High-efficiency Wireless Power Transfer Research Articles

High-Efficiency WPT Systems for Potable Electronics Based on DC-Bias-Voltage- Controlled Variable Capacitor

The power density of wireless power transfer (WPT) systems employed in portable electronics can be enhanced by increasing the operating frequency, but at the expense of lower efficiency due to increased switching and parasitic losses. This paper proposes a method, based on the DC-bias-voltage-controlled variable capacitor (VC) and variable frequency (VF) regulation, to improve the overall efficiency of the WPT system for portable electronics. The method uses VC on the primary side to detune the resonant tank and dynamically regulate the output power, while VF is employed to improve the system efficiency using a look-up table based on a power loss model. A DC bias voltage control module with self-supply ability is employed to control the equivalent capacitance of the VC. A control strategy is proposed to regulate both VC and VF. Experimental results of a 150W prototype are presented to demonstrate that constant output voltage can be maintained despite a wide load range, with an overall efficiency range of 82.1% (@25W) and 92.2% (@150W).

Multitarget Wireless Power Transfer System Strategy Based on Metasurface-Holography Multifocal Beams

Aiming at the critical problem of wireless power transfer (WPT) for the current growing number of wireless devices, the multitarget WPT system based on metasurface-holography multifocal beams and the design methods is investigated and analyzed in this article. As the propagator in near-field synthesis, a new field calculation formula is derived based on the Friis formula between the metasurface unit (MU) and the field point, which breaks the limitation of paraxial approximation and makes metasurface holography applicable in a wide-angle, long-distance, and nonplane space. Besides the phase response of MUs, the amplitude response is also introduced, which achieves the simultaneous amplitude–phase optimization of the metasurface to improve transmission efficiency. A metasurface with holography multifocal beams is first designed and engaged in forming a planar multitarget WPT system, which has been proven to radiate the designed multifocal beam and transfer power to each focal spot uniformly and efficiently after simulated, fabricated, and measured. To verify that the proposed method can achieve a near-field synthesis of multifocal beams in nonplane space over a wide angular range, a metasurface with 23-focus beam distribution on a spherical surface with an elevation angle from <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> 60 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> to 60 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> , omnidirectional azimuth, and 66.67 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\uplambda $</tex-math> </inline-formula> radius is designed and simulated, which shows that the simulated radiation field distribution is in good agreement with the initial preset target field. This work demonstrates a feasible high-efficiency and long-distance WPT strategy for multiple targets in wide-angle and spatial scenarios.

Concave Ferrite Core for Wireless Power Transfer (WPT)

High-efficiency wireless power transfer (WPT) systems can present a perfect solution for fast-charging autonomous guided vehicles (AGV) to improve working hours in high-tech warehouses. Stationary charging stations reduce separation distance, improving coupling factor and power transfer efficiency. Analysis and design of the WPT system focused on maximum power at the load with a SS compensation circuit to reach high efficiency while applying the theory of power transformers design to maximize the power handleability with the physical dimensions. The proposed concept fits small AGVs. This paper proposes a unique ferrite structure for the transmitter ferromagnetic core. This novel shape introduces horizontal angular misalignment resistance due to the transmitter’s omnidirectional concave disc ferrite core combined with an E-core ferrite at the receiver side. The proposed WPT system can output 200 W at 100 kHz. A realistic 3D model has been designed into a symmetrical equivalent to reducing complexity and computational effort. The visualization of the magnetic flux distribution demonstrated that the proposed design has a better path to flow without concentrating flux in small regions, reducing heating losses.

