Extended-range two-coil adaptively reconfigurable wireless power transfer system

It is well known that the power transfer efficiency (PTE) of a two-coil wireless power transfer (WPT) system is maximized at critical coupling under fixed system parameters. However, the two-coil WPT system is critical-coupled with a relatively short distance between the coils. To overcome the problem, a multicoil WPT system has been studied. In this paper, we review the impedance characteristics of the critical-coupled two-coil WPT system and reveal that the conventional three-coil WPT system cannot have the same impedance characteristics as critical coupling. Based on the impedance matching conditions with critical coupling of the two-coil WPT system, we propose a design method of a three-coil WPT system in which the free resonant frequencies of all resonators are changed. The proposed design method is successfully implemented at the operating frequency of 6.78 MHz. The experimental results are in good agreement with the simulated PTEs and show that by changing the free resonant frequencies, a higher PTE can be effectively achieved in comparison to the conventional two-coil and thee-coil WPT systems.

Wireless power transfer (WPT), a technology full of promise, whose electromagnetic compatibility (EMC) has become a vital issue. In this study, we investigated the electro-magnetic emission generated by WPT systems using resonant magnetic field coupling, especially in applications with two-coil configurations. A novel finite element analysis based model for the two-coil WPT system is proposed. Useful information about the distribution of magnetic field can be acquired from the proposed model. The influences of coil distance, coil angle and probe distance on the magnetic field is also studied in detailed. Finally, a two-coil WPT testbed is built to provide an intensively investigation for the electromagnetic emission.

Wireless Power Transfer (WPT) technology is able to transmit electric power from the Tx side to Rx side without any electrical connection, realizing electrical isolation and breaking through the limitations of electric wires. Traditionally, finding the best working point of the WPT system is difficult as there are a great number of coupled parameters to tune. Besides, the experimenter has to be on site to carry out the experiment with limitations such as time, location, safety issue as well as sharing issue. In this paper, a two-coil structure WPT system is integrated into web-based online laboratory NCSLab using a controller and a DAQ (data acquisition) card as well as an user-defined algorithm. With the latest technologies brought in, NCSLab is completely plug-in free for experimentation on the WPT system. The optimum frequency can be easily obtained by setting the system in the sweep-frequency mode using the remote control platform. The remote control platform NCSLab addresses the safety issue and test rig sharing issue by offering experimenter flexibility to carry out WPT experiment anytime anywhere as long as the Internet is available. T he integration of WPT system into NCSLab also provides teachers with a powerful tool for classroom demonstration of state-of-the-art technology.

This paper presents a new resonance circuit design method for LCC-LCC wireless power transfer (WPT) systems that incorporate reactive shielding (SH) coils on the receiver (RX) side to suppress the electromagnetic field (EMF). While reactive SH coils are known to reduce leakage magnetic fields, they alter the equivalent inductance of the system, thereby disrupting resonance conditions. To address this, we derive the changes in the equivalent inductance caused by SH coils and propose a method to re-select the series capacitor on both the RX and TX sides. Furthermore, we investigate the adjustment of the required input voltage to maintain output power with the SH coils. The proposed methodology eliminates the need for a series capacitor on the RX side, simplifies the network, and reduces the magnetic leakage field by up to 55.6%, as verified by the simulation and measurement results. This study provides a new pathway toward compact, EMF-conscious and LCC-based WPT systems.

This paper presents a theoretical analysis, an optimal design method, and experimental results for a wireless power transfer (WPT) system with an intermediate coil. The analytical expression of the dc voltage transfer function is presented and discussed. In a two-coil WPT system, which has low coupling coefficient, the intermediate coil boosts the apparent self-inductance and magnetizing inductance of the primary side at around the resonance frequency of the intermediate coil, so that the apparent coupling coefficient is compensated. The coupling coefficient makes the system efficiency increase and induces bifurcation phenomenon. From the analysis, this paper proposes an optimal design method using the second resonance frequency operation with the bifurcation phenomenon and presents design procedure for high efficiency. A prototype of the WPT system with the intermediate coil is implemented and experimented to verify the validity of the analysis and the proposed design method. The prototype operates at 100 kHz switching frequency and has an air gap between primary and secondary side of 200 mm. An overall system efficiency of 95.57% has been achieved at 6.6 kW of output power.

