Wireless Power Transfer Applications Research Articles

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.

Highly Efficient Wireless Power Transfer System With Single-Switch Step-Up Resonant Inverter

In this article, a highly efficient wireless power transfer (WPT) system is realized by employing the proposed single-switch step-up resonant inverter with a series–series-compensated network. This resonant inverter has the merits of a simplified circuit structure, low component count, inherent voltage boosting, zero-voltage switching (ZVS), and high efficiency. Based on the equivalent circuit model, the two-coil WPT system is analyzed in the frequency domain, which shows that a quasi-constant current (CC) in the primary side and a corresponding quasi-constant voltage (CV) in the secondary side can theoretically be achieved. A systematic top-down design methodology of the entire WPT system is also provided, which serves as a practical design guideline. The proposed solution benefits real WPT applications by generating a clean and stable dc voltage for the built-in dc–dc battery management system of the receiving device. The experimental results show that the maximum efficiency of the WPT system at the rated output power of 11 W is around 86.4%.

Separation-Independent Wearable 6.78 MHz Near-Field Radiative Wireless Power Transfer using Electrically Small Embroidered Textile Coils

Achieving a wireless power transfer (WPT) link insensitive to separation is a key challenge to achieving power autonomy through wireless-powering and wireless energy harvesting over a longer range. While coupled WPT has been widely used for near-field high-efficiency WPT applications, the efficiency of the WPT link is highly sensitive to separation and alignment, making it unsuitable for mobile systems with unknown or loose coupling such as wearables. On the other hand, while ultra-high frequency (UHF) and microwave uncoupled radiative WPT (0.3–3 GHz) enables meters-long separation between the transmitter and the receivers, the end-to-end efficiency of the WPT link is adversely limited by the propagation losses. This work proposes radiative WPT, in the 6.78 MHz license-free band, as a hybrid solution to separation-independent WPT, thus mitigating the losses associated with coil separation. Resonant electrically small antennas were fabricated using embroidered textile coils and tuned using L-matching networks, for wearable WPT. The antenna’s efficiency and near-fields have been evaluated numerically and experimentally. The proposed WPT link achieves a stable forward transmission of S 21 > − 17 dB and S 21 > − 28 dB, independent of coil separation on the XZ and XY planes respectively, in a 27 m 3 volume space. The presented approach demonstrates the highest WPT link efficiency at more than 1-m separation and promises higher end-to-end efficiency compared to UHF WPT.

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.

Research and Development for Industrial Application of Inductive Wireless Power Transfer

Research and Development for Industrial Application of Inductive Wireless Power Transfer

High-Power Wire Bonded GaN Rectifier for Wireless Power Transmission

A novel wire bonded GaN rectifier for high-power wireless power transfer (WPT) applications is proposed. The low breakdown voltage in silicon Schottky diodes limits the high-power operations of microwave rectifier. The proposed microwave rectifier consists of a high breakdown voltage GaN rectifying element for high-power operation and a novel low loss impedance matching technique for high efficiency performance. Wire bonding method is adopted to provide electrical connection between GaN chip and board which induces undesirable inductance. In order to realize high efficiency performance, an impedance matching network is proposed to exploit the unavoidable inductance along with a single shunt capacitor, resulting in a low loss matching circuit to achieve a compact high-power rectifier. The fabricated GaN rectifier exhibits a good performance in the high-power region and can withstand up to 39 dBm input power before reaching the breakdown limit at the operating frequency of 0.915 GHz and load resistance of 100 Ω. It has a compact size and exhibits high efficiency performance in high-power region (achieved a maximum efficiency of 61.2% at 39 dBm), making it suitable for high-power applications like future unmanned intelligent devices and WPT in space applications.

Self-Tuning Multi-Transmitter Wireless Power Transfer to Freely Positioned Receivers

Free positioning of receivers is one of the key requirements for many wireless power transfer (WPT) applications, necessary from the end-user point of view. However, realization of stable and effective wireless power transfer for freely positioned receivers is technically challenging because of the need of complex control and tuning circuits. In this paper, we propose a concept of power channeling via an array of repeaters that enables self-tuning of multi-transmitter WPT systems without any sensing or control elements. The proposed WPT system consists of uncoupled transmitters and uncoupled repeaters which form independent power transfer channels. The proposed self-tuning WPT system is capable of maintaining stable output power with constant high efficiency regardless of the receiver position. Namely, our numerical and experimental results for a four-transmitter WPT system in form of a linear array show the efficiency 94.5 ± 2% for any position of the receiver within 20 mm range.