Multitarget Wireless Power Transfer System Using Metasurface for Quasi-Bessel Beams With Large Half Power Beam Length

We propose a multitarget wireless power transfer (WPT) system using a transmissive metasurface with quasi-nondiffraction Bessel beams. To provide similar powers for more different targets, a special metasurface is designed to achieve uniform energy distributions in the nondiffraction area of Bessel beams, which can increase the half-power beam length (HPBL) by 10% compared with ordinary Bessel beams. A transmissive metasurface unit with transmission coefficient of more than 90% and a phase shift range greater than 360° are presented to form the metasurface at 10 GHz, which further ensures the high efficiency of the generated Bessel beams. Various receiving parameters are analyzed at the energy receiving end to reduce the influence between different targets. Receiving antennas that can harvest energy are designed with different apertures at 10 GHz, and the system efficiencies with different receiving apertures and different spaces between the receiving targets are analyzed to determine the appropriate receiving parameters. Finally, a multitarget metasurface WPT system based on the quasi-Bessel beams is fabricated. Simulation and experimental results show that the system can achieve almost equal WPT efficiency for five targets. More than 91% of the incident power can be modulated to form the quasi-Bessel beams, and the efficiency of each target is greater than 4.4% for the simulated results and greater than 3.3% for the measured results. Analysis of the designed rectifier circuit further proves the rationality of the proposed WPT system, which can be used for multiple charging targets and maintain a uniform power transfer efficiency. The quasi-Bessel beams achieve the high-efficiency multitarget WPTs and simplify the circuit design of receiving targets, making WPT applicable to more scenarios.

A High-Efficiency Wireless Power Transfer System Using Quasi-Z-Source Inverter and Current-Double Synchronous Rectifier for Low-Voltage and High-Current Applications

In low-voltage and high-current wireless power transfer systems (LVHC WPTSs), the loss of power loop is usually large due to the high current. More critically, the equivalent load resistance (ELR) is usually much lower than the optimal load resistance, which leads to serious heating and low system efficiency. This article proposes a new technology, which employs the current-double synchronous rectifier (CDSR) to replace the traditional diode rectifier and dc/dc converter on the secondary side, and thus, the ELR can be adjusted in a large range by regulating the duty cycle of CDSR and the system can always work at the maximum efficiency point. In addition, the traditional dc/dc converter and inverter are replaced by the quasi-Z source inverter (q-ZSI) on the primary side, which can boost the dc-link voltage of inverter and reduce the current of power loop, so as to reduce the heating and improve the system efficiency further. Furthermore, the cooperative working mechanism of q-ZSI and CDSR for high-efficiency and constant power (CP) charging is analyzed thoroughly. Finally, the viability of the proposed topology is verified in a 1-kW prototype with 48-V input and 24–33.6-V output voltage, and the maximum efficiency of the prototype achieves 87.4%.

Guest Editorial: Special Issue on Advanced and Emerging Technologies of High Efficiency and Long-Distance Wireless Power Transfer Systems

Guest Editorial: Special Issue on Advanced and Emerging Technologies of High Efficiency and Long-Distance Wireless Power Transfer Systems

On the Design of Discrete Apertures for High-Efficiency Wireless Power Transfer

This communication deals with the design of discrete apertures (DAs) for wireless power transfer (WPT) within the Fresnel region and provides some design guidelines to enhance WPT efficiency. In fact, although the aperture excitations optimization problem, given two specific continuous or DAs, has been already researched, the effect of antenna arrays design choices (such as the number of antenna elements or their spacings) on the WPT efficiency has not been investigated yet. In order to do that, WPT efficiency formulation has been theoretically analyzed to understand the different antenna parameters contributions to the power transfer efficiency (PTE). After that, the problem of the array excitation optimization in the sense of maximum PTE has been revisited including cases with or without perfect impedance matching. Finally, different array designs and relative achievable PTEs have been compared systematically, and it is shown that arrays with interelement spacing in the order of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.3\lambda $ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.4\lambda $ </tex-math></inline-formula> (tightly coupled arrays) provide the largest PTE (also for oblique WPT cases) when the array excitation is calculated taking into account the impedance mismatching.

A wearable metasurface for high efficiency, free-positioning omnidirectional wireless power transfer

We introduce a design principle of metasurfaces that can form any desired distribution of magnetic field for high-efficiency wireless power transfer centered at 200 kHz, which can be used to efficiently charge implanted medical devices. This metasurface can improve the power transfer efficiency for both single-user and multi-user cases by over tenfold compared to those without the metasurface. Our design enables a robust field distribution to the positions of the transmitting and receiving coils, as well as the geometric distortions of the metasurface itself, demonstrating its feasibility as a wearable device. With our design, the field distribution and subsequent power division among the multiple users can be readily controlled from equal distribution to any selective user(s). When incorporating a three-dimensional unit cell of the metasurface, we theoretically demonstrate an omnidirectional control of the field orientation to achieve a high-efficiency wireless power transfer for multiple users.

