Wireless Power Transmission Link Research Articles

Analytical layout optimization of printed planar coil with variable trace width for inductive wireless power transfer

In the inductive wireless power transmission (WPT) designs of consumer electronics and implantable devices, the printed planar coil in standard manufacture is commonly used. Layout optimization of the coils is one of the important ways to make the power transmission system more efficient. Varying the trace width and turn-to-turn spacing together for the coils is proposed to optimize the maximum achievable power transfer efficiency (𝜂max). An accurate analytical model for the printed square coils is also established as well to speed up the design process. By virtue of this model, an optimal scaling factor of the trace width and the optimal frequency can be quickly estimated. The proposed model is validated by both the simulations (ANSYS HFSS) and experiments. A WPT link of two planar coils with size of 50 mm × 50 mm × 1 mm, operating at 23 MHz, is optimized by using this methodology. After optimization, the measured 𝜂max of the WPT system is increased from 22.60% to 32.74% at a 100-mm transmission distance.

Feasibility Study on Active Back Telemetry and Power Transmission Through an Inductive Link for Millimeter-Sized Biomedical Implants.

This paper presents a feasibility study of wireless power and data transmission through an inductive link to a 1-mm 2 implant, to be used as a free-floating neural probe, distributed across a brain area of interest. The proposed structure utilizes a four-coil inductive link for back telemetry, shared with a three-coil link for wireless power transmission. We propose a design procedure for geometrical optimization of the inductive link in terms of power transmission efficiency (PTE) considering specific absorption rate and data rate. We have designed a low-power pulse-based active data transmission circuit and characterized performance of the proposed inductive link in terms of its data rate and bit error rate (BER). The 1-mm2 data-Tx/power-Rx coil is implemented using insulated bonding wire with diameter, resulting in measured PTE in tissue media of 2.01% at 131 MHz and 1.8-cm coil separation distance when the resonator coil inner radius is 1 cm. The measured BER at 1-Mbps data rate was and in the air and tissue environments, respectively.

A Hybrid Inductive-Ultrasonic Link for Wireless Power Transmission to Millimeter-Sized Biomedical Implants

Ultrasound has recently been utilized for efficient wireless power transmission (WPT) to biomedical implants with millimeter (mm) dimensions and below. However, the power transmission efficiency (PTE) of ultrasonic links reduces significantly in mediums with different acoustic impedances. This brief presents a hybrid inductive-ultrasonic WPT link for powering mm-sized implants that utilizes two cascaded co-optimized inductive and ultrasonic links for WPT through bone/air and tissue, respectively. The ultrasonic link is first optimized based on the implant depth (d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">us</sub> ) and load (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> ) to find the optimal geometries for ultrasonic transducers and operation frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> ). Then, the inductive link is optimized at f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> to drive the transmitter (Tx) transducer. A hybrid link was optimized and measured to transfer power to a receiver (Rx) transducer with the diameter of 1.1 mm, loaded by R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> of 2.5 kQ and located at d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">us</sub> = 3 cm inside castor oil, as the tissue model. The Tx coil was placed in air and spaced from the Rx coil by 3 cm, resulting in a total powering distance (d) of 6 cm. At the optimal f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> of 1.1 MHz, the hybrid link achieved a considerable measured PTE of 0.16%.

Robust Wireless Power Transmission to mm-Sized Free-Floating Distributed Implants.

This paper presents an inductive link for wireless power transmission (WPT) to mm-sized free-floating implants (FFIs) distributed in a large three-dimensional space in the neural tissue that is insensitive to the exact location of the receiver (Rx). The proposed structure utilizes a high-Q resonator on the target wirelessly powered plane that encompasses randomly positioned multiple FFIs, all powered by a large external transmitter (Tx). Based on resonant WPT fundamentals, we have devised a detailed method for optimization of the FFIs and explored design strategies and safety concerns, such as coil segmentation and specific absorption rate limits using realistic finite element simulation models in HFSS including head tissue layers, respectively. We have built several FFI prototypes to conduct accurate measurements and to characterize the performance of the proposed WPT method. Measurement results on 1-mm receivers operating at 60 MHz show power transfer efficiency and power delivered to the load at 2.4% and 1.3 mW, respectively, within 14-18 mm of Tx-Rx separation and 7 cm2 of brain surface.

