Maximum Efficiency Tracking of a Wireless Power Transfer System With 3-D Coupling Capability Using a Planar Transmitter Coil Configuration

High degrees of coupling freedom in wireless power transfer (WPT) has emerged as a prominent research topic. The challenge lies in achieving a compact structural design for such systems. This article introduces a planar transmitter configuration composed of three decoupled and separately controlled converters to enable three-dimensional (3-D) WPT to an arbitrarily positioned receiver above the transmitter. By theoretical analysis based on the coupled mode equations, it is found that there is a direct relationship between the dc input voltages and currents of the inverters for achieving the maximum efficiency. The spatial magnetic field is shaped by controlling the input voltages based on the measured currents to achieve the best magnetic field coupling for maximum efficiency tracking, regardless of the positioning and alignment of the power pickup. A practical system is built with a 10 × 10 cm transmitter (with three decoupled coils) and 3.5 × 3.5 cm (single coil) receiver, and it is demonstrated that the system can achieve a power transfer efficiency about 80% within 3 cm above the transmitter under all 3-D coupling conditions.