A Position-Independent Three-Coil Constant Power Output WPT System

With the increasing popularity of wireless power transfer in applications such as electric vehicle charging and medical device inspection, the system's freedom and transmission distance face greater challenges. This paper introduces a three-coil wireless power transfer (WPT) system that can maintain a constant power output when facing variable coupling coefficients. A control strategy based on LC coil self-freewheeling is proposed, which eliminates the need for the dc-dc converter and construct a negative resistance by using only one inverter. Theoretical analysis demonstrates that this system maintains stable and efficient transmission capability within a wide range of coupling coefficients. To verify the feasibility of the theoretical analysis, an experimental prototype with a stable output of 30W is designed. The result indicates that a constant voltage output can be maintained within a transmission range of 18-30 cm, with the operating frequency fixed at 250.3 kHz and a peak efficiency exceeding 90%.