General optimal design method for series-series resonant tank in loosely-coupled wireless power transfer applications

Conventionally in wireless power transfer applications, symmetrical resonant tank (SRT) type, which employs an identical physical geometry for transmitter coil (Tx) and receiver coil (Rx), has been popular because it minimizes the leakage flux. However, for portable applications it is desirable to minimize size and weight of Rx, while Tx should provide stronger and more uniform magnetic field. Consequently, asymmetrical resonant tank (ART) type, which employs different physical geometries for Tx and Rx, is quite attractive. Compared to SRT type, ART type exhibits higher complexity in analysis and design optimization, because self-inductances of Tx and Rx can be either equal or largely different. Therefore, derivation of analytical design equations for ART type calls for a general analysis. In this paper, general frequency domain characteristics of series-series (SS) resonant tank are analyzed to derive general analytical equations for determining various quantities of interest. Based on the assumption of loosely-coupled condition, analytical design equations are presented. An optimal design method is then proposed and validated. In this paper, an asymmetrical SS resonant tank is optimized using the proposed design method. It is shown in both simulation and experiment that the optimized resonant tank exhibits approximately equal values for voltage gain and current gain under magnetic coupling variation at a desirably fixed frequency. It is also shown that under this condition circulating current gain is approximately at its absolute minimum, which translates in reduced conduction losses. Therefore, an optimal compromise between power delivery robustness and coil-to-coil efficiency can be achieved.