Simulation-Assisted Design Process of a 22 kW Wireless Power Transfer System Using Three-Phase Coil Coupling for EVs

The objective of this paper is to study a 22 kW high-power wireless power transfer (WPT) system for battery charging in electric vehicles (EVs). The proposed WPT system consists of a three-phase half-bridge LC–LC (i.e., primary LC/secondary LC) resonant power converter and a three-phase sandwich wound coil set (transmitter, Tx; receiver, Rx). To transfer power effectively with a 250 mm air gap, the WPT system uses three-phase, sandwich-wound Tx/Rx coils to minimize the magnetic flux leakage effect and increase the power transfer efficiency (PTE). Furthermore, the relationship of the coupling coefficient between the Tx/Rx coils is complicated, as the coupling coefficient is not only dominated by the coupling strength of the primary and secondary sides but also relates to the primary or secondary three-phase magnetic coupling effects. In order to analyze the proposed three-phase WPT system, a detailed equivalent circuit model is derived for a better understanding. To give a design reference, a novel coil design method that can achieve high conversion efficiency for a high-power WPT system was developed based on a simulation-assisted design procedure. A pair of magnetically coupled Tx and Rx coils and the circuit parameters of the three-phase half-bridge LC–LC resonant converter for a 22 kW WPT system are adjusted through PSIM and CST STUDIO SUITE™ simulation to execute the derivation of the design formulas. Finally, the system achieved a PTE of 93.47% at an 85 kHz operating frequency with a 170 mm air gap between the coils. The results verify the feasibility of a simulation-assisted design in which the developed coils can comply with a high-power and high-efficiency WPT system in addition to a size reduction.

High-efficiency and compact WPT receiving antenna design using simulation optimization method

In this paper, a high-efficiency and compact wireless power transfer (WPT) receiving antenna using an flexible printed circuits board (FPCB) with high-Q factor is proposed. The proposed high-efficiency and compact WPT receiving antenna consists of a spiral structure, a lumped element and via through the ground structure. In this paper, in order to design a miniaturized high-efficiency WPT receiving antenna, machine learning technology was used to optimally design the antenna pattern that affects the Q factor. The designed receiving antenna has improved wireless power transmission efficiency with high Q characteristics. The power transfer efficiency is above 88% from distance 10 mm and the resonant frequency is 110 MHz. The sizes of the receiving antenna and the NFC antenna are 30 × 40 mm and 20 × 30 mm, respectively. Machine learning was used to design and fabricate the composite module of a WPT receiving antenna and NFC antenna. Comparison of the experimental results of the proposed receiving antenna with a general receiving antenna confirms the improvement in transmission efficiency.

Mid-Range Wireless Power Transfer System for Various Types of Multiple Receivers Using Power Customized Resonator

This paper presents a high-efficiency wireless power transfer (WPT) system that can charge multiple receivers (Rx's) of various types. For system analysis, the transmitted power and received power levels are derived using the parameters of the resonators based on electro-magnetic (EM) resonance. Using the analysis results, the power customized resonator (PCR) not only for deriving sufficient output power from the transmitter (Tx) but also for appropriately distributing the received power to the various and multiple Rx's is proposed. In addition to the PCR, the Tx unit is designed to work as a load-dependent voltage source using a differential Class-E power amplifier (PA) through a wide range of load impedances. Therefore, the Tx can naturally adapt to the required Tx power levels with a high efficiency corresponding to various Rx configurations without additional tunable circuits or adaptive control schemes. The WPT system, including the load-dependent voltage source for the Tx, full-bridge rectifiers for the Rx's, and the proposed PCR, was designed for the 6.78 MHz frequency band. The proposed system is validated for use with multiple mobile devices by conducting experiments with nine charging cases using three types of Rx's. For all cases, high system efficiencies of 70.7-85.5% were maintained over received power levels of 8.6-45.7 W at a charging distance of 30 mm, and each of the three types of Rx's were experimentally verified to receive sufficient power.

