Wireless power transfer system with an asymmetric 4-coil resonator for electric vehicle battery chagers

This paper proposes a high efficiency wireless power transfer system with an asymmetric 4-coil resonator. It presents a theoretical analysis, an optimal design method, and experimental results. Multi-coil systems which have more than three coils between the primary and secondary side provide the benefits of a good coupling coefficient, a long transfer distance, and a wide operating frequency range. The conventional 4-coil system has a symmetric coil configuration. In the primary side, there are source and transmitter coils, and the secondary side contains receiver and load coils. On the other hand, in the proposed asymmetric 4-coil system, the primary side consists of a source coil and two transmitter coils which are called intermediate coils, and in the secondary side, a load coil serves as a receiver coil. In the primary side, two intermediate coils boost the apparent coupling coefficient at around the operating frequency. Because of this double boosting effect, the system with an asymmetric 4-coil resonator has a higher efficiency than the conventional symmetric 4-coil system. A prototype of the proposed system with the asymmetric 4-coil resonator is implemented and experimented on to verify the validity of the proposed system. The prototype operates at 90 kHz of switching frequency and has 200 mm of the power transmission distance between the primary side and the secondary side. An AC-DC overall system efficiency of 96.56% has been achieved at 3.3 kW of output power.