A resonance frequency tracker and source frequency tuner for inductively coupled wireless power transfer systems

Inductively coupled resonant loops, for use in wireless power transfer applications up to several meters, exhibit the largest transfer efficiency when the loops operate at the strongly coupled regime. At the same time, this largest efficiency is independent of the mutual inductance between the loops. However, the resonance frequencies, at which this largest efficiency occurs, vary with mutual inductance or the distance and orientation between the loops. Therefore, the RF source frequency should be tuned to the resonance frequency of the loops. This paper presents a resonance frequency tracker and source frequency tuner for inductively coupled resonant loops. The inductively coupled loops are represented by a lumped element circuit with a tunable inductor that resembles the variation of mutual inductance. The resonance frequency is tracked by sampling the incident and reflected RF signal using a bidirectional coupler and calculating the input reflection coefficient. The source frequency is tuned until the reflection coefficient is minimized. Each individual part of the overall system is fabricated and tested, and the controller is programmed. The complete tracking & tuning system is simulated based on these measurement results.