A Review of Methods and Challenges for Improvement in Efficiency and Distance for Wireless Power Transfer Applications

Abstract Over the past few years, interest and research in wireless power transfer (WPT) have been rapidly incrementing, and as an effect, this is a remarkable technology in many electronic devices, electric vehicles and medical devices. However, most of the applications have been limited to very close distances because of efficiency concerns. Even though the inductive power transfer technique is becoming relatively mature, it has not shown near-field results more than a few metres away transmission. This review is focused on two fundamental aspects: the power efficiency and the transmission distance in WPT systems. Introducing the principles and the boundaries, scientific articles will be reviewed and discussed in terms of their methods and respective challenges. This paper also shows more important results in efficiency and distance obtained, clearly explaining the theory behind and obstacles to overcome. Furthermore, an overlook in other aspects and the latest research studies for this technology will be given. Moreover, new issues have been raised including safety and security.

Noise Suppression and Precise Phase Control of a Commercial S-Band Magnetron

We demonstrate noise suppression and precise phase control of a commercial 2.45GHz magnetron aiming for wireless power transfer application. The impurity of the microwave spectrum was confirmed to be due to the switching frequency of 76kHz in the power supply. With the decoupling capacitor installed to the output of the high-voltage power-supply driving the magnetron, the spectral purity of the magnetron was significantly enhanced. And we achieved extreme precision in phase-control (peak-to-peak 0.3°) for the high-power (1 kW) microwave magnetron by applying the phase locking loop to the noise-suppressed magnetron system.

RF complex permeability spectra of Ni-Cu-Zn ferrites prepared under different applied hydraulic pressures and durations for wireless power transfer (WPT) applications

Frequency dispersion of RF complex permeability of Ni-Cu-Zn ferrites prepared under different amplitudes and durations of applied hydraulic pressure have been thoroughly investigated. The Ni0.35Cu0.19Zn0.46Fe2O4 ferrites were prepared by conventional ceramic synthesis process. The obtained ferrite powders were pressed under different hydraulic pressures (4 MPa, 8 MPa, 12 MPa) of different durations (30 s, 2 min, 3 min and 5 min). It was noted that the complex permeability spectra change considerably with respect to the application of different amplitudes and durations of hydraulic pressure before sintering. Porosity and grain size play an important role in affecting the permeability and magnetic loss of the sintered ferrite samples. A significant increase of the real part of the complex permeability from 96 to 151 was achieved due to the application of hydraulic pressure of 8 MPa for 30 s as compare to manually pressed sample. The magnetic loss tangent also increased from 0.01 to 0.02 as the imaginary part of the complex permeability increased, which is still acceptable for most of the wireless power transfer (WPT) applications. It is postulated that the hydraulic pressure applied before sintering of ferrite powder-based specimen increases the real part of permeability while influencing magnetic loss tangent much less severely.

A Unique Military Application of Wireless Power Transfer: Wireless Charging Through a Vehicle Seat With Simplified Design Considerations

Noncontact wireless power transfer technology has received much attention lately, including significant funding from the U.S. Depar tment of Defense (DoD). The distances of interest for power transmission range from less than an inch to several hundred feet, depending on the application. This article describes engineering aspects and provides an overview of some of these technologies and their possible applications. We describe a unique military application involving the wireless charging of soldier-carried equipment and present the results of a recent project that addressed this application with a system using a near-field inductive resonance technique. Simplified design considerations are also discussed.

RF Near-Field Coupling System Using Reverse PDN and Considering Electromagnetic Effects

One of the more intriguing applications of wireless power transfer is power delivery for low-power Internet of Things (IoT) devices. By utilizing RF near-field coupling (NFC), an improved combination of small receiver coil area and system efficiency can be achieved, as is ideal for wireless charging for low-power IoT platforms. However, utilizing high efficiency RF NFC comes with a cost—various electromagnetic effects can lead to detuning of the resonant frequency if not properly modeled. In addition, integrating power regulation with high frequency ac–dc conversion can create large ripple voltage supply to a power distribution unit (PDU). This paper investigates the system level issues that arise in efficient, regulated RF NFC systems for milliwatt-scale output power. A method for modeling the various electromagnetic, frequency detuning effects is presented, as well as a selection process for the circuit component values in the design. Furthermore, a reverse power distribution network is designed to integrate the rectifier with a PDU, introducing a target impedance and phase considerations for an RF rectifier. The modeling and integration solutions are experimentally verified through two different RF NFC systems. The systems have measured peak efficiency of 68.4% at 3.3 V output and 58.6% for 1 V output with less than 100 mm $^2$ receiver coil area, demonstrating a unique combination of small coil area and high efficiency compared to other NFC systems to better support low-power IoT devices.

