A new design approach is presented in this paper to show that under certain conditions, in a two-coil wireless power transfer system, the double-sided inductor-capacitor-capacitor (LCC) compensated wireless power transfer (LCC-WPT) system can be more energy efficient than the series-series (SS) compensated wireless power transfer (SS-WPT) system for the same load power, with special attention being paid to the effect that the parasitic coil and capacitor resistances have on the system efficiency. To make a fair comparison between the SS and LCC WPT systems, the direct current (DC) link voltage was adjusted to set equal load power for the two systems whilst using identical transmit and receive coils, coil-to-coil distance and load resistance. The system performance in terms of the system efficiency, the voltage stresses on the components, and the losses in the power devices were analysed for a practical system, comparing the LCC-WPT system and the SS-WPT system with respect to the load resistance. The effect of coil misalignment on the transferred power and efficiency for the two systems was compared. The theoretical proof and the conditions for meeting the objective are derived and practically verified in a two-coil WPT practical prototype, showing good agreement between analysis and experiments.

The frequency splitting phenomenon (FSP) is a critical issue in wireless power transfer (WPT) systems. When the FSP exists, the load power will sharply increase and can be dozens of times of the power obtained at the resonant frequency if the driving frequency varies from the resonant frequency, which seriously affects the system safety. This paper studies the impacts caused by the FSP in detail and then proposes a system control method by utilizing the advantages of the FSP to realize constant power output and zero voltage switching (ZVS) of the DC-AC inverter simultaneously. The splitting frequencies are tracked and the driving frequency is precisely adjusted together with the DC-link voltage. The specific implementation method is given in this work to minimize the impacts on the power transfer performance. An 85 kHz, 3.3 kW WPT system is built up to validate the proposed method. The experimental results show that when the FSP occurs at 150 mm coil-to-coil distance and 5 Ω load resistance, the load power can be hold at 3.3 kW with the DC-link voltage reduced from 327 V to 160 V. Meanwhile, ZVS of the inverter is realized and the inverter efficiency is improved from 95% to 97%.

With regards to wireless power transfer (WPT) systems with relay coils, the power transfer distance, efficiency, and capacity can be effectively improved by tuning the relative position of the relay coil compared with a common two-coil WPT system. However, the cross-coupling between nonadjacent coils can cause the system to no longer resonate even when the compensation capacitors resonant with the corresponding coils. This will cause an increase in reactive power and additional losses in the coils. In this article, the optimal operating conditions of the coaxial fully resonant three-coil WPT system are analyzed first for achieving higher efficiency compared with the two-coil WPT system with the same load and the same power transfer distance, by adjusting the relative location of the source coil and relay coil. An optimization design method for further improving the efficiency of the three-coil WPT system by tuning the compensation capacitor of the relay coil is proposed subsequently. The experimental results show that for a given load (10 Ω in this article), the efficiency of the three-coil WPT system can be improved from 88.8% to 92.3% compared with the two-coil WPT system, and be further improved to 93.3% when the relay compensation capacitor is optimized.

In this article, electromagnetic metamaterials with mu-near-zero (MNZ-MM) have been presented to explain the shielding effect in the wireless power transfer systems. First, the optimized design of the metamaterials unit cell has been carried on to achieve the near-zero permeability at the working frequency of 13.56 MHz. The equivalent circuit model of the MNZ-MM unit cell has been derived, which illustrates metamaterials with near-zero permeability have the great electromagnetic field (EMF) shielding effect. Additionally, the self- and mutual inductances associated with the resonance coils and MNZ-MM unit cells are deduced in detail. Moreover, the simplified human head model is established to investigate the magnetic field cancellation by MNZ-MM on the coil side. To evaluate the performance of the proposed MNZ-MM, the magnetic field strengths of the receiving terminal with different shielding materials have been compared in simulation. Finally, a series of experiments in real-environment are implemented to discuss the shielding effect of the MNZ-MM. The measurement results show the MNZ-MM has better performance in the swine brain sample than the ferrite and aluminum plate on the whole. With the proposed MNZ-MM, we demonstrate a maximum reduction of 17.52 dB in the leakage EMF and basically no effect on transfer efficiency.

This paper analyses the relationship between the coil design parameters and the system performance, including power transfer efficiency and amount, when circular flat spiral coils are adopted in a wireless power transfer (WPT) system. Coil design variables including outer radius, inner radius, channel width and coil turns are thoroughly studied to improve the system performance with a limited maximum outer radius for practical purposes. A two-coil WPT system has been built to verify the analysis, and the experimental results show good consistency with the theoretical calculations and simulation results, which show that the coil design parameters have a significant impact on the system performance, even with the same coil size. In the experiments, the coil-to-coil distance is 150 mm, the maximum coil outer radius is limited in 300 mm, and the DC input voltage and the load resistance are 100 V and 5 Ω, respectively. When the coils are tightly-wound in the most traditional way to maximize the coil size, the coil-system efficiency is 62.6% with only 4.5 W load power. In contrast, the efficiency optimized coil can improve the coil-system efficiency to 91.2% with the outer radius stayed the same. Besides, when the power transfer efficiency and amount are considered simultaneously, the system can achieve 1279 W load power with 85.94% coil-system efficiency.

One of the most difficult problems of the three-coil wireless power transfer (WPT) systems is that the receiving coil is hardly fully aligned with the transmitting and relay coils, which affects the efficiency of the practical system seriously as the coupling exists among the transmitting, relay and receiving coils simultaneously. Therefore, this article presents a new mutual inductance identification and efficiency optimization method for the three-coil WPT system with a switchable relay coil. First, the mathematical model of the three-coil WPT system with and without the relay coil is constructed. Second, the mutual inductance identification method is proposed to analyze the mutual inductances among the three coils by the switchable relay coil, which does not need complicated mathematical calculation strategies and the operating frequency of the system can be held constant. Then the coil-to-coil efficiency of the system is compared with and without the relay coil when the load resistance or position varies. Finally, a coil-to-coil efficiency improvement method is proposed to ensure that the system can always operate under high-efficiency conditions according to the identified mutual inductances of the three-coil WPT system.