This paper presents RX-Sensorless technologies required in a self-induced type of low power wireless power transfer system. Recently, apart from the fields of smartphone and electrical vehicles which have been arduously studied for application of wireless power transfer, wireless power transfer technologies are being applied to various fields. Among them, there is a very rapidly increasing demand for low power systems in displays, body implant medical devices, drones, electrical kickboards and so on. In order to acquire portability of RX side of low power systems, the most critical factors are power density and efficiency. For this purpose, there have been very much ongoing studies to improve the power density on the RX side. Among various research, most frequently used types of methods involve derivation of output voltage using the physical quantity on the TX side while removing sensors on the RX side. However, in the modeling process to derive output voltage, they fail to properly reflect the parasite components of wireless power transfer and also due to the lack of measures in case of being out of the range of resonance frequency, there tends to be a high error in the output voltage which raises an issue. Therefore, this paper aims to suggest a model which reflects the parasite resistance components and the permeability rate of medium for the TX/RX pads of wireless power transfer systems. By reflecting winding losses and eddy current losses of TX/RX pads to a model, it is possible to reflect the parasite resistance components and using the Neumann formula, it is possible to reflect it to the medium between RX/TX pads. As a result, based on the derived model, it is possible to control the output voltage. Prototypes of hardware are designed and implemented for verifications. According to the computer simulation results and test results, output voltage estimates yield an error in the range of less than or equal to 8% and model-based output voltage control also shows the control range with errors of less than or equal to 5%.

The output power of a magnetic resonant coupling wireless power transfer (WPT) system drastically decreases when the distance of couplings coils changes due to impedance mismatch. A maximum power point occurs at only one optimal distance. These characteristics are caused by limitation in design stage. This paper proposes a new impedance tuning method of tracking, for maximum power point, by adjusting resonant frequencies and capacitance values according to the varied distance. The theoretical performance is analyzed using the Thevenin equivalent circuit of the WPT. In the experiment, a prototype of two-coil wireless power transfer system is created. The power transferred to a resistive load for the cases of constant frequency and the proposed impedance tuning method are compared. Simulation and experimental results show that the maximum power point tracking under coil distance changes is made possible. This work can be useful in a design stage of wireless power transfer.

Since the concept of wireless power transfer has been proposed and implemented by Tesla over a hundred years ago, it has obtained lots of breakthroughs and applied in many potential fields especially in implantable biomedical devices [1–2]. The implantable biomedical devices are surgically introduces into the human body to rebuild body function, achieve a better quality of life, and expand longevity. With the development of microelectronics, biotechnology, and materials, the industry of implantable biomedical devices grows up quickly. The researches show that in America 8% to 10% of the people have experienced an implantable medical device, while in industrialized countries 5% to 6% of population have consumption demand in implantable medical devices. How to provide a constant power supply, for implantable medical devices, becomes a restricted problem. In order to guarantee the implanted medical devices to work normally, the energy storage or harvesting components should be included in these devices. In the traditional implantable medical devices, there are two ways to provide power for the devices. The common method is the implantable batteries are used to be as a power supply, and the other method is to transfer the power in vitro to the devices with the lead wires through the skin. For implantable batteries, their lifetime, size and toxic composition will lead to potential hazard to the patients. The transcutaneous wires, as well, will bring infection and reliability problems. In order to address above problems, the technology of wireless power transfer is used in the biomedical devices. The system has two parts in which one part including power supply and the transmitter coil is placed outside the patients’ body, and the other part including the receiver coil is placed in the patients’ body. For this system, the receiver coil should be limited as small as possible and the power transfer efficiency should be as high as possible. Different from the traditional two-coil wireless power transfer system, a novel three-coil system is proposed in this paper. In the novel system, there are two rectangular transmitter coils and one circular receiver coil. To obtain the maximum power transfer efficiency, the system is optimal designed according to the following step as shown in Fig. 1. Firstly, the size and the geometry of the receiver coil are determined. A circular coil with a radius of 10mm is chosen as the receiver coil. Secondly, the distribution of the magnetic field generated by the rectangular coil is analyzed based on the Biot-Savart’s law. Then the relationship between the distance and the magnetic field intensity will be obtained and expressed as a formula according to the analysis result. Finally, the above formula is chosen as the optimization constrains and the power transfer efficiency is chosen as the optimization objective function. After the optimal designing, the optimal parameters of the transmitter coils are obtained and verified through the experimental system as shown in Fig. 2. The power transfer distance is about 30cm, and the power received by the receiver coil can achieve about 5W. The detailed results will be given in the full paper.

