Design and voltage regulation of inductively coupled wireless power transfer circuits with an intermediate coil

An inductively coupled wireless power transfer (WPT) circuit consisting of the transmitter, the receiver, and an additional intermediate coil is studied. It is assumed that the intermediate coil is located coplanar with the transmitter coil. Although it has been known that the intermediate coil serves as a power repeater, its effect on the three-coil WPT circuit characteristics has not been thoroughly investigated. Accordingly, this study analyses how the intermediate coil affects the voltage transfer function and the input impedance of the WPT circuit. Based on the analysis result, design considerations are presented and output voltage regulation via operation frequency adjustment is discussed. A design example for the three-coil WPT circuit is presented and the simulation and experimental results are provided.