High-Efficiency WPT System for CC/CV Charging Based on Double-half-bridge Inverter Topology with Variable Inductors

Efficiency remains a key challenge in wireless charging in academia and industry. In this article, a new wireless power transfer (WPT) system based on a double-half-bridge (DHB) inverter with two variable inductors (VIs) is proposed. Compared with conventional full-bridge (FB) inverters, the DHB inverter can reduce the current through the mosfet s under the same output power and thus, reduce the conduction loss. Next, by adjusting the inductances of the VIs instead of using phase shift (PS) method, the output voltage or current can be controlled, while the circulating current can be eliminated and wide-range zero voltage switching operation can be achieved. Consequently, the power loss can be further reduced. Circuit analysis, VI design, as well as hardware implementation, are provided in detail. A laboratory prototype is built to verify the feasibility of the proposed method. Close agreement is obtained between simulation and experimental results. The maximum efficiency can reach 92.4%, which is 3.65% higher than traditional PS control.

Second Harmonic Exploitation for High-Efficiency Wireless Power Transfer Using Duplexing Rectenna

In this article, a novel duplexing rectenna with harmonic feedback capability is proposed for efficient wireless power transfer (WPT) applications. The proposed duplexing rectenna can harvest the incoming radio frequency (RF) energy efficiently and also make use of an inherent harmonic signal, which is sent back to the RF transmitter (Tx) for positioning in order to guide the radiation patterns from Tx antenna array for optimum WPT. A novel duplexing dipole antenna is designed based on the top loading and capacitive gap effects. It is used to receive RF power at fundamental frequency 0.915 GHz and transmit the second harmonic signals at 1.83 GHz as a feedback. Experimental validation of a complete WPT system with beam-scanning capability has been carried out. It is shown that the fabricated rectifier has realized a maximum RF-dc conversion efficiency of 71% (at 15-dBm input power) and a measured peak second harmonic power of −1 dBm. Thus, the proposed duplexing rectenna can form a closed-loop system by providing its location information for efficient WPT applications.

Experimental Demonstration of Microwave Two-Dimensional Airy Beam Generation Based on Single-Layer Metasurface

In this article, a 2-D Airy beam is experimentally generated at microwave frequency by using a simple and thin metasurface composed of a single-layer of square C-shaped complementary split-ring resonators (C-SRRs). It is shown that the cross-polarization transmission coefficient, including the magnitude and phase, of a single C-SRR unit can be controlled by modifying the single C-SRR unit rotation angle, and this property is herein used to reshape the impinging field in accordance with the desired 2-D Airy beam profile. The designed metasurface is then fabricated to measure the most significant features of the generated microwave Airy beam, including quasi-nondiffraction, self-bending, and self-healing properties, thus achieving good results in agreement with those predicted by theory. The generated microwave 2-D Airy beam with its properties can be widely applied for high-efficiency wireless power transfer, wireless covert communication, and directional communication.

Rollable metamaterial screen for magnetic resonance coupling-based high-efficiency wireless power transfer

AbstractThis paper presents a rollable metamaterial screen for high-efficiency wireless power transfer (WPT) system based on magnetic resonance coupling, which operates at 4.5 MHz. The rollable metamaterial screen with a fully expanded area of 750 mm × 750 mm is located in the middle between transmitter and receiver coils and focuses the magnetic field and, by such a way, significantly improves power transfer efficiency (PTE). The metamaterial screen can be rolled up, e.g. onto the ceiling when it is not used, and thus does not require any designated space for the screen saving space. A WPT system with the rollable metamaterial screen is designed, fabricated, and characterized. Improved PTE is qualitatively and quantitatively verified by light bulb experiments and vector network analyzer measurements. The PTE of the WPT system with the metamaterial screen increases from 36 to 58.52% and 10.24 to 31.36% for the distances between the transmitter and receiver coils 100 and 150 cm, respectively. The effects of lateral and angular misalignments on the PTE of the WPT system are also studied. Obtained results show that the rollable metamaterial screen improves the PTE even at the misaligned condition.

