With the recent advancement and progress in the field of wireless power transfer (WPT), there is an ever increasing demand for high power transfer efficiency (PTE) of the WPT systems and improved transfer distance for the end-users. However, some existing WPT systems have limited PTE and transfer distance as they take the inductive coupling approach, where the PTE dramatically decreases as the distance between (Tx) and receiver (Rx) coils increases. Alternatively, magnetic resonance coupling (MRC) is used as a mid-range WPT approach, for which the insertion of metamaterials (MTMs) between Tx and Rx coils is exploited to improve efficiency. MTMs are artificially engineered materials that show uncommon electromagnetic properties, such as evanescent wave amplification and negative refractive characteristics, which could be utilized for the enhancement of PTE. In this article, a comprehensive review on recent progresses in the MTM-based WPT systems is reported, where previously reported MTM-based WPT systems are compared in terms of various parameters such as configurations, operating frequencies, dimensions and PTE. Also, the PTEs of these systems were plotted as a function of the normalized transfer distance. This review is expected to provide an insight for understanding the trends of the MTM-based WPT systems and serve as a reference for researchers who work on WPT systems and their applications.

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Wireless Power Transfer Systems Using Metamaterials: A Review

This paper methodizes efficiency improvement during lateral misalignment for the wireless power transfer (WPT) system using two coupled semielliptic defected ground structure (DGS) resonators. We design the WPT system using a scaled value of the available mutual coupling between the resonators at perfect alignment. This scaled mutual coupling value enforces over-coupling regime, and the WPT efficiency is lower than the peak one. Then, during lateral misalignment, WPT efficiency improves until it peaks at critical coupling. Next, with further misalignment, the efficiency drops as the system enters loose coupling regime. The proposed method consists of two steps. First, we derive an analytical approach to determine the scaled mutual coupling value for efficiency design during lateral misalignment, and second, we improve the maximum obtainable efficiency by careful designing of the DGS structures’ profile. We verify the proposed method by implementing the WPT system at 300 MHz, which shows a peak efficiency of 80% for a size $D_{o} \times D_{o} = 40 \times 40$ mm2 with a transfer distance $d = D_{o}$ . During misalignment, the efficiency is higher than 50% for a lateral shift up to ±0.75 $D_{o}$ .

Design Approach for Efficient Wireless Power Transfer Systems During Lateral Misalignment

Conventional resonant inductive coupling wireless power transfer (WPT) systems encounter performance degradation while energizing biomedical implants. This degradation results from the dielectric and conductive characteristics of the tissue, which cause increased radiation and conduction losses, respectively. Moreover, the proximity of a resonator to the high permittivity tissue causes a change in its operating frequency if misalignment occurs. In this report, we propose a metamaterial inspired geometry with near-zero permeability property to overcome these mentioned problems. This metamaterial inspired geometry is stacked split ring resonator metamaterial fed by a driving inductive loop and acts as a WPT transmitter for an in-tissue implanted WPT receiver. The presented demonstrations have confirmed that the proposed metamaterial inspired WPT system outperforms the conventional one. Also, the resonance frequency of the proposed metamaterial inspired TX is negligibly affected by the tissue characteristics, which is of great interest from the design and operation prospects. Furthermore, the proposed WPT system can be used with more than twice the input power of the conventional one while complying with the safety regulations of electromagnetic waves exposure.

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Wireless power transfer system rigid to tissue characteristics using metamaterial inspired geometry for biomedical implant applications

This letter presents a separation-misalignment insensitive wireless power transfer (WPT) system. This insensitivity is possible by minimizing the variation of mutual inductance. The transmitter (TX) of the proposed WPT system uses a two-plane printed inductor, and the receiver (RX) is a single-plane one. The TX encloses the RX to achieve minimal mutual inductance variations. Hence, the lost mutual inductance due to the misalignment between the RX and one of the TX inductors is compensated by getting close to/moving away from the other TX inductor. Furthermore, admittance inversion and compensation networks are designed to compensate for the remaining variations. Then, we design and fabricate a prototype at 50 MHz. Each TX inductor has an area of 50 $\text {mm} \times 50$ mm and an initial separation of 30 mm from the 25 $\text {mm} \times 25$ mm RX. The measured efficiency ranges between 72.5% and 80.5% for the separation of ±20 mm from the initial position.

Separation-Misalignment Insensitive WPT System Using Two-Plane Printed Inductors

We propose a dual-band wireless power transfer (WPT) system employing a dual-mode inductor. The dual-mode inductor is possible through enforcing a self-resonance condition by loading an inductor in series by a tank circuit. In return, two distinct resonances are achieved, simultaneously, utilizing a single compensation capacitor as the inductance of the dual-mode inductor appears with a smaller value after its self-resonance. Also, by maintaining the same mutual coupling, the coupling coefficient becomes larger at the higher resonance, which allows for the employment of the same source/load admittance inversion network to achieve maximum power transfer at both of the operating frequency bands, concurrently. We verify the operation by fabricating a dual-band WPT system, which shows measured efficiencies of 70% and 69% at 90.3 MHz and 138.8 MHz, correspondingly. The size of the WPT system is $\boldsymbol {50} \boldsymbol {\times } \boldsymbol {50}$ mm $^{\boldsymbol {2}}$ and has a transfer distance of 40 mm.

