Resonant Wireless Power Transfer System Research Articles

A Real-Time Calibrated Adaptive-ZVS Class-D Power Amplifier for 6.78 MHz Wireless Power Transfer System

This brief presents a class-D power amplifier (PA) with an adaptive zero-voltage switching (A-ZVS) technique for 6.78 MHz resonant wireless power transfer (R-WPT) system. In R-WPT operation, the loading impedance of a PA can be varied by the process tolerance of the LC resonant components or WPT environments, such as the resonant topology, coupling coefficient, and loading condition of the receiver. In particular, the commonly used series-parallel resonant topology suffers from resonant point deviation of primary LC tank, resulting in hard switching of the PA. Consequently, efficiency degradation and electromagnetic interference can occur under multi-MHz operation. The proposed A-ZVS feedback loop calibrates the equivalent resonant capacitance in real-time to achieve ZVS by adjusting the loading impedance to be slightly inductive. The proposed PA was fully integrated except for one switched capacitor used as the tuning element and fabricated in a TSMC 0.18 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> BCD process. The measurement results demonstrated robust ZVS operation with a peak system efficiency of 52.7 % and an enhanced maximum transmitting power of 107 %.

Enhancement of robustness to frequency detuning with wireless power transfer based on Van der Pol resonance

AbstractIn near‐field energy transmission, wireless power transfer based on magnetic coupling resonance is preferred. For efficient power transfer, the quality factor of magnetic coupling resonance wireless power transfer (MCR‐WPT) system should be kept at a high value, which makes the system sensitive to frequency detuning. Different from conventional approaches dealing with the problem in linear MCR‐WPT system, this paper proposes a novel MCR‐WPT system, which is based on Van der Pol nonlinear resonant circuit to address the problem of frequency detuning. A nonlinear resistor is introduced, and the nonlinear Van der Pol MCR‐WPT system is established. Based on the mathematical model and the theoretical analysis, it is found that the frequency band is broadened. To validate the performance of the proposed system, extensive simulation and experimental work are conducted. Unlike the traditional linear system, which only peaks at the resonant frequency, the results show that the output of the proposed system is still high within a certain frequency band when the operating frequency deviates from the original resonant frequency. The impact of frequency detuning caused by variation of capacitance is also studied. It can be concluded that the proposed MCR‐WPT system is effective in coping with the frequency detuning.

Model of fractional-order resonant wireless power transfer system for optimal output

Abstract Wireless Power Transfer (WPT) technology has recently gained popularity in applications and research topics. It enables the transfer of electrical energy from a source to a load without connecting wires physically. The WPT system is commonly studied classically using integer order capacitors and inductors. Nonetheless, such integer order based systems have drawbacks, such as low output power, poor transmission efficiency and sensitivity to parameter variations. This paper proposes a fractional order resonant WPT circuit whereby both the transmitting and receiving ends are composed of a fractional capacitor and inductor to overcome such problems. In this paper, the overall performance is studied based on its output power and efficiency considering a series-parallel topology. The effect of fractional order in fractal elements will be analyzed to observe the optimal combination of components to achieve the maximum output power with higher efficiency. Through a comparative analysis of the results, several combinations of circuit parameters can provide a theoretical understanding for implementing an experimental system.

Design of Spiral Resonator Based on Fractal Metamaterials and Its Improvement for MCR-WPT Performance

To further optimize the electromagnetic characteristics of square spiral resonator (SSR) in wireless power transfer (WPT) system, the SSR fractal finite element model is established in the high frequency structure simulator (HFSS) simulation software based on Koch curve and Hilbert curve. The mechanism of the effect of Koch and Hilbert fractal structure on frequency and permeability characteristics of SSR is analyzed in depth. The inductance of the resonant coil is calculated by the Maxwell software and then a magnetically coupling resonant WPT (MCR-WPT) system operating at 6.78 MHz is constructed in HFSS. The influence mechanism of the spiral metamaterial (MM) structure combined with the Koch/Hilbert fractal on the transfer performance of MCR-WPT system is analyzed. The simulation and experimental results show that the power transfer efficiency (PTE) of the system can be improved effectively when the negative permeability/hybrid MM slab (NMS/HMS) based on Koch/Hilbert/initial SSRs is inserted in the middle of the transfer distance. In the experiment, the PTE of the HMS based on Hilbert fractal can be improved by up to 21.62%.

