Fully Detuned Four-Coil Structure Enables Efficient Wireless Power Transfer Over Long Distances

Addressing the issue of low efficiency in long-distance wireless power transfer due to a very small coupling coefficient, it is common to place relay coils at a certain distance between the transmitter and receiver to improve efficiency through resonance. However, this reduces the effective transmission distance. This paper presents a new structure that places the relay together with the transmitter and receiver, significantly increasing the effective gap for wireless transmission and greatly improving transfer efficiency through fully detuned configuration. First, the equivalent circuit of the four-coil structure with a relay is analyzed to obtain the equivalent mutual inductance and equivalent self-inductance, facilitating subsequent circuit analysis. Second, the losses of the four-coil structure under fully detuned conditions are analyzed to explore the changes in losses under different detuning states. Finally, the system achieves 120W of wireless power transfer at a transmission distance of 1 meter and a distance-to-diameter ratio of 2, with a coupling mechanism transmission efficiency of 87.6%.