A Coil Detection System for Dynamic Wireless Charging of Electric Vehicle

Application of wireless power transfer while an electric vehicle (EV) is in motion can significantly reduce the battery storage capacity. A major challenge in implementation of dynamic wireless power transfer (DWPT) is an automatic detection of EV to avoid loss in efficiency and alleviate any safety concerns. This paper proposes a novel coil detection method for segmented DWPT. Detection of the EV ahead of its arrival will initiate energizing of the transmitter buried inside the road to enable just-in-time transfer of power. At low speeds, communication can be a reliable method to power up the transmitter coil. However, at high speeds on highways, the communication latency time for the detection of an EV is long and hence impractical. This paper proposes a low-cost and low-power EV detection system based on a novel orthogonal coil arrangement to detect EVs traveling at high speeds. The proposed detection system was tested on a laboratory scale prototype for verification purpose. For high-speed verification, simulation in PLECS was conducted to test the functionality of the proposed system.

A Real-Time Range-Adaptive Impedance Matching Utilizing a Machine Learning Strategy Based on Neural Networks for Wireless Power Transfer Systems

In this article, the implementation of a machine learning (ML) strategy based on neural networks for the real-time range-adaptive automatic impedance matching of wireless power transfer (WPT) applications is discussed. This approach for the effective prediction of the optimal parameters of the tunable matching network and the selection of range-adaptive transmitter coils (Tx) is introduced in this article, aiming to achieve an effective automatic impedance matching over a wide range of relative distances. We propose a WPT system consisting of a tunable matching circuit and three Tx coils that have different radii and are simultaneously controlled by trained neural network models, returning an output set of matching capacitances as well as the optimal single transmitter among the three transmitters. In addition, a proof-of-concept prototype of the entire real-time range-adaptive automatic impedance-matching system is built and characterized. Finally, the proposed approach achieves a power transfer efficiency (PTE) of around 90% for ranges within 10–25 cm.

A Novel Coding Metasurface for Wireless Power Transfer Applications

We propose and implement a novel 1-bit coding metasurface that is capable of focusing and steering beam for enhancing power transfer efficiency of the electromagnetic (EM) wave-based wireless power transfer systems. The proposed metasurface comprises 16 × 16 unit cells which are designed with a fractal structure and the operating frequency of 5.8 GHz. One PIN diode is incorporated within each unit cell and enables two states with 180 ° phase change of the reflected signal at the unit cell. The two states of the unit cell correspond to the ON and OFF states of the PIN diode or “0” and “1” coding in the metasurface. By appropriately handling the ON/OFF states of the coding metasurface, we can control the reflected EM wave impinged on the metasurface. To verify the working ability of the coding metasurface, a prototype metasurface with a control board has been fabricated and measured. The results showed that the coding metasurface is capable of focusing beam to desired direction. For practical scenarios, we propose an adaptive optimal phase control scheme for focusing the beam to a mobile target. Furthermore, we prove that the proposed adaptive optimal phase control scheme outperforms the random phase control and beam synthesis schemes.

Full-Range Soft-Switching Pulse Frequency Modulated Wireless Power Transfer

This article proposes and implements a full-range soft-switching pulse frequency modulated (PFM) wireless power transfer (WPT) system, which not only offers high efficiency but also high controllability, selectivity, and security. Although the phase-shift control (PSC) has been widely used in various WPT applications, the high-frequency hard-switching operation will inevitably degrade the system performances in terms of power losses, and hence, efficiency during power regulation. Thus, a soft-switching PFM is newly conceived to suppress the switching frequency and power losses while maintaining full-range soft switching for effective power control, hence improving the system efficiency. In addition to realizing the controllability, the full-range soft-switching PFM can be further derived to implement the selectivity and security. The system efficiency of a multiobjective PFM-WPT system prototype can reach 86.27% at 220-W output power and 135-mm transfer distance. During wireless power control, it can effectively be improved by up to 5.68% as compared to its counterpart using the PSC. Theoretical analysis, computer simulation, and hardware experimentation are given to verify the feasibility of proposed full-range soft-switching PFM-WPT system.

