Resonant Wireless Power Transfer Research Articles

Parametric study of metamaterial-inspired resonator for wireless power transfer

A metamaterial-inspired resonator for free-positioning wireless charging of several devices is proposed and numerically studied. The resonator is based on orthogonal metallic wires arrays deposited on a high permittivity low loss dielectric substrate. Given a fixed area of 50 × 50 cm, its resonant frequency can be tuned in a wide frequency band from 5 MHz to 45 MHz. A parametric study is performed to reveal the influence of resonant frequency, dielectric permittivity and number of wires on Q-factor.

A Compact Magnetically Dispersive Surface for Low-Frequency Wireless Power Transfer Applications

We present a compact, magnetically dispersive engineered surface for resonant inductive wireless power transfer (WPT) that significantly enhances the performance of an inductive link, increasing the efficiency and/or working distance. The proposed metasurface measures 6 cm $\times 6$ cm $\times0.1$ cm, achieving a greatly reduced thickness over a 3-D metamaterial, especially relative to the long wavelength at the chosen operating frequency of 5.77 MHz, which is in the range of typical wireless biomedical implants. Such feature is obtained by virtue of an extremely compact connected double-spiral unit cell, which provides high inductance values despite the small size. We prove through numerical simulations that the proposed metasurface considerably lowers the electric field level with respect to traditional two- or three-coil WPT systems, while maintaining the same magnetic field level at the receiver. In addition, we conducted efficiency measurements on a fabricated prototype, confirming the performance of our design. The excellent efficiency enhancement, combined with the resulting low electric field, makes the proposed metasurface an attractive solution for a number of WPT applications, especially where the exposure in terms of specific absorption rate (SAR) is one of the major concerns, such as in biomedical implants and rechargeable devices.

Research on load adaptive impedance matching of magnetic resonant wireless power transfer system

Research on load adaptive impedance matching of magnetic resonant wireless power transfer system

Interference Analysis of Metal Conductors for Target-Resonance-Based Detection Method

Frequency bifurcation phenomenon is an important input characteristic of the magnetically coupled resonant wireless power transfer (MCR-WPT) system in overcoupling situation. In this paper, frequency bifurcation phenomenon in MCR is extended to the general transmitter-receiver coupling system, and the two main factors affecting the phenomenon are analyzed. Finally, the phenomenon is introduced into the field of signal detection, and a noncontact detection method based on target coil resonance is proposed in this paper. Since the proposed method is a kind of electromagnetic method, the detection system is highly susceptible to the surrounding metal conductors. Therefore, the detection performance of this method under three metal-environment conditions is analyzed, respectively. Simulations and experiments show that the metal barrier can weaken the detection because of the eddy current effect, but as long as the barrier does not completely isolate the two coils, the method still has strong penetrability. Besides, the metal plate placed parallel outside the transmitter has less influence on the detection when compared with the metal barrier, and the metal plate by the side of the transmitter can even enhance the detection effect. As a consequence, this method has strong stability in metal environment.

Calculation and analysis of near-field magnetic spiral metamaterials for MCR-WPT application

Mu-negative and mu-near-zero metamaterials have great potential for magnetically coupling resonant wireless power transfer (MCR-WPT) application. It is well known that square and circular spiral structures are often applied in MCR-WPT system. However, there is rarely comparative analysis in detail. Here we present the electrical Lorentzian-like model of square and circular spiral resonators to accurately predict the − 1 and 0 points of effective permeability. Combined with semi-empirical equations, analytical process is completely given out. Particularly, all losses of unit cell are calculated in detail. Higher losses are observed in self-resonance circular spiral resonator, resulting in lower quality factor (Q). Measurement and full-wave simulation verify the calculated accuracy of effective permeability. To further analyze two spiral metamaterials, their magnetic fields (Hz) are calculated. By the comprehensive comparison of lumped capacitor and self-resonance metamaterials, the near-field metamaterial with lumped capacitor possesses the better performance. This model can accelerate the design process of near-field metamaterials. These results have the guidance meaning in MCR-WPT application.

