Optimal design method to achieve both good robustness and efficiency in loosely-coupled wireless charging system employing series-parallel resonant tank with asymmetrical magnetic coupler

Abstract

In wireless charging applications, it is often desirable to minimize the weight and size of the receiver coil (Rx) while the transmitter coil (Tx) may be larger so as to provide a stronger and/or more uniform magnetic field and to accommodate the magnetizing current flowing in the Tx coil. Furthermore, in many applications, the receiver coil geometry is often limited by the dimensions of the movable battery-powered system, whereas the transmitter coil geometry can be larger. Consequently, the need for asymmetrical coupler implementation is increasing. In this paper, detailed analysis of series-parallel (SP) resonant topology of asymmetrically-implemented resonant tank (ART) type is performed in the frequency domain to explain various trade-offs and to provide detailed design criteria. In the analysis, general expressions for voltage gain and current gain are used to derive design-oriented equations that ensure various desirable characteristics in the frequency domain. These equations can be used for optimally designing SP resonant tanks of ART type as well as symmetrically-implemented resonant tank (SRT) type. In this paper, it is shown that using the presented design equations to meet the design criteria allows achieving high robustness in power delivery characteristics under coupling (k) variation with only a small increase in circulating current in a coupler.