Maintaining constant power transfer while keeping near-unity transfer efficiency at varying transfer distances is a major challenge for existing wireless power transfer (WPT) system with multiple repeaters. In order to overcome the problem, this article proposes a novel WPT mechanism with multiple repeaters based on the concept of parity-time symmetry. First, the coupled-mode model of this WPT relay system is established. Then, the steady-state transfer characteristics of the proposed WPT system with an odd and even number of repeaters are analyzed. The theoretical analysis shows that whether an odd or even number of repeaters are inserted between the transmitting and receiving coils, the proposed system automatically achieves constant output power and transfer efficiency against the variation of the transfer distance without any tuning or feedback within a certain distance. The prototype with one repeater and two repeaters is implemented to verify the validity of the theoretical analysis. Experimental results show that the prototype with one repeater can transfer power with an invariant transfer efficiency of 91% and a constant output power of 15 W within a transfer distance of 420 mm. Similarly, the prototype with two repeaters transfers constant power of 15 W over a transfer distance ranging from 420 to 500 mm, and the transfer efficiency is constant near 89%.

Nowadays, much effort focuses on research for the multireceiver wireless power transfer (WPT) system based on magnetic resonance. However, its output characteristics are significantly deteriorated by the variation of transfer spacing, loads, and cross-couplings, which exerts harmful effects on the performances of cascading bucks before loads if needed. Inspired by the work of a single-receiver WPT system based on the parity time symmetric model, in this article, the self-oscillating source is first applied to the multireceiver WPT system to tackle these drawbacks. Based on the coupled-mode theory, the proposed system's modeling is established and deduced to analyze its performance. The model analysis shows how the system can achieve zero-voltage switching simply via choosing the minimum dead time and leading time whose calculation method is provided. Meanwhile, a transmitting coil for a uniform magnetic field is designed to improve the free-positioning performance. The analysis is validated on a 36-W prototype, including the effects of the variation of transfer spacing, cross-couplings, and loads on the output characteristics, efficiency, and operating frequency. The proposed system is demonstrated to provide a much more robust power transfer than the conventional magnetic-resonant multireceiver WPT system, which helps extend the transfer distance and improve the system efficiency.

Wireless charging of automated guided vehicles (AGVs) has been widely used because of its high security and convenience. However, due to the inevitable misalignment between the coupling coils, the charging system cannot maintain the constant current (CC) or constant voltage (CV) output. Therefore, this article proposes a misalignment-tolerant fractional-order wireless power transfer (FOWPT) system with CC or CV output. According to the principle of the fractional-order autonomous circuit, the natural resonant frequency and operating frequency can be adjusted adaptively with the change of transfer distance and load. Benefiting from the frequency characteristics and the new implementation method of the FOWPT, the system maintains CC and CV output, and the horizontal and vertical misalignment tolerances are within 44.4% and 22.2%, respectively. Therefore, the proposed system improves the freedom and convenience of charging, and provides a new wireless charging solution for AGVs. Future article will focus on more concise modeling and implementation methods of fractional-order systems, and improving the performance of the FOWPT.

In recent years, the parity–time (PT) symmetry principle has been introduced into wireless power transfer (WPT) systems, which can improve the stability of WPT within a certain range. In this article, we analyze the generalized PT-symmetry conditions of four basic WPT topologies based on circuit theory (CT) and coupled mode theory (CMT). The results show that the series–parallel (SP) topology meets the PT-symmetry condition based on the CMT, although it does not meet the condition obtained by the CT. The symmetry between the energy of the two-resonance system is established in the SP topology. According to this unique characteristic, a constant-voltage WPT system for spinal cord stimulation is designed, which is independent of the transfer distance and load in a wide range.

This brief presents a fractional coupled model of fractional-order wireless power transfer (FOWPT) system based on the fractional calculus. By applying coupled-mode theory (CMT), dynamic characteristics of fractional systems can be described clearly while avoiding complex circuit models. The theoretical simulation results indicate that the FOWPT system has better characteristics and greater design freedom. Furthermore, the fractional coupled model is in good agreement with the circuit model and experiment, which proves the correctness of the generalized fractional coupled mode equations. Therefore, the fractional coupled model provides a valuable tool for the further analysis of FOWPT systems and may be used in other fractional-order resonant circuits.

For wireless power transfer (WPT) systems, parameter fluctuations and misalignment of coils will lead to frequency detuning, which will in turn affect system performance and output accuracy. In general, the conventional frequency-tuning strategies rely on adjustable capacitors or inductors, which increase the control complexity or require additional modules. Therefore, this paper proposes a fractional-order wireless power transfer (FOWPT) system with adjustable fractional order. The proposed system with misalignment and detuning tolerance can provide a constant current (CC) or constant voltage (CV) output. First, based on the coupled mode theory, the frequency and output characteristics are analyzed. Then, a frequency-tuning strategy based on the fractance element is proposed. Operation modes and equivalent impedance of the fractance element are analyzed. On this basis, control strategies of CC and CV are designed for the transmitter and receiver, respectively, Finally, an 864-W prototype is set up to validate the proposed frequency-tuning strategy and control strategy. The proposed system has robustness to misalignment and parameter fluctuations, which can improve efficiency and output accuracy.