Analysis and design of wireless power transfer system with an intermediate coil for high efficiency

This paper presents a theoretical analysis, an optimal design method and experimental results for a wireless power transfer (WPT) system with an intermediate coil. The analytical expression of the DC voltage transfer function is presented and discussed. In a two coil WPT system, which has low coupling coefficient, the intermediate coil boosts the self-inductance and magnetizing inductance of the primary side at around the resonance frequency of the intermediate coil, so that the coupling coefficient is compensated. The coupling coefficient makes the system efficiency increase and induces bi-furcation phenomenon. From the analysis, this paper proposes an optimal design method using the second resonance frequency operation with the bi-furcation phenomenon and presents design procedure for high efficiency. A prototype of the WPT system with the intermediate coil is implemented and experimented to verify the validity of the analysis and the proposed design method. The prototype operates at 100 kHz switching frequency and has an air gap between primary and secondary side of 200mm. An overall system efficiency of 95.57% has been achieved at 6.6kW of output power.