Null-Free Wireless Power Transfer System Based on a Coil Array to Compensate for 2-D Misalignment

In this study, a coil array technology is proposed to compensate for 2-D misalignment between coils, regardless of its direction, in a wireless power transfer (WPT) system at 6.78 MHz. The primary coil is printed on one side of the substrate, while the assistant coil is printed on the other side in a 2 x 2 array configuration. Depending on the degree of misalignment, the coil pair with the best alignment can be switched on. Thus, high efficiency can be maintained without increasing the size of the coils. Experimental results for a 47 x 47 mm(2)-sized coil system demonstrate null-free operation that maintains transfer efficiency of more than 80% with diagonal misalignment of up to 98% relative to the largest coil dimension. Regardless of the direction of the misalignment, its efficiency is 80% or higher with up to 72% misalignment, and 50.5% or higher with extremely high misalignment (up to 120%). The results indicate that the proposed coil array technology successfully addresses the notorious misalignment issue in a near-field WPT system, regardless of the direction of the misalignment, without increasing the coil size. Furthermore, the results for an asymmetric system in which the coil array technology is adopted only on the Tx side reveal the feasibility of the proposed method for practical applications.