Simulation-Assisted Design of a Bidirectional Wireless Power Transfer with Circular Sandwich Coils for E-Bike Sharing System

Abstract

A wireless power transfer (WPT) system with bidirectional power flow control for charging batteries and drawing electric power from batteries is presented in this paper for e-bike sharing applications. The proposed WPT consists of a bidirectional DC-DC converter and a sandwiched coil set, which includes primary winding with ferrite pad in charging stations and a movable secondary coil without ferrite pad, which is inserted into the primary coils to receive or transmit energy, on the e-bike. Hence, the proposed coil set could provide high coupling factor, better tolerance of misalignment, and a compact size that enables easy installation in the current mechanism of bike-sharing systems. The Maxwell 3D is adopted to design coil sets, which are based on the known resistance of Litz wire, and the effect of the ferrite pad and misalignment are analyzed to achieve good performance. To select a suitable converter, the characteristics of predesigned coils are simulated using Maxwell 3D, MATLAB and PSIM. Then, a full-bridge CLLC converter with the same resonant frequency for charging and discharging modes is chosen to provide minimal loss by using the synchronous rectifier on the 48-V side and voltage gain without the loading effect. Finally, a digital signal processor-based digitally controlled CLLC converter is constructed to verify the performance of the proposed WPT, which can provide 200 to 48 V/500 W charging and discharging functions with constant current/constant voltage modes and fast-response current control. In addition, the maximum efficiency is 96% at 52 V and 75% rated load in charging mode.