Magnetic Resonance Wireless Power Transfer System Research Articles

Axial magnetic quadrupole mode of dielectric resonator for omnidirectional wireless power transfer

To achieve omnidirectional wireless power transfer (WPT) with high efficiency, a high Q-factor transmitter generating homogeneous magnetic field is crucial. Traditionally, orthogonal coils of different shapes are used to realize transmitters. In this paper, we develop an omnidirectional magnetic resonant WPT system based on a dielectric disk resonator with colossal permittivity and low loss operating at the axial magnetic quadrupole mode. The constant power transfer efficiency of 88% at the frequency of 157 MHz over the transfer distance of 3 cm for all azimuth angles of a receiver is experimentally demonstrated. The possibility of multi-receivers charging is also studied demonstrating a total efficiency of 90% regardless of azimuth angle between two receivers with respect to the transmitting disk resonator. The minimized exposure of biological tissues to the electric and magnetic fields as well as a low specific absorption rate is observed that makes the WPT system safer for charging with higher input power.

Gallium Nitride Power Devices in Magnetically Coupled Resonant Wireless Power Transfer Systems

The main function of power devices is to convert electrical energy through high-speed switching, such as AC/DC, high and low voltage conversion, etc. Therefore, the performance of the device directly affects the performance of the power electronic device, thereby further affecting the conversion efficiency of electrical energy. The arrival of the 5G era has greatly increased the demand for gallium nitride (GaN). The development of the wireless communication market has made GaN play a key role in many aspects of human activities. The main aim of this article is to research the application of GaN power devices in magnetically coupled resonant wireless power transfer (WPT) systems. This paper first introduces the related concepts of GaN power devices, magnetic coupling resonance and WPT, and analyzes the construction of magnetic coupling resonance WPT system in detail. Secondly, the energy transmission structure is analyzed, the system is designed and the system loss is calculated and analyzed. The system loss experiment shows that by using the output capacitor of the GaN device, the dead time is optimized, the conduction loss of the switch tube is reduced, the efficiency of the converter device is improved, and the practical application of the GaN device is provided.

Reduction in Human Interaction with Magnetic Resonant Coupling WPT Systems with Grounded Loop

Wireless power transfer (WPT) systems have attracted considerable attention in relation to providing a reliable and convenient power supply. Among the challenges in this area are maintaining the performance of the WPT system with the presence of a human body and minimizing the induced physical quantities in the human body. This study proposes a magnetic resonant coupling WPT (MRC-WPT) system that utilizes a resonator with a grounded loop to mitigate its interaction with a human body and achieve a high-efficiency power transfer at a short range. Our proposed system is based on a grounded loop to reduce the leakage of the electric field, resulting in less interaction with the human body. As a result, a transmission efficiency higher than 70% is achieved at a transmission distance of approximately 25 cm. Under the maximum-efficiency conditions of the WPT system, the use of a resonator with a grounded loop reduces the induced electric field, the peak spatial-average specific absorption rate (psSAR), and the whole-body averaged SAR by 43.6%, 69.7%, and 65.6%, respectively. The maximum permissible input power values for the proposed WPT systems are 40 and 33.5 kW, as prescribed in the International Commission of Non-Ionizing Radiation Protection (ICNIRP) guidelines to comply with the limits for local and whole-body average SAR.

Optimization and Analysis of Multilayer Planar Spiral Coils for the Application of Magnetic Resonance Wireless Power Transfer to Wearable Devices

In this study, small multilayer planar spiral coils were analyzed and optimized to wirelessly charge an in-ear wearable bio-signal monitoring device in a wine-glass-shaped transmitter (Tx) based on magnetic resonance wireless power transfer (MR-WPT). For analysis of these coils, a volume filament model (VFM) was used, and an equivalent circuit formulation for the VFM was proposed. The proposed method was applied to design effective multilayer coils with a diameter and height of 6 and 3.8 mm, respectively, in the wearable device. For the coils, a printed circuit board having a 0.6 mm thick dielectric substrate and a 2 oz thick copper metal was used. Moreover, the coils on each layer were connected in series. The dimensions of the double-, four-, and eight-layer coils were optimized for the maximum quality factor (Q-factor) and coupling efficiency. The operating frequency was 6.78 MHz. The optimal dimensions for the maximum Q-factor varied depending on the number of coil layers, pattern width, and turn number. For verification, the designed coils were fabricated and measured. For the four-layer coil, the coupling efficiency and Q-factor using the measured resistance and mutual inductance were 58.1% and 32.19, respectively. Calculations showed that the maximum Q-factor for the four-layer coil was 40.8 and the maximum coupling efficiency was 60.1%. The calculations and measurement were in good agreement. Finally, the entire system of the in-ear wearable bio-signal monitoring device, comprising a wine-glass-shaped transmitter, the designed receiving coil, and a monitoring circuit, was fabricated. The measured dc-dc efficiency of the MR-WPT system was 16.08%.

