Multi-MHz IPT Systems for Variable Coupling

In typical multi-MHz inductive power transfer (IPT) systems, a change in coupling or load resistance can significantly deteriorate the end-to-end efficiency due to a deviation from the optimal load of the IPT link and suboptimal operation of the resonant inverter due to the loss of soft switching condition. This paper proposes solutions for an IPT system to operate efficiently when large changes in coupling take place. To achieve high power efficiency independent of coupling, we utilize inherent regulation properties of resonant converters to avoid losing soft switching for any coupling value, and present the optimal load to the IPT link at the maximum energy throughput coupling. A probability-based model is introduced to assess and optimize the IPT system by analyzing coupling as a distribution in time, which depends on the dynamic behavior of the wireless charging system. The proposed circuits are a Class D rectifier with a resistance compression network in the receiving end and a load-independent Class EF inverter in the transmitting end. Experiments were performed at 6.78 and 13.56 MHz verifying high efficiency for dynamic coupling and variable load resistance. End-to-end efficiencies of up to 88% are achieved at a coil separation larger than one coil radius for a system capable of supplying 150 W to the load, and the energy efficiency was measured at 80% when performing a uniformly distributed linear misalignment of 0-12.5 cm, corresponding to a receiver moving at a constant velocity over a transmitter without power throughput control.