Tunable Metamaterial Slab for Efficiency Improvement in Misaligned Wireless Power Transfer

This letter presents a tunable metamaterial (MTM) slab for high-efficiency wireless power transfer (WPT) in misaligned conditions, which operates at 6.78 MHz, an AirFuel Alliance Standard frequency. The tunable MTM slab is located between transmitter (Tx) and receiver (Rx) coils serving as a tunable lens to help focus the magnetic field toward the place where the Rx coil is located. This tunable slab consists of two types of unit cells which can be independently tuned to have proper negative refractive indices. The experimental results show that the tunable MTM slab effectively focuses magnetic fields, thereby improving the power transfer efficiency (PTE) of WPT even in a misaligned condition.

Compact coupled resonators for small size dual‐frequency wireless power transfer (DF‐WPT) systems

This work introduces a novel design for high efficiency and compact size dual-frequency wireless power transfer (DF-WPT) systems. The proposed design is composed of two identical transmitting (Tx) and receiving (Rx) substrates. Each substrate has a microstrip feed line, a C-shaped coupled resonator on the top layer and a spiral coupled defected ground structure on the bottom layer. The system is analysed using J-inverters for the dual-frequencies (DFs) operation. The measured results show that the proposed DF-WPT system with a compact size of 20 × 20 mm2 is able to provide a stable WPT characteristic with total efficiencies of 91.2 and 79.4% at 0.28 and 0.49 GHz, respectively with a transmission distance of 6 mm. The proposed system achieved good agreements between the analytical design procedure, the circuit model, the electromagnetic simulations, and the measurements.

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 &gt; − 17 dB and S 21 &gt; − 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.

Battery-Less Location Tracking for Internet of Things: Simultaneous Wireless Power Transfer and Positioning

We propose a battery-less location tracking system that enables 3-D positioning of an Internet of Things (IoT) device powered by the radio frequency (RF) wireless power transfer (WPT). In the proposed system, a power beacon is equipped with a phased antenna array that has the dual purposes of high-efficiency WPT and phase-based accurate positioning. In order to enhance the efficiency of the RF WPT, we propose a beam focusing algorithm that dynamically controls the respective phases of antenna elements to place the focal point of the electro-magnetic (EM) wave onto the target IoT device. We also propose a phase-based positioning algorithm that requires only one multiantenna anchor point for determining the distance as well as the direction from the anchor point. We analyze the Cramer–Rao lower bound (CRLB) of the phase-based positioning with a single multiantenna anchor point, and show that the distance from the anchor point can be estimated as long as the IoT device lies within the radiative near-field region. We propose a joint location tracking and WPT algorithm that performs 3-D positioning and beam-focused WPT in a unified way. We have built a real-life testbed with a large-scale antenna array with 64 antenna elements for testing the proposed algorithm. The experimental results show the effectiveness of the proposed algorithm in a real environment.

A 5.8-GHz High-Power and High-Efficiency Rectifier Circuit With Lateral GaN Schottky Diode for Wireless Power Transfer

In this letter, we propose to implement a microwave lateral GaN Schottky barrier diode (SBD) in a designed 5.8-GHz rectifier circuit for future high-power and high-efficiency wireless power transfer. The low-pressure chemical vapor deposition SiN-passivated lateral GaN SBD demonstrates a low turn- on voltage of 0.38 V, a low on -resistance of 4.5 Ω, a low junction capacitance of 0.32 pF at 0-V bias, and a high breakdown voltage of 164 V, which are essentials for a high-efficiency and high-power rectifying application. By incorporating this lateral GaN SBD in a well-designed 5.8-GHz rectifier circuit, an unprecedented combination of high efficiency and high power is achieved simultaneously. The rectifier circuit demonstrates a high RF/dc conversion efficiency ( η RF/DC) of 71 ± 4.5% with an input power ( P in) of 2.5 W and η RF/DC = 50 ± 4.5% with P in = 6.4 W per single diode, showing the great promise of embracing lateral GaN SBD for future wireless high-power transfer application.