Two‐port network approach for a wireless power transfer link using a cascade of inductively coupled resonators

SummaryA wireless power transmission link consisting of a transmitting and a receiving resonator wirelessly connected by means of N relay resonators is investigated, and a 2‐port network approach is proposed. The presented approach exploits the properties of periodic networks to derive useful closed‐form formulas that can be used to match the wireless energy link with respect to generic reference impedances.

A Figure-of-Merit for Design and Optimization of Inductive Power Transmission Links for Millimeter-Sized Biomedical Implants.

Power transmission efficiency (PTE) has been the key parameter for wireless power transmission (WPT) to biomedical implants with millimeter (mm) dimensions. It has been suggested that for mm-sized implants increasing the power carrier frequency (fp) of the WPT link to hundreds of MHz improves PTE. However, increasing fp significantly reduces the maximum allowable power that can be transmitted under the specific absorption rate (SAR) constraints. This paper presents a new figure-of-merit (FoM) and a design methodology for optimal WPT to mm-sized implants via inductive coupling by striking a balance between PTE and maximum delivered power under SAR constraints (PL,SAR). First, the optimal mm-sized receiver (Rx) coil geometry is identified for a wide range of fp to maximize the Rx coil quality factor (Q). Secondly, the optimal transmitter (Tx) coil geometry and fp are found to maximize the proposed FoM under a low-loss Rx matched-load condition. Finally, proper Tx coil and tissue spacing is identified based on FoM at the optimal fp. We demonstrate that fp in order of tens of MHz still offer higher PL,SAR and FoM, which is key in applications that demand high power such as optogenetics. An inductive link to power a 1 mm 3 implant was designed based on our FoM and verified through full-wave electromagnetic field simulations and measurements using de-embedding method. In our measurements, an Rx coil with 1 mm diameter, located 10 mm inside the tissue, achieved PTE and PL,SAR of 1.4% and 2.2 mW at fp of 20 MHz, respectively.

Design and Optimization of Ultrasonic Wireless Power Transmission Links for Millimeter-Sized Biomedical Implants.

Ultrasound has been recently proposed as an alternative modality for efficient wireless power transmission (WPT) to biomedical implants with millimeter (mm) dimensions. This paper presents the theory and design methodology of ultrasonic WPT links that involve mm-sized receivers (Rx). For given load (RL) and powering distance (d), the optimal geometries of transmitter (Tx) and Rx ultrasonic transducers, including their diameter and thickness, as well as the optimal operation frequency (fc) are found through a recursive design procedure to maximize the power transmission efficiency (PTE). First, a range of realistic fcs is found based on the Rx thickness constrain. For a chosen fc within the range, the diameter and thickness of the Rx transducer are then swept together to maximize PTE. Then, the diameter and thickness of the Tx transducer are optimized to maximize PTE. Finally, this procedure is repeated for different fcs to find the optimal fc and its corresponding transducer geometries that maximize PTE. A design example of ultrasonic link has been presented and optimized for WPT to a 1 mm3 implant, including a disk-shaped piezoelectric transducer on a silicon die. In simulations, a PTE of 2.11% at fc of 1.8 MHz was achieved for RL of 2.5 [Formula: see text] at [Formula: see text]. In order to validate our simulations, an ultrasonic link was optimized for a 1 mm3 piezoelectric transducer mounted on a printed circuit board (PCB), which led to simulated and measured PTEs of 0.65% and 0.66% at fc of 1.1 MHz for RL of 2.5 [Formula: see text] at [Formula: see text], respectively.

Optimal resonance configuration for ultrasonic wireless power transmission to millimeter-sized biomedical implants.

In order to achieve efficient wireless power transmission (WPT) to biomedical implants with millimeter (mm) dimensions, ultrasonic WPT links have recently been proposed. Operating both transmitter (Tx) and receiver (Rx) ultrasonic transducers at their resonance frequency (fr) is key in improving power transmission efficiency (PTE). In this paper, different resonance configurations for Tx and Rx transducers, including series and parallel resonance, have been studied to help the designers of ultrasonic WPT links to choose the optimal resonance configuration for Tx and Rx that maximizes PTE. The geometries for disk-shaped transducers of four different sets of links, operating at series-series, series-parallel, parallel-series, and parallel-parallel resonance configurations in Tx and Rx, have been found through finite-element method (FEM) simulation tools for operation at fr of 1.4 MHz. Our simulation results suggest that operating the Tx transducer with parallel resonance increases PTE, while the resonance configuration of the mm-sized Rx transducer highly depends on the load resistance, Rl. For applications that involve large Rl in the order of tens of kΩ, a parallel resonance for a mm-sized Rx leads to higher PTE, while series resonance is preferred for Rl in the order of several kΩ and below.

