Load-Independent Push–Pull Class E$ ^2$ Topology With Coupled Inductors for MHz-WPT Applications

This article investigates the modeling and implementation of the load-independent operation for the push–pull Class E topology with coupled inductors. The analytical modeling is first presented and provides a new design methodology, which distantly improves the soft-switching performance, load regulation, and efficiency compared with the conventional inductor-coupled Class E topology. An LCC-S resonant Class E$ ^2$ converter, which combines the load-independent inductor-coupled inverter and active rectifier, is proposed for MHz-wireless power transfer (MHz-WPT) applications. The active inductor-coupled Class E rectifier maintains soft switching, constant voltage gain, and zero-phase-angle input at various loads. A three-winding magnetic structure is also proposed to improve the coupled inductors by enhanced self-inductance, which features more effective harmonic suppression with lower magnetic losses and volumes. By the proposed methodology and the improved magnetic structure, the converter features negligible harmonic contents and maintains soft switching with a constant voltage conversion ratio over the entire load range. A 6.78-MHz WPT prototype was built to verify the proposed methodology with detailed parameter design and hardware implementation. The prototype realizes load-independent zero-voltage switching and maintains an approximately constant voltage gain with +5/$-$4% variance from no-load to 320-W output power. The measured dc–dc efficiency reaches 89.3% at 210 W.