Design of a 13.56 MHz Inductive Power Transfer System With Closed-Loop Output Regulation and Active Soft-Switching

A multi-MHz Inductive Power Transfer (IPT) system was designed and implemented so that it can maintain constant output voltage for dynamic changes in load and coupling factor. Traditionally, output regulation is achieved by using an additional converter between the rectifier and the load or with a synchronous rectifier. However, in this work, output regulation and soft-switching over a wide range of loads and coil misalignments is achieved exclusively from the primary-side converter requiring only a passive rectifier on the secondary. A Multi-Frequency Averaging (MFA) model of the Class EF inverter, combined with First Harmonic Approximation (FHA) of the Class DE rectifier was developed to determine the dc/dc gain of the IPT system as a function of switching frequency. This is experimentally validated on a 13.56 MHz system with a separate wireless communications link to provide feedback to the primary-side closed-loop controller. The measurements showed that the closed-loop system achieves higher system efficiency over load and coupling variations than an open-loop system, whilst maintaining a constant output voltage.