Inductive power transmission to millimeter-sized biomedical implants using printed spiral coils.

The operation frequency (f) has been a key parameter in optimizing wireless power transmission links for biomedical implants with millimeter (mm) dimensions. This paper studies the feasibility of using printed spiral coils (PSCs) for powering mm-sized implants with high power transmission efficiency (PTE) at different fps. Compared to wire-wound coils (WWCs), using a PSC in the implant side allows batch fabrication on rigid or flexible substrates, which can also be used as a platform for integrating implant components. For powering an implant with 1 mm diameter, located 10 mm inside the tissue, the geometries of transmitter (Tx) and receiver (Rx) PSCs were optimized at different fPs of 50 MHz, 200 MHz, and 500 MHz using a commercial field solver (HFSS). In simulations, PSC- and WWC-based links achieved maximum PTE of 0.13% and 3.3%, and delivered power of 65.7 μW and 720 μW under specific absorption rate (SAR) constraints at the optimal fp of 50 MHz and 100 MHz, respectively, suggesting that the performance of the PSC-based link is significantly inferior to that of the WWC-based link.

Rigorous design of matched wireless power transfer links based on inductive coupling

AbstractThis paper focuses on a near‐field wireless power transmission link consisting of two magnetically coupled inductances. The case of a resonant coupling realized by adding appropriate compensating capacitances is solved. By using a network formalism, the link is modeled as a two‐port network and rigorously analyzed in the case where both the input impedance and the load are specified. In particular, it is demonstrated that there is just one optimum design of the network that allows maximizing both the efficiency and the active power on the load. Closed‐form design formulas for the optimum design are presented and validated by circuital simulations.

Optimal Design of Wireless Power Transmission Links for Millimeter-Sized Biomedical Implants.

This paper presents a design methodology for RF power transmission to millimeter-sized implantable biomedical devices. The optimal operating frequency and coil geometries are found such that power transfer efficiency (PTE) and tissue-loss-constrained allowed power are maximized. We define receiver power reception susceptibility (Rx-PRS) and transmitter figure of merit (Tx-FoM) such that their multiplication yields the PTE. Rx-PRS and Tx-FoM define the roles of the Rx and Tx in the PTE, respectively. First, the optimal Rx coil geometry and operating frequency range are identified such that the Rx-PRS is maximized for given implant constraints. Since the Rx is very small and has lesser design freedom than the Tx, the overall operating frequency is restricted mainly by the Rx. Rx-PRS identifies such operating frequency constraint imposed by the Rx. Secondly, the Tx coil geometry is selected such that the Tx-FoM is maximized under the frequency constraint at which the Rx-PRS was saturated. This aligns the target frequency range of Tx optimization with the frequency range at which Rx performance is high, resulting in the maximum PTE. Finally, we have found that even in the frequency range at which the PTE is relatively flat, the tissue loss per unit delivered power can be significantly different for each frequency. The Rx-PRS can predict the frequency range at which the tissue loss per unit delivered power is minimized while PTE is maintained high. In this way, frequency adjustment for the PTE and tissue-loss-constrained allowed power is realized by characterizing the Rx-PRS. The design procedure was verified through full-wave electromagnetic field simulations and measurements using de-embedding method. A prototype implant, 1 mm in diameter, achieved PTE of 0.56% ( -22.5 dB) and power delivered to load (PDL) was 224 μW at 200 MHz with 12 mm Tx-to-Rx separation in the tissue environment.

