Analysis and Design of a Wireless Power Transfer System With an Intermediate Coil for High Efficiency

TLDR

In two‑coil wireless power transfer, a low coupling coefficient limits efficiency, but inserting an intermediate coil near its resonance boosts apparent inductance and compensates the coupling, thereby improving efficiency and inducing a bifurcation phenomenon. The study aims to develop and validate an optimal design method for a high‑efficiency wireless power transfer system employing an intermediate coil operating at its second resonance frequency. The authors derive a dc voltage transfer‑function expression, design a system operating at the intermediate coil’s second resonance, and build a 100 kHz prototype with a 200 mm air gap to test the theory. An overall system efficiency of 95.57 % was achieved at 6.6 kW output power.

Abstract

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 apparent self-inductance and magnetizing inductance of the primary side at around the resonance frequency of the intermediate coil, so that the apparent coupling coefficient is compensated. The coupling coefficient makes the system efficiency increase and induces bifurcation phenomenon. From the analysis, this paper proposes an optimal design method using the second resonance frequency operation with the bifurcation 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 200 mm. An overall system efficiency of 95.57% has been achieved at 6.6 kW of output power.