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Planar contactless battery charging platform is an emerging technology that has the potential of unifying the charging protocols of portable consumer electronic products. In this paper, a new hybrid structure which consists of a coil and a spiral winding is proposed for improving the uniform magnetic field distribution over the charging surface. An analysis into an optimal design of the number of turns and the dimension of the spiral winding is presented for a given concentrated coil. The uniform magnetic field distribution of the designed prototype is measured by an EMC scanner and by an energy-receiving coil. Based on circuit modeling and analysis, the inverter circuit topology and particularly the resonant compensation tank is designed for maximizing power transfer for multi-load applications. A design procedure is proposed and verified by the experiments. An efficiency of about 80% has been achieved for the coupling structure itself when four loads are charged on the platform simultaneously
Liu et al. (Sun,) studied this question.