Frequency drift insensitive broadband wireless power transfer system

The resonant frequency of coils in a four-coil magnetic coupling resonance wireless power transfer (MCR-WPT) system has a direct impact on transfer coefficient. For efficient power transfer, most of the systems operate at high-Q resonant state, causing a narrow bandwidth and reducing the system tolerance to resonant frequency deviation. To deal with the problem, a four-coil MCR-WPT system characterized with broadband performance is investigated. Based on the equivalent circuit, the transfer coefficient and the input impedance are firstly calculated. The transfer characteristic at different resonant frequency under different transfer distances is discussed. By comparing the number and variance of the extreme values of transfer coefficient under different resonant frequencies, the resonant frequency for excellent broadband transmission is determined. To validate the broadband performance, frequency drift caused by capacitance variations is investigated at the resonant frequency of 19 MHz under the transfer distance of 45 mm. The results indicate that the MRC-WPT system generally maintains broadband transmission and the transfer coefficient at the excitation frequency of 19 MHz remains above 0.9 when the resonant frequency is deviated. Experiments have been performed to validate the excellent broadband performance. The results are generally consistent with the simulation analysis.

Active Rectification For the Optimal Command of Bidirectional Resonant Wireless Power Transfer Robust to Severe Circuit Parameters Deviations

In this article, a method resorting to active rectification for the optimal control of a bidirectional resonant wireless power transfer system is proposed. Benefiting from a dual active bridge topology and without requiring any additional dc-dc converter, the real and the imaginary parts of the equivalent load impedance are modulated simultaneously and independently, for achieving optimal operating conditions despite circuit parameters deviations. Based on a first harmonic analysis, the methodology and its beneficial impacts against variations in the windings mutual inductance or against mistuned resonant capacitor are presented and illustrated. By an argued discussion, the proposed method extent is enlarged to changes in any circuit parameters. Time-domain simulations of the converters topology and command demonstrates the method effectiveness by achieving the system maximum efficiency. Controllability restrictions preventing the compensation of the most severe parameters variations are highlighted. Associated with the limited voltage producible by conventional full-bridge converters, the latter restrictions are surmounted with success by considering Z-source topologies for the front-end and back-end converters.

Frequency Splitting and Transmission Characteristics of MCR-WPT System Considering Non-Linearities of Compensation Capacitors

The frequency splitting phenomenon and transmission characteristics have been research hotspots in the field of magnetically coupled resonance wireless power transfer (MCR-WPT). In this paper, non-linear dynamics theory was innovatively introduced into the research, and non-linear coupled transmission dynamics modelling of the MCR-WPT system was established considering the non-linearities of the compensation capacitor. The mechanism of the frequency splitting phenomenon of the MCR-WPT system was revealed through systematic mathematical analyses based on the modelling. The analysis results showed that the system usually has dual natural frequencies which are low resonance frequency and high resonance frequency. Based on non-linear dynamics theory, the transmission characteristics of the system with different non-linear parameters were discussed comprehensively in relation to the modelling. The results of the numerical simulations and theoretical analyses showed that non-linear parameters can cause the jumping phenomena with the output responses, and the output responses in the vicinities of the lower resonance frequencies were extremely sensitive to changes in the coupling coefficient. According to analyses of the linear and non-linear systems, the energy transmissions performed in the vicinity of the high resonance frequency had a wider working frequency band and a better transmission stability under non-linear conditions.

Effects of Number of Relays on Achievable Efficiency of Magnetic Resonant Wireless Power Transfer

In magnetic resonant wireless power transfer (WPT), relays are placed between a transmitter and a receiver as a means of increasing both the operating distance and the power transfer efficiency, but this makes the analysis of the WPT system more complicated. In this letter, we derive a mathematical expression for the power transfer efficiency with an optimal load resistance based on an equivalent circuit model, and analyze the effects of varying the number of relays on the power transfer efficiency that can be achieved. By means of circuit-level simulations and experiments under a variety of scenarios, we verify the accuracy of our analysis, and we also confirm that there is an optimal number of relays for maximizing the achievable power transfer efficiency for a given end-to-end distance.