高速スイッチング技術を利用した磁界共振型非接触給電の低次高調波における漏洩磁界低減

高速スイッチング技術を利用した磁界共振型非接触給電の低次高調波における漏洩磁界低減

New Magnetic Coupling Pad with Circular Geometry for Wireless Power Transfer Applications

In Resonant Inductive wireless power transfer systems (RIPT), the magnetic coupler plays a fundamental role. That assists in passing of power without wires. In wireless power transfer systems (WPT), couplers are designed to achieve various parameters such as power transfer efficiency, high power density, transferrable distance, and tolerant towards misalignment. In addition, minimizing leakage flux, low weight, less cost, and space occupied. To achieve these parameters, researchers proposed different types of coils with different shapes. Those circular coils are more popular due to their unique features. Compared to other coils, these coils lack in power transfer distance. In this paper, a distinct magnetic coupler with circular geometry proposed to preserve some extent of its features. In proposed, a circular quadrature coil added with two circular coils shaped to the circular coil. Proposed topology analyzed using ANSYS Maxwell Finite Element Analysis 3D simulation and compared with different combinations chosen better performing combination.

Planar Omnidirectional Wireless Power Transfer System Based on Novel Metasurface

In this article, we propose a novel metasurface-basedmagnetic coupling resonant wireless power transfer (MCR-WPT) system to realize planar omnidirectional WPT at a mid-range distance. The resonator devices—dielectric-constant disks or planar helixes—are located on both sides of the metasurface, which comprises conducting wires placed in a planar sunburst pattern. Transmission efficiency of >40% is achieved on a circular plane with a diameter of ∼40 cm. In addition, the transfer efficiency is insensitive to the placement of a human hand near the proposed MCR-WPT system. The maximum permissible input powers for the coil and dielectric resonator versions of the proposed MCR-WPT system are 0.83 and 2.56 W, respectively, in compliance with the limit prescribed in the International Commission of Non-Ionizing Radiation Protection guidelines. The proposed design may help realize many potential WPT applications, especially for the desktop wireless charging of household devices, such as mobile phones, tablets, and LED desk lamps.

Compensation Optimization of the Relay Coil in a Strong Coupled Coaxial Three-Coil Wireless Power Transfer System

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.

A Systematic Design Method for a Wireless Power Transfer System Based on Filter Theory

One of the major considerations for a short- or intermediate-distance wireless power transfer (WPT) system is its robustness against variations of the distance, displacement, misalignment between the transmitting and receiving coils, and variations of the loads. Designing such a robust system has not been easy since most methods require some forms of trial-and-errors or optimization that present different degrees of uncertainty. In this article, we propose a design method to overcome the above issue: we make the power transfer efficiency (PTE) function of the WPT equivalent to that of a modified filter. Then, we apply a well-established filter theory to develop a systematic WPT design method; it can keep the PTE stable within the specified ranges of the distance changes, displacements, misalignments, and load variations. We take the Chebyshev filter as an example to verify the proposed method and build and test the prototype. Experiments show that when distance and misalignments occur, the prototype can maintain the efficiency of around 90% with less than 5% in variations as predicted by the theoretical design. In comparison, the conventional magnetically coupled resonant WPT system can only maintain the efficiency of 82% with more than 20% in variations. Therefore, the proposed design method presents a new efficient and optimized way to develop a robust WPT system for practical applications.