Selection of high transfer stability and optimal power-efficiency tradeoff with respect to distance region for underground wireless power transfer systems

Magnetic resonant wireless power transfer (MR-WPT) has become a significant method for powering the measurement units in landslide monitoring boreholes. This paper focuses on the effects of quality factor, coupling distance and load on the power transfer stability, sensitivity and performance of an MR-WPT system. Firstly, through a numerical model of an MR-WPT, the effect of quality factor and coupling distance on the output power, transfer efficiency and frequency splitting phenomenon of MR-WPT systems have been comparatively analyzed. The relationship between the load and the coupling distance region corresponding to optimal transfer performance has been studied. Results show that improving the quality factor of the coils is beneficial for the transfer performance and stability of WPT system. Furthermore, through the selection of the load, a coupling distance region with high transfer stability and an optimal tradeoff between power and efficiency can be obtained. Finally, the above theoretical simulation results have been verified by experimental results.

Effect of Vertical Metal Plate on Transfer Efficiency of the Wireless Power Transfer System

Power transfer efficiency is an important issue in wireless power transfer (WPT). In actual applications, the WPT system may be exposed to a complex electromagnetic environment. The metal which is inevitably or accidentally close to the system will impact the power transfer efficiency. Most previous research has aimed at the effect of the metallic sheet paralleled to the resonant coil. This paper focuses on the effect of the metallic plate perpendicular to the resonant coils. Firstly, based on the theoretical analysis, a simulation model is setup using COMSOL Multiphysics. The efficiencies of the double-coils magnetic resonant WPT system with the presence of the parallel and vertical aluminum plate are studied comparatively. Efficiency improvement is observed with the vertical plate while the reduction appeared with the presence of the parallel plate. The vertical metallic plate has shown a magnetic field shielding effect according to the magnetic field distribution. It can reduce the radial magnetic field and enhance the axial magnetic field. Then, the effects of the position and size of the vertical plate are studied. It is found that the transfer efficiency has a preferable improvement when the vertical aluminum plate with a larger size is placed between the resonant coils and near outer edge of the windings. Finally, the experiment is carried out to verify the effect of the vertical aluminum plate on the WPT system.

Modeling and Experimental Validation of a Butterfly-Shaped Wireless Power Transfer in Biomedical Implants

Wireless power transfer (WPT) presents a safe and robust method for powering biomedical implants. The range and misalignment tolerance restrictions of the WPT system are the main factors, conducive to the transfer efficiency degradation. Especially, the implanted device is invisible and difficult to arrange in alignment with the transmitting coil. This paper proposes a butterfly-shaped transmitting (BS-Tx) coil of a magnetic resonance WPT system for biomedical implants. The BS-Tx coil prototypes with the changing driven current directions, parallel and anti-parallel current directions, are verified and analyzed. Besides, an equivalent circuit model of a coupling two-coil system with an optimal pair of capacitors method for the matching network is mathematically derived to obtain the maximized power transfer efficiency. The BS-Tx coil enhances the efficiency of the system in the distance, angle, and axial misalignment tolerances. A practical WPT system is fabricated and measured to verify the transfer efficiency through the real biological tissue and exhibits a significant efficiency improvement. Finally, a hypothetical context of retinal prosthesis application is demonstrated as a potential application to retinal implants. The proposed system shows a low specific absorption rate, that is satisfied the human safety regulations.

Experimental and Numerical Study of a Magnetic Resonance Wireless Power Transfer System Using Superconductor and Ferromagnetic Metamaterials

In this paper, a magnetic resonance wireless power transfer (WPT) system with two coils using superconductor and ferromagnetic (SC-FM) metamaterials was constructed and tested. SC-FM metamaterials were used in the WPT system to improve the transfer efficiency of the magnetic resonance WPT system. Three kinds of resonance coils were designed and the effects on the WPT system of different resonance coils using SC-FM metamaterials were investigated. This involved designing and conducting several kinds of WPT experiments with, and without, the SC-FM metamaterials. The results are then presented and analyzed. It is shown that the SC-FM metamaterials are able to improve the transfer efficiency of a WPT system.

