Influence of Asymmetric Coil Parameters on the Output Power Characteristics of Wireless Power Transfer Systems and Their Applications

Nowadays, it is a trend to update electronic products by replacing the traditional wire charging with emerging wireless charging. However, other features of the products must generally be left unchanged, which limits the options in receiving coil design. As a result, asymmetric coil designs should be adopted in wireless charging systems. In this paper, a wireless power transfer system with asymmetric transmitting and receiving coils is modelled using circuit theory. The output power of the asymmetric system is analyzed, and the conditions of the maximum power splitting phenomenon are addressed in detail. Cases for different resonant frequency conditions are elaborated. The splitting frequencies and critical coupling coefficient are obtained, which are different and more complicated compared with the symmetric counterparts. Asymmetric coil designs can be adopted based on the proposed method, so that adequate and efficient output power transfer can be realized. Finally, the asymmetric coils design is utilized in an experimental prototype in order to contactlessly charge a portable power tool lithium-ion battery pack with 18 V DC and 56 W output through 220 V AC input, without altering its original configuration, and the correctness of proposed analysis can thus be verified.