Radially Periodic Metasurface Lenses for Magnetic Field Collimation in Resonant Wireless Power Transfer Applications

In this paper, a new procedure and topology of magnetic metasurface lens are proposed for improving the performance of Resonant Wireless Power Transfer systems. Firstly, three subwave-length unit cells are optimized in order to get negative magnetic permeability in the same frequency, each one with a different refractive index, and they are experimentally characterized. Then, the Transformation Optics technique is applied in order to find the unit cell arrangements which lead to the magnetic field focusing in a given direction. For this purpose, two coordinate transformations are proposed, and the effective electric permittivity and permeability that generate these profiles are calculated. It is shown from simulations and measurements that the refractive index gradient produced by the radial disposition of the unit cells can lead to a magnetic field manipulation similar to the optical converging and diverging lenses. Both metamaterials lenses are built, and the calculation of the magnetic flux density as a function of the measured induced voltage in a probe coil verifies their effects on the magnetic field. Finally, their performance in a resonant wireless power transfer system is tested, and improvements in terms of efficiency and range are presented. The proposed design method and the lenses that were developed demonstrate that metasurface lenses can improve efficiency without reducing the range, once these lenses are positioned close to the transmitter coil. Besides that, this method can reduce the losses due to misalignment between coils once the field can be collimated in a specific direction.

Genetic-Algorithm-Based Optimization of a 3D Transmitting Coil Design with a Homogeneous Magnetic Field Distribution in a WPT System

In magnetically coupled resonant wireless power transfer (MCR-WPT) systems, the nonhomogeneous magnetic field of the transmitting coil can lead to frequency splitting phenomena and lower efficiency. In this paper, a 3D transmitting coil (TX) with a homogeneous magnetic field distribution is proposed. The proposed coil structure consists of two layers with different numbers of turns per layer, i.e., with different current distributions. To achieve a homogeneous magnetic field distribution with a high magnetic field value and a low profile of the 3D coil structure, the optimal layer placement and current distribution were optimized using a genetic algorithm (GA). The prototype of the optimized coil was fabricated, and its magnetic field distribution was measured. The measurement results agreed more than 95% with the simulation results. The measured homogeneous area was at least 12.5% larger than reported in the literature. By using a different current distribution, the profile of the 3D coil structure was successfully reduced by 29% and the average magnetic field value was increased by 25% compared to our previous work.

Metamaterials for Improving Efficiency of Magnetic Resonant Wireless Power Transfer Applications

In this article, we investigate a compact metamaterial structure for enhancing a magnetic resonant wireless power transfer (WPT) system operated at 6.5 MHz. A thin magnetic metamaterial (MM) slab placed between the transmitter (Tx) and receiver (Rx) coil can improve WPT efficiency. The metamaterial unit cell is designed by a ten-turn spiral resonator (10T-SR) loaded with an external capacitor. The resonant frequency of MM unit cells can be easily controlled by changing the capacitor value. By using the optimization approach, we achieve a significant WPT efficiency improvement at a mid-range distance. The results showed an enhancement of the magnetic field in the WPT system when MM slab was present. This demonstrates the ability to amplify the evanescent wave of MM slab, thereby improving the WPT efficiency. The transmission coefficients of WPT system at 60 cm increased from 0.5 to 0.76 with MM slab, which corresponds to a 46% improvement.

Wireless power transfer system with enhanced efficiency by using frequency reconfigurable metamaterial

The wireless power transfer (WPT) system has been widely used in various fields such as household appliances, electric vehicle charging and sensor applications. A frequency reconfigurable magnetic resonant coupling wireless power transfer (MRCWPT) system with dynamically enhanced efficiency by using the frequency reconfigurable metamaterial is proposed in this paper. The reconfigurability is achieved by adjusting the capacitance value of the adjustable capacitor connected in the coil of the system. Finite element simulation results have shown that the frequency reconfigurable electromagnetic metamaterial can manipulate the direction of the electromagnetic field of the system due to its abnormal effective permeability. The ultra-thin frequency reconfigurable metamaterial is designed at different working frequencies of 14.1 MHz, 15 MHz, 16.2 MHz, 17.5 MHz, 19.3 MHz, 21.7 MHz and 25 MHz to enhance the magnetic field and power transfer efficiency (PTE) of the system. Frequency reconfigurable mechanism of the system with the frequency reconfigurable metamaterial is derived by the equivalent circuit theory. Finally, further measurement which verifies the simulation by reasonable agreement is carried out. PTE of the system by adding the metamaterial are 59%, 73%, 67%, 66%, 65%, 60% and 58% at different working frequencies. PTE of the system with and without the metamaterial is 72% and 49% at the distance of 120 mm and the frequency of 15 MHz, respectively.

