The Effect of Relay Coil in Four-coil Wireless Power Transfer System

Wireless power transfer (WPT) system based on magnetic resonant coupling can realize the mid-range wireless power transfer, but the efficiency will decrease rapidly with the transfer distance increasing to the long-range. In this letter, a relay coil is utilized to improve the performance of WPT system over a relative long distance. The circuit models of WPT system with or without relay coil are built and the scattering parameter is used to evaluate the transfer performance. In four-coil system, the frequency splitting is analyzed and the critical coefficient is given. In five-coil system, the positions of relay coil under different distances are investigated and using FEM software the model is corrected. Then, the design constraint of relay coil is derived. Based on the parameters used in model analysis, the simulations and experiments are carried out and the results confirm the model analysis, which indicate that the relay coil can effectively extend the transfer distance with a relative high efficiency or increase the transfer efficiency at a given distance