Variation of spatial scales and load resistances have significant effects on the transmission power of magnetically coupled resonant (MCR) wireless power transfer (WPT) system. To realize maximum power transmission under arbitrary spatial scale and different load resistances, a control scheme for automatic maximum power point tracking (MPPT) operation was presented in this paper. Based on the transmission power characteristics of a two-coil WPT system with a series resonant topology, a SEPIC converter was introduced to cascade at the receiving side to achieve MPPT for MCR WPT system. Adopting perturbation and observation (P&O) method to adjust equivalent input resistance of SEPIC converter dynamically, the proposed scheme tracked the maximum transmission power operating point by matching the corresponding mutual inductance with varying spatial scales. The experimental results had shown the validity of the theoretical analysis and the feasibility of the proposed scheme.

A Wireless Power Transfer (WPT) system, known for its contactless power delivery, is extensively used for power supply in spacecraft applications. Achieving efficient and stable power transfer necessitates the integration of DC/DC converters on both the primary and secondary sides of WPT systems for power conversion and control. Traditional efficiency optimization methods primarily focus on impedance matching within the wireless power resonance network, often neglecting the overall efficiency optimization of multi-stage DC-DC and WPT systems. This oversight results in suboptimal overall system efficiency despite optimal efficiency in the wireless transmission segment. Additionally, the time-varying nature of mutual inductance and load parameters during power transmission in WPT systems presents challenges for maximum efficiency tracking and power control. This paper introduces a multi-level coordinated control efficiency optimization method for WPT systems utilizing the particle swarm optimization (PSO) algorithm. This method takes into account the transmission losses across all power conversion units within the WPT system, establishing a mathematical model for the joint optimization of overall system transmission efficiency and power. The PSO algorithm is then employed to solve this optimization model using estimated mutual inductance and load values. By adjusting the DC/DC converters on both sides, the method ensures optimal overall system efficiency and consistent power transmission. Experimental results indicate that under varying load and mutual inductance conditions, a Series–Series (SS) compensated WPT system using this method achieves a 200 W power output with maximum efficiency tracking, a power output error of 0.63%, and an average transmission efficiency of 86.2%. This demonstrates superior power transmission stability and higher efficiency compared to traditional impedance matching methods.

The wireless power transfer (WPT) system based on parity-time (PT) symmetry has the advantages of robustness, stable, and efficient power transmission. However, the region of PT symmetry is limited, which cannot satisfy some practical requirements. Multiple transmitting coils have been shown to improve the misalignment tolerance of WPT systems. Towards this end, a parity-time-symmetric-based WPT (PT-based WPT) system with double-transmitting coil is presented in this article. First, an approximate equivalent circuit model of this PT-based WPT system was developed. Second, compared with the conventional single transmitting coil PT-based WPT system, the proposed system expands the PT symmetry region and provides higher output power and transmission efficiency. Furthermore, the mutual inductance between two transmitting coils can be designed to meet the requirements of different scenarios, which improves the freedom of system design. Finally, a 30 W prototype is designed to evaluate the proposed WPT system. Experimental results show that the critical coupling coefficient is reduced by about 12.5% at <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> = 0.51, which implies a longer transmission distance. Moreover, the maximum output voltage fluctuation in the strongly coupled region is only 3.5%, while the transfer efficiency is approximately constant, which implies stable and efficient power transfer.

