Tunable Coil-Links for Multiple-Receiver Wireless Power Transfer System With Arbitrary Position and Power Division

This study demonstrates tunable coil links for a wireless power transfer system with arbitrarily-located multiple receivers and arbitrary power division between them. Both the Tx and Rx coils are composed of a main coil and an array of assistant coils loaded with varactors. By tuning the varactors to control the resonant characteristics of each assistant coil, the Tx coil can steer the near field, and the Rx coil can reconfigure its electrical size. Consequently, the proposed system maintains the high overall efficiency regardless of the position of the receivers and the power division ratio. For the verification, a two-receiver system at 6.78 MHz is designed and fabricated with a 2 x 1 assistant coil array. A thorough investigation is performed for various cases, which include when the two receivers of the same size are located symmetrically with respect to the center of the Tx coil and two receivers of different sizes, located asymmetrically. For all cases, the overall efficiency is evaluated for various receiver locations and power division ratios when the tunable coil links are adopted on both Tx and Rx coils, only on either Tx or Rx coil. The conventional system provides the minimum transfer efficiency of 0% for symmetric case, as well as 12.8% for asymmetric case with respect to the receiver locations. On the other hand, results of the proposed system show minimum efficiency of 83.7% and 83.5% for symmetric and asymmetric case, respectively. More importantly, while the conventional system showed strong dependence not only on the Rx locations but also the power division ratio, the proposed system maintains the efficiency with little, if any, variation regardless of the two. For example, while the power division ratio is varied from 5:1 to 1:5, the proposed system remains a very high efficiency of up to 91.0% with only 0.3% variation.