Design and Implementation of Dual-Mode Inductors for Dual-Band Wireless Power Transfer Systems

This brief proposes a design methodology based on the admittance ( ${J}$ -) inverters for a dual-band wireless power transfer (WPT) system that employs two cascaded circulars defected ground structure (DGS) resonators with different capacitive loading to guarantee distinct resonances. A single microstrip line excites the two DGSs, and when two DGS resonators are coupled back to back, it transforms to a dual band pass filter leading to WPT at both bands. Each of the DGS resonators has independent coupling. Thus, the realized ${J}$ -Inverters are designed independently. Also, we employ a single stub for the matching. This stub appears with a different length according to the operating frequency; hence, an independent external coupling is achieved at both frequencies. A compact size of 30 mm ${\times } \,\, 15$ mm is fabricated achieving a WPT efficiency of more than 71% at a power transfer distance of 16 mm for both bands (0.3 and 0.7 GHz).

Dual-Band Defected Ground Structures Wireless Power Transfer System With Independent External and Inter-Resonator Coupling

This letter proposes a wireless power transfer (WPT) system using coplanar waveguide fed rectangular defected ground structure (DGS) resonators. One of the advantages of rectangular DGS resonator is that it has higher quality (Q-) and coupling (K-) factor compared to the H-shape or semi-H-shape DGS resonators. When two DGS resonators are coupled back-to-back, it transforms to a bandpass filter with tight coupling, resulting in the transfer of power wirelessly. The fabricated WPT system of size $35.8~\text {mm} \times 20$ mm achieves, for the first time, a figure of merit of more than one at a WPT distance from 40 to 44 mm.

1.06 FoM and Compact Wireless Power Transfer System Using Rectangular Defected Ground Structure Resonators

This paper presents a compact dual-band wireless power transfer (WPT) system using overlapped defected ground structure for biomedical applications. One band for power and another for data transfer. The proposed defected ground structure can channel two distinct resonant frequencies. As a result, the fabricated WPT device has almost 50% reduction in the size without changing other performances compared to ref. [9]. The fabricated device is 15 mm ×15 mm. The measured efficiency is 71% and 73%, respectively at 0.45 GHz and 0.95 GHz at WPT distance of 12.5mm.

Compact Dual-Band Wireless Power Transfer Using Overlapped Single Loop Defected Ground Structure

This paper presents a novel design for a compact dual-band wireless power transfer (WPT) system through biological tissues using dual-reference defected ground structure (DGS) resonators. Overlapping is used to minimize the size to be suitable for biomedical applications. The proposed DGS resonator exhibits bandstop characteristics at two different frequencies depending on the excitation reference-plane. Then, two coupled sets of the proposed dual-reference DGS, in a back-to-back configuration, achieves a dual-band WPT system. The sizes of the receiver and transmitter are 15 mm × 15 mm and 18 mm × 18 mm, respectively. The measured efficiencies are 60.3% at 381 MHz and 54.3% at 750 MHz when the receiver is embedded in chicken breast within a depth of 10 mm, while the overall WPT distance of 11 mm

Efficient and Compact Dual-band Wireless Power Transfer System through Biological Tissues Using Dual-Reference DGS Resonators

This work presents a dual-band wireless power transfer (WPT) system with high figure of merit $(FoM)$. One band can be used for power transfer and another for data transfer in biomedical applications. A Bow-tie Defected Ground Structure (DGS) resonators with high quality (Q-) factor and coupling (k-) factor is utilized for ensuring high efficiency. An equivalent circuit model using the admittance (J-) inverters is utilized to design the admittance inversion and compensation networks. The measured result shows that the proposed WPT system is able to transfer the power at both of the bands with a highFoM, making a suitable candidate for compact near filed WPT applications.

Fairus Tahar

Papers

Dual-Band Defected Ground Structures Wireless Power Transfer System With Independent External and Inter-Resonator Coupling

1.06 FoM and Compact Wireless Power Transfer System Using Rectangular Defected Ground Structure Resonators

Compact Dual-Band Wireless Power Transfer Using Overlapped Single Loop Defected Ground Structure

Efficient and Compact Dual-band Wireless Power Transfer System through Biological Tissues Using Dual-Reference DGS Resonators

High FOM Dual Band Wireless Power Transfer using Bow-tie Defected Ground Structure Resonators