Theoretical and Numerical Design of a Wireless Power Transmission Link With GaN-Based Transmitter and Adaptive Receiver

In this paper, we describe a rigorous theoretical approach to the circuit-level nonlinear design of an entire inductive resonant wireless power transfer (IR-WPT) system, including the transmitter and receiver nonlinear subsystems. Starting from a novel analytical characterization of the inductive resonant link, the system efficiency is parametrically computed as a function of a set of circuital parameters, including the power levels to be transferred. These quantities are then used as design goals inside the nonlinear optimization of the transmitter and receiver blocks. By adopting the last generation miniaturized enhanced-mode AlGaN/GaN-power field-effect transistor and fast Schottky diodes, a Class-D amplifier and a full-bridge rectifier followed by a switching dc-dc Buck converter that acts as load impedance transformer are designed in a single optimization process at 6.78 MHz. Thus, the transmitter and the receiver are directly connected by the IR two-port network, and the system is capable to adapt to variable distances between the resonators of the IR-WPT link. The choice of the Class-D topology for the transmitter and the adaptability of the active receiver enable to get rid of inter-stage matching networks, which can severely reduce the overall efficiency, especially in high power transfer environments. With the proposed IR-WPT system, up to 44 W of transferred power and a peak of 73% dc-to-dc efficiency were obtained with an input dc voltage VDC=30 V at a link distance D=5 cm. Numerical and experimental results are discussed, demonstrating the accuracy of the proposed design procedure.

THEORETICAL ANALYSIS OF RESONANT WIRELESS POWER TRANSMISSION LINKS COMPOSED OF ELECTRICALLY SMALL LOOPS

This paper presents an analytical method to calculate the scattering parameters of a wireless power transmission link composed of electrically small single loop resonators. The proposed method takes into account all the difierent couplings in the structure. First, the method is presented and used to flnd the S-parameters for links composed of circular and rectangular resonators. The model is then used to flnd the optimal topology for a given transmission distance. Validation of the model is done by comparing its results with experimental measurements. Based on this model, a software used for the design of wireless power transmission links has been developed and is presented. Finally, demonstrations that this model produces excellent results are provided. At resonant frequency, an accuracy better than 2% is reached.

Wireless power transmission technology state of the art the first Bill Brown lecture

This first Bill Brown SSP Technology Lecture covers the state-of-the-art (SOA) in wireless power transmission (WPT) technology including microwave and laser systems for the transfer of CW electric power, as related to eventually developing Space Solar Power (SSP) systems. Current and past technology accomplishments in ground based and air and space applied energy conversion devices, systems and modeling information is presented, where such data are known to the author. The purpose of the presentation is to discuss and present data to encourage documenting and breaking the current records, so as to advance the SOA in WPT for SSP. For example, regarding DC to RF and laser converters, 83% efficient 2.45 GHz cooker-tube magnetrons with 800 W CW output have been developed by CPII and the Russians. Over 50% wall-plug efficient 1.5 kW/cm 2 CW, water-cooled, multibeam, solid state laser diode bar-arrays have been developed by LLNL at 808 nm wavelength. The Germans have developed a 36% efficient, kW level, single coherent beam, lateral pumped semiconductor laser. The record for end-to-end DC input to DC output power overall WPT link conversion efficiency is 54% achieved during the Raytheon-JPL experiments in 1975 for 495.6 W recovered at 1.7 −m range, at 2.4469 GHz The record for usefully recovered electric power output (as contrasted with thermally induced power in structures) is 34 kW DC output at a range of 1.55 km, using 2.388 GHz microwaves, during the JPL-Raytheon experiments by Bill Brown and the author at Goldstone, CA in 1975. The GaAs-diode rectenna array had an average collection-conversion efficiency of 82.5%. A single rectenna element developed by Bill Brown demonstrated 91.4% efficiency. The comparable record for laser light to DC output power conversion efficiency of photovoltaics is 59%, for AlGaAs at 1.7 W and 826-nm wavelength.

Case study in Reunion Island

This paper presents the status of a case study that is being carried in Reunion Island, France, to investigate the opportunity to use a point-to-point wireless power transmission link to deliver electricity to a small isolated village called Grand-Bassin. The different steps followed since this case study was first started are briefly reminded. The last stage consisting in the development of an indoor industrial prototype is presented. Perspectives are given towards the emergence of a semi-private organization whose first role would be to evaluate the viability of the concept of using WPT as a way to extend the conventional electricity grid in order to provide energy to remote places not only in Reunion Island, but in many places around the world. This organization would also have as a role, to head the industrial engineering and development of point-to-point WPT prototype equipment.