Characteristics of the Magnetic Resonance WPT System Applying a Frequency Converter

Recently, studies on the commercialization of magnetic resonance wireless power transfer (WPT) systems are underway domestically and internationally. The magnetic resonance WPT technology enables power transmission between transmitter and receiver coils with the same resonance frequency. If the resonance frequencies of the transmitter coil and receiver coil are different, the efficiency of the WPT will rapidly decrease. Various organizations, such as A4WP, have been striving to standardize resonance frequencies, but the countries around the world are sticking to their own distinctive standards based on the frequency bands available in their territories. Therefore, each country uses a slightly different resonance frequency band. In this paper, a frequency converter that enables WPT when the transmitter and receiver coils have different resonance frequencies is proposed. The frequency converter consists of two coils, and its structure is very simple. The two coils transmit power using the magnetic induction method, and frequency variation is possible using variable capacitors. It was confirmed from the experiment that WPT is possible even if the resonance frequencies of the transmitter and receiver coils are different.

MR-WPT With Reconfigurable Resonator and Ground for Laptop Application

A magnetic resonance wireless power transfer (MR-WPT) system with a reconfigurable planar receiver (Rx) to optimize transfer efficiency (TE) for laptop application has been investigated. The proposed MR-WPT system consists of a general four-coil system: a source coil and a transmitter Tx) coil in a Tx part and an Rx coil and a load coil in an Rx part. To enable performance comparison, the Rx coil uses either a rectangular loop resonator or a slit ground resonator (SGR) and both resonators are connected with a p-i-n diode to reconfigure and optimize the MR-WPT system’s TE along the transfer distance between the Tx and Rx parts. Mobile devices, including laptops, need a ground plane for practical use; therefore, the Rx coil is replaced by only an SGR, which operates as both a resonator and a ground. This letter, experimentally and theoretically proves that a reconfigurable MR-WPT system with an SGR has sufficient efficiency without the ferrite sheet typically used in practical MR-WPT applications.

<italic>In Vivo</italic> High-Efficiency Wireless Power Transfer With Multisine Excitation

This paper presents a systematic design of a high-efficiency magnetic resonant wireless power transfer (MR-WPT) system for biomedical implants based on the unconventional multisine (MS) transmission waveform. The MS waveform featuring a high peak to average power ratio can boost the system efficiency with an elaborate design. Optimizations on flexible coil, system model, and rectifier design are all considered to strengthen its efficiency improvement as well as eliminate its drawbacks. With all the proposed optimizations implemented, the MS waveform has been applied for the first time in practical MR-WPT system for in vivo power delivery. A 6.78-MHz MR-WPT system with 18-mm implant depth is realized for wirelessly powering a spinal cord stimulator. It achieves an overall system efficiency (both coils and rectifier included) of 50.7% with a three-tone 230-kHz tone spacing MS waveform postimplantation in the rodent model. Low specific absorption rate and the tissue temperature rising from electromagnetic fields in the body are also verified to guarantee a safe and practical wireless power link for biomedical implants.

3D‐spatial efficiency optimisation of MR‐WPT using a reconfigurable resonator‐array for laptop applications

In this study, magnetic resonant wireless power transfer (MR-WPT) system with a reconfigurable resonator using RF switches (PIN diodes) was studied to improve and optimise the transfer efficiency (TE) in a limited three-dimensional (3D) zone for laptop applications. Generally, the TE of the MR-WPT system is decreased by a 3D spatial variation (distance and location) in between the transmitter (Tx) source and the receiver (Rx) load . To improve and optimise the TE for this spatial variation, the authors utilised RF switches in a planar arrayed coils in the Rx. The TE was optimised by the status of the coil resonator (modes 1-6, open, and short) in the Rx. The average TE of the optimal mode increased >5% comparing with that of a certain mode in the whole range of the spatial variation ( x , y , z directions; 80 × 80 × 40 cm 3 ). In addition, they analytically and experimentally demonstrated the TE optimisation by the practical laptop model with the proposed MR-WPT system installed into a desk ( yz -plane) and the wall ( z -direction). Through this research, the proposed reconfigurable resonator with RF switches was shown to be possible for use as a reconfigurable efficiency optimiser in the MR-WPT system.

