A Three-Coil Wireless Power Transfer System using Self-Resonant Open-Bifilar Coils

A common trend in the field of wireless power transfer consists in the use of more than two coils in order to extend the link distance in terms of efficiency, in which the 3-coil system is of great interest due to its high efficiency. In these circuits, usually, the preferred method of compensation is the series (capacitor and inductor in series), mainly due to its simplicity. Another promising topic is the use of coils tuned at their self-resonant frequency, mainly because it could avoid the use of external capacitors to achieve the resonant state in the involved circuits. However a multiple-coil self-resonant frequency with series compensation system was not yet presented in literature. In this paper, using open-bifilar coils, we present a self-resonant 3-coil wireless power transfer system that behaves as common series compensated circuit. Practical results confirm the validity of proposed solution, in a proof of concept methodology, that is, without any optimization strategy applied. For this sake, three different distances between transmitter and relay coil are adopted, while a 10 ohm resistor is used as load, allowing a maximum efficiency of 47% at a distance of 8 cm between relay and load coil (coils diameter are 15 cm).