Analysis and parameters optimization of a contactless IPT system for EV charger

This paper discusses the characteristics of a series-series compensated inductive power transfer system (IPT) with theoretical analysis and experimental results. To maximize system efficiency, two-stage structure, which includes a PFC stage and a resonant DC-DC stage operating in ZVS region, is proposed. One of the major design challenges in implementation of a practical contactless EV charger is the variation of the coupling condition of the loosely coupled transformer. This combined with the battery charger´s wide range load variation makes parameters design of the resonant DC-DC stage more complex. To optimize the parameter design for all coupling and load conditions, a parameter sweeping method is proposed. The design procedure searches parameters set to minimize the averaged primary current while keeping the voltage stress across the primary capacitor below the preset limit; both the high coupling and low coupling conditions are considered. To validate the analysis, a 4 kW resonant DC-DC prototype was built and tested. At 4 cm gap distance with 0.526 coupling coefficient, an efficiency of 97% for the DC-DC stage was achieved with output power range from 0.8 kW to 4 kW.