Optimization of resonant inductive links for wireless power transfer

A shunt-series mixed resonant coupled structure for the wireless power transfer (WPT) applications is proposed. If the coils are designed to have proper inductance values, the power transfer efficiency depending on distance has proportional relation only to the shunt capacitors. It enables that the proposed structure facilitates tracking the maximum WPT efficiency according to distance. In the experiment, two pairs of resonant square coils on PCB with different turn ratio (5/10 turn) are compared for 6.78 MHz operation. Both coils have almost same maximum WPT efficiency by optimizing the series and shunt capacitors. But the 10-turn coil only demonstrates aforementioned relation. The proposed structure with 10-turn coil shows that the efficiency of 77.7 % is achieved at a distance of 30 mm.