A 3.5 kW wireless charger for electric vehicles with ultra high efficiency

In this paper, the design and hardware realization of a 3.5kW inductive power transfer system to charge electric vehicles is presented. The commercial success of such a wireless charger is highly dependent on the energy conversion efficiency. The presented design consists of a circular receiver coil mounted on a car and a larger circular transmitter coil fixed to the ground. It achieves an efficiency as high as 96.5% from 400 V_DC to 400 V_DC. This figure includes the necessary power electronics circuits on the transmitter and receiver side and is achieved over an air gap of 110mm between those coils. Even with a lateral misalignment of 100mm between transmitter and receiver and an increased air gap of 160mm the efficiency was determined to be 94.3%. Due to the limited space available on the vehicle the dimensions of the receiver coil are a mere 240×240 mm, which results in a coupling coefficient of less than 10 %. With these results the efficiency is no longer inferior to a wire connected charging system. Instrumental to achieve this performance is a careful optimization of the transmitter and receiver geometry and the proper tuning of the electric part of the circuit. Both aspects are discussed in detail in this paper.