Performance Comparison between Three-Coil and Four-Coil Based Wireless Power Transfer

In this paper, we formulate three-coil and four-coil wireless power transfer systems mathematically using an equivalent circuit model, and derive a power transfer efficiency for each system. We fabricate inductive and resonant coils with spiral shape, and verify the accuracy of our analysis via efficiency measurements using a vector network analyzer under a variety of environments. Through the comparison of the performance of the two systems, we also confirm that there is a condition where each system shows better performance, which provides the effective use for the additional resonant coil. For example, the four-coil system achieves higher power transfer efficiency than the three-coil system if the distance between resonant coils is longer or the distance between inductive and resonant coils is shorter. Otherwise, the three-coil system outperforms the four-coil system.