Free-positioning wireless power transfer using multiple coupling coils in a transmitter

The paper presents an effective impedance matching technique using switchable multiple coupling coils in a transmitter for an effective magnetic resonant wireless power transfer (WPT) system supporting free-positioning wireless charging. The system consists of a set of Tx coupling coils for impedance matching, Tx and Rx self resonant coils, and a load coil. The coupling coils has different sizes for proper mutual inductance between the Tx resonant coil and the coupling coils to satisfy optimal impedance matching conditions are varied according to misalignment between Tx and Rx coils. The proposed system is analyzed based on circuit theory and then a figure of merit (FoM) of the system is used to design the coupling coils. For verification, a WPT system of planar rectangular multi-loop coils with three coupling coils is fabricated for 6.78 MHz. The Tx and Rx resonant coils are 20 × 10 cm and 10 × 10 cm, respectively. The results show that measured efficiency is well consistent with calculation and efficiencies are nearly uniform regardless of Rx position.