An Accurate Equivalent Circuit Model of Metasurface-Based Wireless Power Transfer Systems

In this article, we introduce a general analytical procedure to unambiguously characterize a metasurface through its lumped circuital equivalent in resonant inductive Wireless Power Transfer (WPT) applications. The proposed model incorporates the finite extent of the slab, as well as the WPT near field operative regime and the presence of the particular driving/receiving coils arrangement, providing quantitative and easy-to-handle parameters which can be manipulated to achieve WPT performance enhancement. We first develop the theoretical background aimed at the lumped parameters extraction, which reveals, for WPT applications, more accurate and robust with respect to the conventional sub-wavelength homogenization theories based on infinite slab extent and impinging plane wave hypotheses. We provide some general guidelines for the design of metasurfaces for WPT performance enhancement based on the derived circuit model; afterwards, we numerically design a test-case consisting of two resonant coils (driver and receiver, respectively) with an interposed passive metasurface to verify the developed theory. Finally, we show some measurements performed on a fabricated prototype, that present an overall excellent agreement with both the lumped model and the numerical simulations.

Design and Optimization of Magnetic Coupled Resonant Wireless Power Transfer System at Weak - coupling Region

Frequency splitting is one of the hot topics in the research of magnetic coupling wireless power transfer technology. Its purpose is to improve the transmission efficiency by eliminating the frequency splitting at the over-coupled region of the system. Frequency splitting does not occur at the weak coupling region. The transmission characteristics of the system in weak-coupled region are also of research significance. The mathematical model of the system’s transmission characteristic was established by equivalent circuit theory. Several key factors that affect the transmission characteristics of the system, self-inductance of the coil, mutual inductance between the coils, load resistance and so on, were obtained. A magnetically coupled resonant wireless power transfer system is designed and A method of coil design is proposed. The transmission efficiency of the system at the weak-coupling region was improved by optimized the design of the coil. Simulation and experimental results had good consistency. The results show that the transmission performance of the system at the weak-coupling area was improved.

Optimal Design of a 3-Coil Wireless Power Transfer System for Deep Micro-Implants

This paper presents a design methodology for a 3-coil magnetic resonance wireless power transfer (WPT) system with a long transfer distance (up to 20 cm) and a small implanted receiver (RX) (2mm in diameter) at a specific frequency. The methodology aims to find out the optimal value of dimensional parameters (i.e., coil diameter, gap interval and coil turn number) of the transmitter (TX) coils to maximize the magnetic field strength at the target distance while keeping the coil self-resonant frequency (SRF) twice of the target operational frequency. Firstly, the circuit model of the TX circuits is developed, which include a single-turn coupling coil and a multi-turn primary coil. Secondly, the co-dependences between the dimensional parameters are analyzed, which shows the dominant factors and secondary factors of each dimensional parameter, and how the optimal values of dimensional parameters are changed by these factors. Based on the analysis, design flow of the TX circuit is proposed to decide the optimal values of dimensional parameters given the transfer distance, source voltage, operational frequency and wire diameter. Using the design flow, optimal values of dimensional parameters are predicted for 20-cm transfer distance and 16-cm transfer distance and are verified with finite element analysis (FEA) software COMSOL Multiphysics. With the optimal TX design, the power received by a 2-mm ferrite core solenoid RX is calculated. At 20 cm transfer distance, up to 4.3 mW can be achieved in air, and 0.8 mW can be achieved in conductive human tissue.

