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Optimized Coil Design for Electric Vehicle Charging With Medium Voltage Input

Trade-off between receiver coil loss and transformer loss factor
Fig.1. Trade-off between receiver coil loss and transformer loss factor
With low maintenance requirements, inductive power transfer (IPT) has been an appealing solution to battery chargers of electric vehicles (EVs). Yet, the large volume for installation and the high-power loss can limit the power level for wireless chargers in EVs. In this paper, an optimization method to minimize the receiver coil size of a 50 kW IPT system with medium voltage (MV) input is proposed, which improves the power density of the receiver side of the IPT system and makes it possible to better integrate the receiver coil into the EVs. Normally, the IPT transformer, which includes the receiver coil and transmitter coil, is optimized to realize the highest power transfer efficiency. In the optimization process, the power loss of the transformer is calculated with the quality factor of the receiver coil (Q2) and transmitter coil (Q1) as well as the receiver side considering the equivalent load resistance (QL). An optimal QL to obtain the minimum coil efficiency with Q1 and Q2 fixed is derived which will determine the optimal inductance value of the receiver coil. A desired system efficiency will then determine the quality factor of the two coils, which leads to the final physical realization of the coils. Meanwhile, in the optimization examined in this paper, the trade-off between the receiver coil loss and transformer loss factor is considered and shown with pareto front. As Fig. 1 shows, the transformer loss factor decreases as the receiver coil loss decreases. Therefore, an optimal design point could be found so that the reduced receiver coil loss could satisfy the reduced receiver coil radius, while sacrificing the transformer loss a bit. Further investigation also shows that increasing the transmitter size will continue to reduce the receiver coil loss with the same transformer efficiency. Finally, the results further show that with a larger transmitter coil size, the smaller receiver coil acquires a similar loss reduction effect for both the coil and the transformer when compared with identical coil size geometry.
Receiver coil loss v.s. transmitter coil radius
Fig.2. Receiver coil loss decreases with increased transmitter coil radius