Numerical Analysis of Magnetic-Shielding Effectiveness for Magnetic Resonant Wireless Power Transfer System**Supported by the National Natural Science Foundation of China under Grant No 51377185.

Magnetic radiation phenomena appear inevitably in the magnetic-resonance wireless power transfer (MR-WPT) system, and regarding this problem the magnetic-shielding scheme is applied to improve the electromagnetic performance in engineering. In this study, the shielding effectiveness of a two-coil MR-WPT system for different material shields is analyzed in theory using Moser’s formula and Schelkunoff’s formula. On this basis a candidate magnetic-shielding scheme with a double-layer structure is determined, which has better shielding effectiveness and coils coupling coefficient. Finally, some finite element simulation results validate the correctness of the theoretical analysis, and the shielding effectiveness with the double-layer shield in maximum is 30 dB larger than the one with the single-layer case.

Frequency Split Elimination of Short Range Wireless Power Transfer System by Active Matching Tuning Circuit

Objective: The main theme of this paper is to eliminate the frequency split in short range Wireless Power Transfer (WPT) system by magnetic resonance coupling. Analysis: The common basic two coils based magnetic resonance WPT system is taken for the analysis; the frequency and transfer coefficient are plotted for different dimensions of the coil. Using genetic algorithm, the lumped impedance parameters are determined, which is used for the tuning of the active matching circuit. Based on the experimental results it has been observed that the frequency split can be eliminated by tuning the matching circuit properly for the corresponding distance and higher efficiency can be achieved for short range. Findings: The frequency split elimination techniques can be used in dynamic transmitter and receiver wireless charging system. Application: This system can be employed in short range wireless power transfer such as neural implants, capsule robots, micro-robots and electric vehicle charging and so on.

Magnetic Resonant Three-Coil WPT System Between Off/In-Body for Remote Energy Harvest

In this letter, the effect of the human body in a magnetic resonance wireless power transfer (MR-WPT) system for wearable and implantable devices is studied. To implement the experiment, muscle-mimicking liquid is configured with parameters similar to those of the human body. The transmitter part is located outside the human body (in the air), and the receiver part is implanted in the liquid. The proposed 3-coil MR-WPT system, which operates at 6.78 MHz shows effectively higher transfer efficiencies compared with a 4-coil MR-WPT system at near transfer distance which is the predicted position for a pacemaker. The results indicate that the proposed system with optimized receiver using air-pocket has potential applications in off/in-body devices that benefit from remote energy harvesting, especially at near distances such as pacemakers.

Development that resonates

Professor Chang Won Jung from Seoul National University of Science and Technology talks to us about his research on resonators. The team used copper wires optimise the transparent film resonators The need for charging electronic devices has been becoming a critical issue. As the number of portable devices has increased, due to the popularity of smart phones and tablets, the ability to conveniently charge them has become essential. In their laboratory in Seoul, researchers have improved on the current technology and applied it to other areas including wireless energy harvesting. This process is called wireless power transfer (WPT) and is a transmission method of electricity from a source to a load without conductors, usually via an electromagnetic field. There are two main types of wireless power transfer, Magnetic Resonance WPT (MR-WPT), and Magnetic Inductive WPT (MI-WPT). These rely on the occurrence of resonance magnetic coupling. In Magnetic Resonance WPT, electric energy is transferred in the magnetic field, whereas Magnetic Inductive WPT operates in the near field. Generally, the transfer efficiency of MI-WPT is higher than that of MR-WPT over short distances, but the transfer distance of MR-WPT is longer than that of MI-WPT. MI-WPT has been studied extensively and the technologies developed are now available commercially, including chargers for mobile devices. However, in the case of the MR-WPT, several problems remain and more research is needed before they can be used commercially. One of these problems is efficiency. By adding capacitors to the MR-WPT system the efficiency can be increased as the resonators correspond to the power energy at the same resonant frequency. The resonant frequency of the resonator depends on inductance and capacitance. For example, increasing the inductance and capacitance of the resonator reduces the resonant frequency. It is not easy to reduce the resonant frequency using inductance and keep the system small enough for mobile use. Adding capacitors adjusts resonant frequency and allows the system to remain small. Therefore, matching the resonant frequency between resonators could help to increase the transfer efficiency of MR-WPT system. Until now transparent WPT hasn't been widely researched because of its poor conductivity. The letter highlighted in this feature presents new advances in the field. The team looked into applications of transparent film resonators such as smart windows and mobile devices. They found that as the transparent WPT system needed be small it wasn't big enough to fabricate the WPT system on. They solved this problem by designing a new, compact, WPT system using copper. Their initial test samples performed badly but through precision processing and optimisation of fabricating conditions they improved the functionality of transparent WPT. This progress paves the way for future novel uses of WPT. Transparent WPTs are incredibly useful due to their transparency. It allows them to be placed anywhere where electricity is needed; from small sensors and mobile devices, to smart vehicles and smart windows. The team are now working on wearable WPT for biomedical sensors that are implanted in the human body and needed continuous energy to function. These sensors could be among the first applications of this project. The technology will allow advancements in smart objects such as phones and glasses The group have been improving transfer efficiency of WPT by changing materials and designing spiral patterns. To give free movements of WPT, they are now researching the alignment of WPT and how to compensate for when the WPT is out of alignment by using impedance matching and optimisation. There are many possibilities for the future use of WPT. WPT technologies could be widely used in various fields, not only in the electronic field but also medical and clothing fields. Professor Jung hopes that in the future many people will be using this technology in their everyday lives, as devices such as mobile phones or pacemakers would be able to charge anywhere.