무선전력 전송효율 음영영역 제거와 최대효율 전송을 위한 3D 스캐닝 MR-WPT 시스템

This paper proposes a 3D scanning magnetic resonance wireless power transfer system to eliminate dead-zones and improve transmission efficiency. This system consists of an oscillator, power amplifier, microcontroller unit, personal computer, pan-tilt motor, transceiver coil, rectifier, and wireless communication units. This system provides the maximum voltage to the receiver within a short timeframe using the zone-based maximal transmission efficiency searching process algorithm. From the measurement results, it can be observed that the time to find the maximum transmission efficiency with this algorithm is 11 times faster than that of the full scan by 5° and that the position error of maximum transmission efficiency is less significant.

Modeling and Design of a Transmission Coil and Four Cascaded Receiving Coils Wireless Charging Structure With Lateral Misalignments

Magnetic resonance wireless power transfer (WPT) is an emerging technology that may create new applications for wireless power charging. However, the mutual inductance fluctuations resulting from lateral misalignments are the major factors hindering the promotion of this technology. In this paper, a structure of a transmission coil and four cascaded receiving coils for wireless charging systems is proposed to suppress fluctuations in mutual inductance with lateral misalignments. The mathematical model of the proposed structure with lateral misalignments is built based on the equivalent circuit method. The expressions of the output voltage and the efficiency are then derived by solving the system equivalent equations. In addition, a method of compensation capacitance is proposed to suppress the effect of the cross-coupling mutual inductance on the voltage and efficiency. The primary advantage of the proposed structure is that the sum of the mutual inductances between the receiving coils and the transmission coil is nearly constant with lateral misalignments. Therefore, the output voltage and efficiency can also be kept nearly constant by using the proposed structure with lateral misalignments. Simulation and experimental results validating the proposed method are shown.

A Fractional-Order Resonant Wireless Power Transfer System With Inherently Constant Current Output

Wireless power transfer (WPT) using magnetic-coupling resonant has been proved to be a promising transmission method. At present, most of the existing resonant WPT systems adopt integer-order inductors and capacitors. In fact, in addition to integer-order components, there also exists fractional-order inductors and capacitors, which possess richer characteristics than integral-order inductors and capacitors. However, there are few studies on the application of fractional-order components in resonant WPT system. Therefore, this paper proposes a fractional-order resonant WPT system, which is based on a fractional-order resonant circuit only incorporating a fractional-order capacitor. The coupled-mode theory model of the proposed WPT system is established, and the impact of different loads and orders on transfer characteristics is explored. The theoretical analysis demonstrates that the proposed system can achieve constant current output that is independent of load, just by choosing appropriate parameters of the fractional-order capacitor. Finally, an experimental prototype of the fractional-order resonant WPT system is built, and the experiment results verify the correctness of the theoretical analysis.

Design and development of wireless power transfer system for implantable cardioverter defibrillator

Wireless power transfer system is playing an important role in biomedical applications by transferring power wirelessly to the implants present in the body. In this paper, magnetic coupling resonance wireless power transfer system is used to recharge battery of implantable cardioverter defibrillator. The proposed system operates at 300 KHz frequency. The LCC-C compensation topology is used to tune the coils for operating the system at stable resonant frequency and to obtain higher efficiency. In this paper, we have investigated the performance of different coil structures in multiple layer configurations and we offer recommendations on the best suitable configuration for effective transfer of power.

Energy-Concentrating Optimization Based on Energy Distribution Characteristics of MCR WPT Systems With SS/PS Compensation

Spatial energy distribution characteristics (EDCs) as well as power and efficiency should be considered to ensure the electromagnetic safety of people nearby when analyzing some magnetically coupled resonant wireless power transfer systems, which is difficult for circuit theory. In this article, we first propose an energy-concentrating effect (ECE) of unit transmitting active power (TAP) based on the EDCs of the system with series-series/parallel-series compensation, and we find that the area integral of the active power density on the infinite plane perpendicular to the transmission direction between the coils equals the TAP of the system, which is consistent with the result obtained by circuit theory. Several couplers are designed and optimized based on the ECE. Simulation and experimental results reveal that the optimized inner-outer ring (IOR) and disc-coil couplers increase the ECE by 10% and 32%, respectively, compared with the original spiral-coil coupler. Furthermore, the electromagnetic exposure of the human body in the same position around the three couplers indicates that the maximum induced electric field and specific absorption rate in the human body around the optimized IOR and disc-coil couplers are substantially decreased, and thus the two couplers increase the maximum allowable TAP of the system from 297 to 379 W and to 700 W.

