Megahertz-range wireless power transfer has become a promising approach for increasing spatial freedom of charging. This paper proposes a cylindrical-shaped coil, which can produce the homogeneous magnetic field in a plane. The coil consists of two helical windings, which are wound to guide the current in opposite directions. A single power source is used to excite the transmitter without any current amplitude or phase control circuits. Furthermore, since the parasitic capacitance of the coil is not negligible due to the complexity of the coil shape, we develop a general equivalent model with parasitic capacitance for the analysis of complex coils. The system efficiency of the proposed omnidirectional wireless power transfer device is validated by a practical experiment. The measured DC-to-DC efficiency of approximately 72.4% and the load power of 13 W are demonstrated for the proposed wireless charging system at 6.78 MHz operating frequency. Finally, we verify that the electromagnetic exposure satisfies the safety regulations.

The use of multiple transmitters (Tx) has been an attractive solution for enabling free-positioning wireless power transfer (WPT) in a large area. One of the key challenges in such multi-Tx WPT systems is the cross-coupling between Tx coils. This letter proposes a compact self-decoupled Tx coil solution for multi-Tx WPT systems. The measured cross-coupling between Tx coils has been reduced by 95% compared to conventional coil designs. Additionally, an integration method for a compensation inductor with the main Tx coil is proposed. Therefore, the coil design proposed in this letter facilitates creation of fully modular and scalable Tx coils for multi-Tx WPT systems. The proposed designs are experimentally verified in a laboratory prototype.

The complicated resonant operations of class Φ 2 topology bring challenges for accurate design and performance optimization, hindering the full utilization potential of converters. Considering the narrow design freedom in traditional methods with almost fixed duty cycle D, this article widens the design options of push-pull class Φ 2 converters through frequency-harmonic analysis. A full selection freedom of D ∈ (0, 0.5) is discussed analytically, providing ample space for optimization based on any required performance indices. From 1.98E5 analytical results, we found six numerical equations that fully decouple the interconnected relations between each circuit parameter and D. The proposed numerical method allows rapid circuit design and component selection with a high accuracy regardless of the system power or load voltage. Parasitic effects are discussed and incorporated into the design approach as correction steps. Finally, we introduce performance analysis based on an example wireless power transfer (WPT) system, providing in-depth studies on the optimization regarding efficiency, power output capability, and component selection. Experimental results validate the accuracy and efficiency of the proposed design method based on a 100-W WPT system at 6.78 MHz frequency. Both inverter and rectifier present load-independent soft-switching operations, with converter efficiency over 93%. The system provides 83% dc-dc efficiency at full load. Index Terms-MHz, resonant power converters, wireless power transfer (WPT), zero-voltage switching (ZVS), zero-voltagederivative switching (ZDS). I. INTRODUCTIONW IRELESS power transfer (WPT) is increasingly becoming a popular technique for diverse applications including electric vehicles, consumer electronics, and industrial applications [1], [2]. WPT systems working at multi-MHz frequencies have the advantage of realizing high efficiency at weak Manuscript