Analysis of -Parameters in Magnetic Resonance WPT Using Superconducting Coils

After Prof. Marin Soljacic of Massachusetts Institute of Technology proposed the magnetic resonance wireless power transfer (WPT) in 2007, magnetic-resonance-type WPT has been tried to be applied to various charging equipment. The need for research of magnetic resonance WPT technology is increasing owing to the expectation on its application to electric vehicle and electric railway. In this paper, we used the superconducting coil in order to improve the transmission efficiency of the magnetic resonance WPT system. We found that the superconducting coil indicated higher transmission efficiency and $Q$ -factor bigger than that of the normal conductor coil through the analysis of the $S$ -parameters between the superconductor and normal conductor coils. If this research will be continued, it is expected to be applicable to high-power transmission systems.

Efficiency optimising scheme for wireless power transfer system with two transmitters

The highest power transfer efficiency (PTE) of the magnetic resonance wireless power transfer (WPT) system with two transmitters (TXs) and a single receiver located at an arbitrary position can be obtained by adjusting the amplitude ratio and the phase difference (PD) of the signals of two adjacent TXs. The coupling coefficient between two coils is calculated, and is used to establish an equivalent circuit model for a WPT system with two TXs. Using the circuit model, the PTE of the WPT system (which is a function of the amplitude ratio and PD) is extracted. The optimum conditions of these values to achieve the highest power efficiency can be analytically determined. The experimental results agree well with the simulated results based on the analysis. The result shows that the suggested optimised transmit scheme can achieve the highest PTE for a magnetic resonance WPT system with two TXs.

Rapid design approach of optimal efficiency magnetic resonant wireless power transfer system

A rapid design approach to achieve the optimal efficiency in a magnetic resonant wireless power transfer (MR-WPT) system is presented. A concise design guideline is provided to determine the optimal efficiency condition and related coupling coefficients. Experimental validations are demonstrated by both simulation and measurement results of a 6.78 MHz MR-WPT system prototype with the maximum efficiency of 76.1% at a 50 mm distance between transmitter and receiver.

Human Exposure to Close-Range Resonant Wireless Power Transfer Systems as a Function of Design Parameters

In this study, human exposure to close-range wireless power transfer (WPT) systems operating in the frequency range 0.1-10 MHz with coil diameters up to 150 mm are investigated. Approximation formulae, which include scaling factors derived from numerical simulations that take variations of complex human anatomies into consideration, are proposed to conservatively estimate human exposure with respect to the most authoritative exposure guidelines. The approximation has been verified for two precommercial prototype WPT systems, the first of which, a 5-W system operating at 100 kHz, has been evaluated in this study; the second system been verified was reported in a separate study and operates at 6.78 MHz with a nominal current of 5.4 A rms. Based on the results obtained, the optimal operational frequency range for WPT with respect to compliance with exposure safety guidelines is revealed to be ca. 1-2.5 MHz. In summary, this study provides novel and insightful information for the design of an exposure-compliant close-range magnetic resonant WPT system.