Transfer characteristics of the MCR-WPT system using two series transmitting coils

PurposeThe impedance compensation approaches have been adopted to achieve the maximum output power and transfer efficiency in many magnetic coupling resonance wireless power transfer projects. However, it remains a challenge to obtain the constant output power and transfer efficiency in a fixed-frequency mode during variations in transfer distance and orientation of the coils. In this paper, using two series transmitting coils to achieve the constant output power and transfer efficiency is used.Design/methodology/approachFirst, the circuit model is established and transfer characteristics are studied. Second, using the two series transmitting coils to achieve the constant output power and transfer efficiency is investigated. Finally, the experimental system is designed; it can optimize the transfer performances by itself; the constant output power and transfer efficiency are achieved in the fixed-frequency mode.FindingsWhen the receiving coil moves between the two series transmitting coils, the tolerance of the output power and transfer efficiency is less than 5 per cent.Research limitations/implicationsWhen a receiving coil is placed between the two series transmitting coils, there are space limits. The receiving coil only shifts between the two transmitting coils.Practical implicationsHowever, the rail guide vehicle may achieve constant output power and transfer efficiency when it moves on the rail guide. So, this topology may provide a practical solution.Originality/valueIn this research, the three-coil MCR-WPT system including two series transmitting coils is presented. In a fixed-frequency mode, the constant output power and transfer efficiency is achieved in experiments during variations in transfer distance and orientation of the coils. The fluctuation of the output power and transfer efficiency is less than 5 per cent.

Single-Stage Regulated Resonant WPT Receiver With Low Input Harmonic Distortion

Resonant rectifier topologies would be a promising candidate for achieving simple, compact, and reliable single-stage wireless power transfer (WPT) receiver if not for the lack of good DC regulation capability. This paper investigates the problems that prevent the feasibility of single-stage DC regulation in resonant rectifier topologies. A possible solution is the proposed differential resonant rectifier topology, of which the rectifier is designed to have a relatively constant AC voltage, and that phase shift control is used to achieve relatively good output regulation. Design considerations on the reactive component sizing, magnetic component design, frequency and phase synchronization, small signal modelling, and closed-loop feedback control design, are discussed. Experimental results verified that the proposed WPT receiver system can achieve single-stage AC rectification and DC regulation while attaining the key features of low harmonic distortion in its AC output voltage, continuous DC current, and zero-voltage-switching (ZVS) operation over a wide operating range.

Strategy of Topology Selection Based on Quasi-Duality Between Series–Series and Series–Parallel Topologies of Resonant Inductive Coupling Wireless Power Transfer Systems

Series–series (SS) and series–parallel (SP) topologies are widely used in resonant inductive coupling wireless power transfer systems for various applications. However, the selection of an appropriate topology to achieve higher output power or higher efficiency is typically difficult because design optimization of the circuit parameters (e.g., characteristic impedance, load resistance, and mutual inductance) for each topology is generally separately discussed using different equivalent circuits with multiple resonance modes. Therefore, the purpose of this study involves proposing a simple strategy to select an appropriate topology. The proposed strategy is based on quasi-duality between the SS and SP topologies that are elucidated from the novel equivalent circuits derived using Lagrangian dynamics. Based on the quasi-duality, the output power and efficiency of the SP topology are calculated via the equivalent circuit of SS topology. Thus, the quasi-duality offers a simple comparison between the SS and SP topologies. The proposed strategy selects an appropriate topology by comparing only the equivalent ac load resistance, which is the ac resistance including the rectifying circuit and the load resistance, the characteristic impedance, and the ac load resistance that achieves the maximum efficiency or maximum output power of the SS topology. Experiments verify the appropriateness and effectiveness of the proposed strategy.