Comparison and selection strategy of compensating topologies in two coil resonant wireless power transfer system

In this study comparisons of different compensation topologies are investigated for two coil resonant wireless power transfer systems. Compensation circuits are examined individually according to system parameters such as efficiency, equivalent impedance, frequency, load resistance, and phase angle. This paper aims to compares the system variables in order to address the constraints in the system applicability regarding the compensation topology selection in Wireless Power Transfer (WPT) systems. Topology selection schemes related to circuit parameters are given. The main motivation of this study is that it presents a suitable topology selection scheme and flow diagram according to applications with various voltages, currents, power and loads. Simulations performed for four main topologies under various load conditions concludes that choosing the proper compensation topology for appropriate load is essential. Simulation studies are carried out with the Simulink software. The results obtained from here are validated with both Matlab calculation codes and C # calculation codes. The analyses according to the frequency in various load conditions have shown that variation of efficiency depends on the compensation topology of the receiver side. Moreover, in this study, it has been revealed that the topology of the transmitter side only affects the equivalent impedance together with the amount of power drawn from the input hence it has no effect on the efficiency and load characteristics. Consequently in the case of working with low load resistance such as an electric vehicle or mobile phone charging, topologies with series compensation on the receiver side can be preferred. Correlatively, topologies with parallel compensation on the receiver side can be evaluated as suitable for high load resistance, low current, and low power operations such as biomedical appliance charging.

Optimization of Low-Frequency Magnetic Metamaterials for Efficiency Improvement in Magnetically Coupled Resonant Wireless Power Transfer Systems

With the extensive applications of magnetically coupled resonant wireless power transfer (WPT), improving the power transfer efficiency (PTE) of WPT systems is critical. Magnetic metamaterials (MTMs) can effectively improve the PTE. However, currently the design of MTMs is mainly based on microwave or optical theory, and their applicability in low-frequency WPT systems needs to be further explored. In this paper, a fast and effective method for the design and optimization of low-frequency MTM is provided. Firstly, based on circuit theory and mutual inductance calculation, the PTE of WPT systems is concluded in an equation that concerns the position of a 2-D MTM slab and added compensation capacitance of MTM unit coils. Two WPT systems with large differences in coil size of the transmitter (Tx) and receiver (Rx) or the same size are considered, and a directly separating variables method is proposed to optimize the MTM slab position and the compensation capacitance of unit coils to maximize PTE when the MTM slab position changes axially between Tx and Rx. It is found that the optimal MTM slab positions for the two systems are remarkably different, and the PTE impressively presents a capacitance splitting phenomenon with the change of compensation capacitance. Finally, two MTM-enhanced WPT systems are established to verify the theoretical calculation. The experiments show that the maximum PTE is significantly increased by 144% and 31% respectively by adopting optimized compensation capacitance compared with that without MTM for the two systems with different or same transceiver coil size.

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.

