High-efficiency power transfer at a long distance can be efficiently established using resonance-based wireless techniques. In contrast to the conventional two-coil-based inductive links, this paper presents a magnetically coupled fully planar four-coil printed spiral resonator-based wireless power-transfer system that compensates the adverse effect of low coupling and improves efficiency by using high quality-factor coils. A conformal architecture is adopted to reduce the transmitter and receiver sizes. Both square architecture and circular architectures are analyzed and optimized to provide maximum efficiency at a certain operating distance. Furthermore, their performance is compared on the basis of the power-transfer efficiency and power delivered to the load. Square resonators can produce higher measured power-transfer efficiency (79.8%) than circular resonators (78.43%) when the distance between the transmitter and receiver coils is 10 mm of air medium at a resonant frequency of 13.56 MHz. On the other hand, circular coils can deliver higher power (443.5 mW) to the load than the square coils (396 mW) under the same medium properties. The performance of the proposed structures is investigated by simulation using a three-layer human-tissue medium and by experimentation.

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Strongly coupled four‐coil wireless power transfer (WPT) system is presented in this letter. Conformal architecture is used for both transmitter (TX) and receiver (RX) coils. Thus, a high mutual coupling coefficient is achieved between the coplanar coils. Furthermore, the quality factors of primary and secondary coils are improved. The proposed WPT system features a compact structure for biomedical applications and provides a measured efficiency greater than 79% when the distance between the TX and RX is 10 mm at a resonance frequency of 13.56 MHz. The designed system is also simulated for 10 mm human tissue medium using HFSS, which generates a power transfer efficiency of 55.1%. The simulated results are also verified by experimentation. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:1861–1866, 2016