Synchronous Operation of High-Frequency Inductive Power Transfer Systems Through Injection Locking

High-frequency inductive power transfer systems can be designed for operation with high tolerance to misalignment and large air-gaps, making it possible to operate in highly dynamic environments. Most examples in the literature use a single active transmitter and a single passive receiver (active–passive approach). Such systems are limited to unidirectional power flow and are susceptible to detuning of the transmitter due to changes of reflected reactance stemming from diode nonlinearities. This also limits the range of coupling over which the system can be operated efficiently. Therefore, there is a significant potential for expanding the range of applications of inductive power transfer systems by moving to an active–active configuration. This will enable bidirectional power flow, power routing through several nodes, and on-the-fly retuning to eliminate reflected reactances. One of the greatest challenges in achieving an active secondary in an inductive power transfer (IPT) system is obtaining a stable frequency and phase reference for the synchronous rectifier/transceiver with respect to the transmitter coil current and, hence, the magnetic field. Various methods for synchronization have been proposed in the literature, but they either require a separate, out-of-band communication link, or are difficult to scale to MHz operation. This article describes an alternative to the existing solutions, using an injection-locked oscillator to provide optimal phase tracking. A series of candidate feedback configurations are also proposed to provide high system resilience. In this work, the basic principles of injection locking are described as applied to synchronous IPT transceivers, and the experimental results are presented demonstrating its application to a bidirectional back-to-back Class-EF configuration operating at 13.56 MHz, with coupling factors ranging from 1.9% to 8.4% and power levels of up to 25 W.