In multi‐input and multi‐output (MIMO) wireless power transfer (WPT) systems, the destructive interference between multiple transmitters and receivers brings a serious impact on power transmission efficiency. This paper proposes a voltage control strategy to maximize the transmission efficiency of a MIMO WPT system by controlling the amplitude and phase of the transmitter voltages. However, the complexity of the realistic WPT circuit topology and the uncertainty in the related parameters obstruct the establishment of an accurate closed‐form description of the WPT model to optimize the transmission efficiency of the system. In this paper, a simple multi‐port network model is introduced to simulate a MIMO system to avoid the complex circuit topology and a large number of circuit elements. To determine the parameters of the proposed model, a simple yet efficient methodology, by sequentially changing the excitations of the input ports and sampling the responses at the out ports, is proposed. A voltage control methodology to optimize the transmission efficiency of a MIMO wireless power transfer system is finally developed. To validate the proposed model and voltage control methodology, a three‐input and three‐output WPT prototype is designed and fabricated and used as the experimental platform. The performances of the proposed voltage control methodology are compared to both the experimental ones and those without the proposed voltage control methodology.

In the existing pieces of literature about the three-coil wireless power transfer (WPT) system, a three-coil structure is proposed to increase the efficiency compared to the two-coil WPT system. The cross-coupling among three coils, compensation capacitor of the repeating coil (RPC) and the rectified load are considered incompletely. The circuit model of a three-coil WPT system with rectified load is developed by taking the cross-coupling of all coils and the compensation capacitor of RPC into consideration in this paper. The relationship among system efficiency, compensation level, and rectifier load is deduced. The detailed mathematical analysis and compensation capacitor selection of RPC are investigated to improve the system efficiency. An experimental study demonstrates that the selected compensation capacitor of RPC achieves a higher efficiency than resonant compensation in a three-coil WPT system with different cross-coupling conditions and rectifier load conditions. System efficiency is promoted from 73.9% (resonant compensation capacitor for RPC) to 84.2% (selected compensation capacitor for RPC).

The magnetic fields (MF) generated in close proximity to high power wireless power transfer (WPT) systems generally exceed exposure limits and might also violate basic restrictions (BR). However, there are as yet no available standards describing methods to measure the MF generated by WPT systems. In this study, a new method for demonstration of compliance with the BR in the close proximity to sources at frequencies from 3 kHz to 10 MHz is proposed. This approach mitigates overestimation by incident field assessment in strong field gradients close to WPT systems. The spatial peak MF amplitude and gradient generated by WPT systems are measured with a field gradient probe at the closest location to the source. The induced fields and absorbed power are then calculated on the basis of specific transformation functions for testing of compliance with the BR of all international safety guidelines.

The magnetic coupling of a practical wireless power transfer (WPT) system is greatly influenced by the variations in the alignment of the magnetic couplers. For a given coupling range, a WPT system is expected to output the required rated power with a given load. However, this requirement cannot be easily met by the widely adopted series–series or inductor–capacitor–capacitor compensation topologies in a conventional two-resonator WPT system. This article proposes a design methodology for three-resonator WPT systems to achieve a constant output power in a wide coupling range. The basic characteristics of the three-resonator WPT system are reviewed and the proposed design methodology is presented in detail. A 3.3-kW three-resonator WPT prototype is designed and built, achieving nearly constant output in the given misalignment range. The measured dc–dc efficiency of the prototype reaches 94.47 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> at the rated power.