A wireless charging system for electric vehicles has two parts which are located inside and outside the vehicle respectively, and energy is transmitted from the outside part to inside part through a loosely coupled transformer. The energy transmission efficiency is directly related to the power conversion efficiency of the entire wireless charging system. This paper aims to improve the transmission efficiency of the DC transformer of the wireless charging system through studying compensation design method of DC transformer. A dual-tap rectifier is applied at the secondary side of the transformer, and a capacitor is connected in series on the primary side. Two capacitors are connected in series on the secondary side. By quantitative analysis on DC transformer efficiency, the relationship among efficiency, switching frequency and compensation parameter is obtained. The compensated DC transformer realizes soft switch and further improves transformer efficiency. Finally, simulation and experiment on the wireless charging system with magnetic induction are conducted to verify the improved transformer design. The simulated and experimental results show that the average compensated DC transformer efficiency has been improved by 1.248%. Thus the designed DC transformer can effectively improve the energy transmission efficiency, and reduce voltage stress of the power device.
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