Free-Positioning Wireless Power Transfer to Multiple Devices Using a Planar Transmitting Coil and Switchable Impedance Matching Networks

In this paper, an efficient free-positioning wireless power transfer (WPT) system for charging multiple devices is proposed. For the system, a planar transmitting (Tx) coil that consists of a spiral coil and concentric multiple parallel loops and switchable impedance matching networks (IMNs) at a transmitter are developed. Using the Tx coil, mutual inductance between the Tx coil and the receiving (Rx) spiral coils is made uniform in the effective charging area. The parallel loops are placed in the interior of the Tx coil, and the magnetic field is enhanced and controlled by adjusting the space in each parallel loop. For optimal design of the coil, an analysis method using an equivalent circuit is explained in detail. It is found that the current of one loop of a parallel loop is more than the input current and negative current flows on the other loop, while the total current of each parallel loop is constant. An array of parallel capacitors with a switch as the switchable IMN is used for the maximum power transfer efficiency corresponding to the number of devices. The reflection coefficients of the fabricated WPT system are under -10 dB at 6.78 MHz, regardless of the number of devices and positions. The WPT system has the maximum power transfer efficiencies of 93% for two devices and simultaneously transfers uniform power to up to four devices with high efficiency.