An updated applied formulation for the Goubau transmission line

The Goubau line is a unique transmission line that consists of only a single conductor. Its unique properties make it a solid candidate for low-loss RF propagation and wireless power transfer applications. However, the applied theory of this line that has been established thus far is missing key details of the line's characteristics. Namely, no discussion has been made on the Goubau line's ohmic current distribution or its effective characteristic impedance that would be used in RF/microwave circuit analysis scenarios. Furthermore, the method previously used to solve the waveguide mode is laborious and obsolete when considering current computational methods. In this work, an updated, efficient mode solution process for the Goubau line will be presented, and the current, loss, and characteristic impedance of the line will be discussed. Additionally, all other waveguide parameters such as wave velocity, guide wavelength, propagation constant, etc. will be investigated.

High rectification sensitivity of radiofrequency signal through adiabatic stochastic resonance in nanoscale magnetic tunnel junctions

Rectification is an important stage in electronic circuits for any wireless radio frequency power transfer application. Currently, Schottky diodes are widely used as rectifiers; however, they are inefficient at low power levels of microwatts or less (providing maximum sensitivities around 4 mV/μW). Nanoscale magnetic tunnel junctions can serve as alternative rectifiers by utilizing the so-called spin-torque diode effect, demonstrating a much higher rectification sensitivity (200 mV/μW) compared to Schottky diodes. However, for this mechanism to work, the signal frequency must match the ferromagnetic resonance frequency, which typically lies in the gigahertz range. For signals in the megahertz range or lower, Schottky diodes remain the only option for rectification. Here, we demonstrate a mechanism based on thermally activated adiabatic stochastic resonance in magnetic tunnel junctions to produce low frequency (up to tens of megahertz) signal rectification at low input power (submicrowatt), with a sensitivity of up to 35 mV/μW—higher than state-of-the-art Schottky diode rectifiers at this frequency and power range. These findings suggest magnetic tunnel junctions as potential alternatives to Schottky diodes for low frequency and low power applications.

A tunable superconducting LC-resonator with a variable superconducting electrode capacitor bank for application in wireless power transfer

The frequency alignment of resonators is essential for wireless power transfer (WPT) systems to achieve strongly coupled magnetic resonances. Here, we present a novel tunable superconducting resonator for WPT applications. The tunable resonator is composed of a high-temperature superconducting (HTS) coil and a variable HTS electrode capacitor bank based on two series YBa2C3O7−x–SrTiO3–YBa2C3O7−x thin films. The HTS coil serves as a wireless transmitting tool for power delivery, while the HTS electrode capacitor bank provides a frequency-tuning element for the resonator. At 77 K, the HTS materials cause low loss and thus achieve a high quality factor, while the incipient ferroelectric SrTiO3 substrate with HTS electrodes has a nonlinear capacitance dependence on the applied electric field with no hysteresis. A voltage supply applies a continuous DC bias to both capacitors synchronously via the coil, adjusting the total capacitance and thus the resonant frequency. Our results indicate that the tunable resonator can achieve a tuning range of 12% from 775 kHz, offering a frequency-tuning functionality to superconducting WPT technology.

A Method to Eliminate Discrete Inductors in a Class-E Inverter Used in Wireless Power Transfer Applications

Wireless power transfer (WPT) systems have attracted tremendous attention over the past decade. Most compact, low-power, WPT systems, however, utilize ferrites to shield and redirect the magnetic fields. Using a magnetic material adds the weight and system bulk. In this article, we present a method to design the WPT coils in which the leakage and magnetizing inductances are utilized in other parts of the circuit. This eliminates the need for discrete inductors commonly required in a resonant inverter, which additionally improves overall efficiency. Utilizing this technique, we demonstrate a small and lightweight 300-W 48-to-48-V WPT system with above 90% efficiency. We further propose a method to eliminate the input inductor required by the class-E inverter by utilizing the input power supply wires. Employing this method, we demonstrate a printed circuit board (PCB)-based WPT circuit operating at 40.68 MHz, utilizing no discrete inductors other than the WPT coils themselves and weighing only 7 g. This article describes the design methodology as well as the procedure to eliminate the discrete resonant inductors used in the implementation of our designs.