Multifrequency and Multiload MCR-WPT System Using Hybrid Modulation Waves SPWM Control Method

To satisfy independent and controllable power supply requirements for loads with different frequencies and power levels, a multifrequency and multiload (MFML) magnetic coupling resonant wireless power transfer (MCR-WPT) system using hybrid modulation waves sinusoidal pulsewidth modulation (HMW-SPWM) control method is proposed in this article. Based on SPWM, loading a hybrid modulation wave formed by superimposing multifrequency modulation waves to drive the inverter, so the multifrequency hybrid current on the primary side can be obtained. According to the principle of mutual inductance coupling, inductive power with different frequencies is obtained and then separated by multiresonant networks on the secondary side for loads with different frequencies. First, the structure and working principle of a MFML MCR-WPT system controlled by HMW-SPWM are introduced. Then, taking a dual-frequency and dual-load MCR-WPT system as an example, the system is mathematically modeled. Besides, parameters design criteria for reducing interfrequency interference is studied. After that, load characteristics and power factor are analyzed to select appropriate parameters. Furthermore, dynamic characteristics are analyzed. Finally, theoretical results are validated by experiments. The experimental results show that the MFML MCR-WPT system controlled by HMW-SPWM realizes independent and controllable wireless power supply for loads with different frequencies and power levels.

Design and Experimentation of a Novel Five Coil Asymmetric Magnetic Resonance Wireless Power Transfer System

This paper introduces a wireless power transfer system using a 5-coil asymmetric topology to mitigate range limitations when powering devices requiring small coils. The efficiency of a traditionally coupled wireless power transfer system falls sharply at distances beyond the diameter of the coils used. In order to power devices requiring small packaging at a distance, this limitation must be eliminated. Magnetically coupled coils have demonstrated the ability to extend power transfer distances beyond that of traditional coupling techniques; however, power transfer is still limited by coil size. We utilized the power transfer capabilities demonstrated by a 4-coil magnetic resonant transfer system and introduced a fifth coil to extend the transfer range. Coil asymmetry was then introduced to overcome coils radius limitations. This power transfer technique was demonstrated by charging a cell phone at a distance of 61 cm. The 5-coil transfer system operates at 40% efficiency from amplifier output to rectifier input. We develop an end-to-end power transfer system including a power amplifying source, 5-coil transfer system, and load including rectification to feed the cell phone. Circuit models and equations are introduced for all three stages with an emphasis on the asymmetric 5-coil system. Key parameters characterizing the 5-coil wireless transmission segment are also derived. Individual and system-wide efficiency limitations are discussed, and areas of future work are presented.

Variable-Frequency Pulse Width Modulation Circuits for Resonant Wireless Power Transfer

In this paper, we develop a variable-frequency pulse width modulation (VFPWM) circuit for input control of 6.78-MHz resonant wireless power transfer (WPT) systems. The zero-voltage switching control relies on the adjustments of both duty cycle and switching frequency for the class-E amplifier used in the WPT as the power transmission unit. High-frequency pulse wave modulation integrated circuits exist, but some have insufficiently high frequency or unfavorable resolution for duty cycle tuning. The novelty of this work is the VFPWM circuit design that we put together. A voltage-controlled oscillator (VCO) of radio frequency and capacitor-coupled difference amplifiers are used to simultaneously perform the frequency and duty cycle tuning required in resonant WPT applications. Different circuit topologies of VFPWM are compared analytically and numerically. The most favorable circuit topology, enabling independent control of the frequency and duty cycle, is employed in experiments. The experimental results demonstrate the validity of the novel VFPWM, which is capable of operating at 6.78-MHz and has a duty ratio adjustable from 20% to 45% of the range applicable in the resonant WPT applications.

Mechanism of wireless power transfer system waveform distortion caused by nonideal gallium nitride transistor characteristics

Gallium nitride (GaN) field-effect transistors have low ON resistance and switching losses in high-frequency (>MHz) resonant wireless power transfer systems. Nevertheless, their performance in the system is determined by their characteristics and operation mode. A particular operating mode in a 6.78-MHz magnetic resonant wireless transfer system that employs class-D GaN power amplifiers in the zero-voltage switching mode is studied. Two operation modes, the forward mode and the reverse mode, are investigated. The nonideal effect under the device-level dynamic resistance and thermal effect are also analyzed. The dynamic resistance under different operation modes is demonstrated to have different generation mechanisms. Finally, the device characteristics with system operating conditions are combined, and the effects of temperature and dynamic resistance under different operating conditions are evaluated.