method is the selection of an appropriate nominal coupling that will minimize the maximum resonant capacitor voltage and maintain high system efficiency. The coordinated control strategy allows for minimum component stress and high efficiency over the entire operating region. Moreover, pulse density modulation is employed to achieve soft switching during the VC-ω0 mode. A comparison with pure VC-ω0 and pure SRFC is presented to highlight the advantages of CVFC in terms of maintaining a higher efficiency and a wider controllable power range during large variations in coupling conditions. The effectiveness and feasibility of the proposed method are validated by simulations and laboratory measurements from a small-scale experimental prototype. With a peak efficiency exceeding 96.9%, the proposed control strategy maintains consistently high efficiency, surpassing 95.5% across the majority of its operating range.
