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This article develops explicit design of an impedance matching network (IMN) to enhance the robustness of megahertz (MHz) wireless power transfer (WPT) systems against variations in their final load and coil coupling. Unlike the existing trial-and-error-based solutions, analytical derivations are conducted to calculate the parameters of a two-port T- or $\bf \Pi$ -IMN. The load-pull technique is first applied to determine a target impedance ray that represents a desired loading condition of the power amplifier. Based on the analytical derivations, the three degrees of the design freedom of the IMN are then fully utilized to accurately match the starting point and direction of the target impedance ray. Different features of a final MHz WPT system, such as load-independent output voltage and high tolerance to coil misalignment, are further achieved by having different target impedance rays. Performance enhancement of an experimental 6.78-MHz WPT system is investigated under largely varied final load (10–50 $\mathbf{\Omega }$) and coil coupling (0–53%). With an explicitly designed IMN, the maximum efficiency drops are reduced from 18% to 6% under the changing final load and 21% to 3.9% under the changing coil coupling (0–40%), respectively. An actual application of wireless drone charging is also included to demonstrate the practicability of the proposed IMN design [ABSTRACT FROM AUTHOR]

Wireless power transfer (WPT) systems operating at several MHz are known to be advantageous for realizing high spatial freedom of power transfer and compact and lightweight designs. The combined use of multiple transmitting (Tx) coils for magnetic field shaping is expected to further increase the spatial freedom. This article studies an extendable planar Tx-coil array architecture. This architecture provides new conveniences for the MHz WPT systems to efficiently charge devices in 3D space. Time-domain and phasor-domain modelings are carried out in turn to analyze the magnetic field shaping effect under the current phase shift modulation of the Tx coils. An overlap design of the Tx-coil layout is also developed to minimize the cross coupling between the coils, which reduces the interference between Tx coils. Finally, the above concepts are experimentally implemented. The results clearly demonstrate the new advantages of the planar Tx-coil array-based magnetic field shaping, in terms of spatial freedom of the power transfer, efficiency, and output power capability, when charging receivers with six-degrees-of-freedom positions and orientations in 3D space. For instance, a perpendicular receiver in the center of the Tx-coil array can be charged with 82% dc–dc efficiency and 45 W, which is difficult for a conventional single Tx-coil solution. [ABSTRACT FROM AUTHOR]

In megahertz (MHz) wireless power transfer (WPT) systems, a varying reflected reactance due to a changing final load deteriorates the overall system performance, such as efficiency and output voltage stability. This article aims to achieve a compact and robust MHz WPT system working over a wide range of load. A novel concept of reactance compression design is proposed to compress the variation of the reflected reactance through positioning of a “reactance window.” Efficient receiving-side parameter design procedures are then developed to best compress the reactance variation and satisfy maximum output power requirement in a wide load range but without adding any other hardware. This makes it possible to directly apply the existing voltage-source design using a modified Class E power amplifier (PA) and current-source design using an LCC transformation network. An edge inductor also becomes worthy to replace the bulky air-core infinite inductor required by the modified Class E PA, thereby further improving circuit compactness. Finally, the receiving-side design concept and the edge inductor are experimentally implemented to verify their performance over a 5–50- $\Omega$ load and with the maximum final output power of 20 W. The results well validate the improved reactance compression, output voltage stability, efficiency, and harmonic distortions over the wide load range. [ABSTRACT FROM AUTHOR]