A hybrid array resonator structure for Wireless Power Transfer (WPT) is presented. The proposed structure exploits an array of four identical coils surrounded by a larger coil to generate uniform magnetic field. As a result, a nearly constant and high efficiency is achieved over a large misalignment region. The quasi-uniform magnetic field distribution of the proposed structure is analytically derived. For demonstration, one prototype operating at 40 is designed for implantable medical devices (IMDs). According to experimental results, the developed WPT system exhibits an efficiency of 53% when the receiver and the transmitter are aligned, while the efficiency is 44% when they operate with a lateral misalignment of 10 . As per a conventional WPT system using a single spiral resonator as transmitter and receiver, the efficiency is 47.2% in aligned condition, while, it is 30.5% for a 10 lateral misalignment. The reported results demonstrate that the proposed design approach provides a high misalignment tolerance. This represents an attractive future for applications, such as the recharge of embedded devices, where the alignment between the transmitter and the receiver is difficult to guarantee.

This paper provides a rigorous theoretical formulation to obtain an inductive resonant wireless power transfer (WPT) link with load-independent output voltage or current. This is a crucial working condition for wireless battery recharging, where there is no deterministic knowledge of load variation with respect to the battery charging level. The ideal lossless and the realistic lossy configurations are considered for both voltage- and current-excited WPT links. The link transfer matrix (ABCD matrix) is used to determine the operating frequencies where a load-independent output voltage or current can be obtained. It is shown that in the lossy cases that an almost load-independent behavior can be achieved, provided that the load resistance lies in a suitable range and analytical conditions on the load resistance value have been derived. The analytical relationships obtained by this theory are validated by means of both circuit simulation and experimental data at 13.56 MHz.

This article presents a new class of filters, conceived for the fabrication with additive manufacturing technologies, with a wide stopband. The resonators are made of a coaxial line where supports are the coupling elements between them. The wide stopband is obtained using two different solutions. The first is the use of stepped-impedance resonators, whereas the second is the use of quarter wavelength resonators. Several prototypes have been built, and the experimental results show the validity of the solution.

Capacitive wireless power transfer can be realized by mutually coupled capacitors operating at a common resonant frequency. An optimal load exists that maximizes either the efficiency or the power transfer to the load. In this work, we utilize the frequency bifurcation effect to propose a frequency agile mode that allows for a nearly coupling-independent regime. We analytically determine the operating conditions of the coupling-independent mode based on the different system gains. In this way, we obtain a solution that achieves nearly constant efficiency and power transfer, even at varying coupling. We compare our results to inductive wireless power transfer where a perfect coupling-independent mode is achievable.

This paper analyzes a resonant inductive wireless power transfer link using a single transmitter and multiple receivers. The link is described as an $(N+1)$–port network and the problem of efficiency maximization is formulated as a generalized eigenvalue problem. It is shown that the desired solution can be derived through simple algebraic operations on the impedance matrix of the link. The analytical expressions of the loads and the generator impedances that maximize the efficiency are derived and discussed. It is demonstrated that the maximum realizable efficiency of the link does not depend on the coupling among the receivers that can be always compensated. Circuital simulation results validating the presented theory are reported and discussed.

Wireless power transfer with multiple transmitters can have several advantages, including more robustness against misalignment and extending the mobility and range of the receiver(s). In this work, the efficiency maximization problem is analytically solved for a capacitive wireless power transfer system with multiple coupled transmitters and a single receiver. It is found that the system efficiency can be increased by adding more transmitters. Moreover, it is proven that the cross-coupling between the transmitters can be eliminated by adding shunt susceptances at the input ports. Optimal values for the input currents and receiver load are determined to achieve maximum efficiency. As well the optimal load, the optimal input currents and the maximum efficiency are independent on the cross-coupling. By impedance-matching the internal conductances of the generators, the maximum-efficiency solution also becomes the one that provides the maximum output power. Finally, by expressing each transmitter–receiver link with its kQ-product, the maximum system efficiency can be calculated. The analytical results are verified by circuital simulation.