Wireless Power Consortium Research Articles

Wireless Power Transfer: A Paradigm Shift for the Next Generation

The first generation of wireless power transfer (WPT) standard Qi, launched in 2010, contains a wide range of transmitter and receiver designs with the aim of maximizing compatibility to attract many manufacturers to share the same standard. Such compatibility feature (i.e., interoperability) has not only attracted over 400 company members in the Wireless Power Consortium (WPC), but also facilitated a fast-growing wireless power market for a decade. The WPC is now expanding the scope of WPT applications to mid-power and high-power applications up to several kilowatts while the Society for Automobile Engineers also set the SAE standard for wireless charging of electric vehicles (EVs) up to tens of kilowatts. Without compromising compatibility, the authors share in this paper their views on the need for a paradigm shift from compatibility to optimal performance in terms of maximum energy efficiency for the entire charging process and minimum charging time. This paradigm change is imminent and important in view of the increasing power of WPT applications. Several enabling technologies essential to the paradigm shift will be addressed.

A Design of Wireless Power Receiver With Gate Charge Recycled Dual-Mode Active Rectifier and Step-Down Converter With 88.2% System Efficiency for Power Management IC

This article presents a power management integrated circuit for a wireless power receiver unit. A dual-mode high-efficiency active rectifier design is based on alliance for wireless power (A4WP) and wireless power consortium (WPC) standards. A gate charge recycling technique is proposed in the dual-mode active rectifier so that the current generated by the switching of the high side gate driver can be recycled to the rectifier output voltage in order to enhance efficiency. A step-down dc–dc converter with a proposed bootstrap circuit and dynamic pull-up resistor gate driver circuit is designed. The chip is implemented in a 0.18-μm bipolar-CMOS double-diffused metal-oxide-semiconductor process. The total die area, including the pad, is 2.64 mm × 2.6 mm. The measurement results show that the active rectifier achieves peak power conversion efficiency (PCE) of 94.7% and generates an output voltage of 7.68 V in WPC mode and 9.1 V with 93.4% efficiency in A4WPC mode. The input voltage varies from 10 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P-P</sub> –12 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P-P</sub> . Similarly, the step-down dc–dc converter obtains a peak PCE of 93.2% at a load current of 500 mA while producing an output voltage of 5 V. The overall system efficiency at A4WP and WPC mode are 86.7% and 88.2%, respectively.

Ranking Qi Wireless Power Transmitters by Efficiency

The Wireless Power Consortium (WPC) Energy Star Task Force (ESTF), comprised of engineers from major cellphone and consumer electronics manufacturers, test houses, and others, developed a proposed efficiency-test protocol for the U.S. Environmental Protection Agency (EPA), intended to rank Qi wireless power transmitters on the basis of power-transfer efficiency. Key to this test protocol is the assumption that a single, optimally designed test receiver may be used to accurately assess transmitter efficiency and enable ranking. The possibility exists that phones using different power receiver designs (e.g., different “friendly metal” profiles, different electrical designs) will interact with transmitters and report different rankings for a given transmitter, thus invalidating the proposed test protocol. Experiments to prove or disprove the assumption that a single optimized receiver can be used as a proxy to fairly compare and rank the real-world power-transfer efficiency performance of Qi wireless power transmitters were conducted. As a result of these experiments, the WPC has concluded that because of receiver–transmitter interaction, it is not possible to rank transmitters by efficiency for more than one receiver design but decreasing the transmitter’s standby power could save as much energy over a 24-h period as improving full-power efficiency by 10%.

Future Networks With Wireless Power Transfer and Energy Harvesting [Scanning the Issue

This month’s special issue provides an overview of the state-of-the-art technology and theory for future networks with wireless power transfer and energy harvesting. Wireless communication using radio frequency (RF) radiation has been around for over 100 years and has shaped our society significantly over the past 40 years. However, wireless is not just about communications. For very short ranges, wireless power supply by inductive power transfer has become a reality through several commercial products and standards (Wireless Power Consortium, Power Matters Alliance, Alliance for Wireless Power, and Rezence). Far-field wireless power transfer (WPT) and wireless energy harvesting (WEH) via RF (as in wireless communications) on the other hand could be used for a longer range. Although it has long been seen as a potential for powering low-power devices, it is only recently that wireless (via RF) power has been recognized as a promising technology to cope with the explosion of low-power devices in future networks. Driven by the reduction in the energy needs of electronic devices (remember Koomey’s law according to which, in 20 years, a device will require 10 000 times less energy to compute a given task) and the advent of trillions of internet device objects [Internet of Things (IoT)], there is a need to rethink the design of the network of the future so that wireless can reach its full potential not only to carry information but also to transfer energy.

A 15-W Triple-Mode Wireless Power Transmitting Unit With High System Efficiency Using Integrated Power Amplifier and DC–DC Converter

In this article, we present an integrated triple-mode wireless power transmitter unit (TWPTU) circuit supporting all three alliance for wireless power (A4WP), wireless power consortium (WPC), and power matters alliance (PMA) standards, simultaneously, for high-efficiency battery charging application. The triple-mode power amplifier (PA) utilizes the adaptive gate splitting technique and free-wheeling control to achieve high efficiency in triple modes. The programmable supply voltage from two high-efficiency dc–dc converters supplies the PA. The output power of the PA is measured by the proposed high accuracy current sensor and controlled through different communication protocols depending on their operating standards. The WPC/PMA modes are based on the amplitude-shifting-key and frequency-shifting-key modulations for the communication between the transmitter and the receiver, while in the A4WP mode, to control the output power of the PA, the bluetooth-low-energy (BLE) unit adjusts the supply voltage through the provided step-up dc–dc converter. A step-down dc–dc converter with pulse-skipping modulation reduces the power consumption of the TWPTU during charge waiting time. The demodulator for in-band communication of WPC/PMA and interintegrated circuit for BLE communication of A4WP is fully implemented. This chip is fabricated using a 180-nm bipolar-CMOS-DMOS (BCD) process with laterally diffused <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> options and has a die size of 5 × 3.5 mm. The proposed architecture achieves the output power level of 25.8 W. The measurement results show the maximum system efficiencies of the TWPTU in WPC/PMA and A4WP modes are 88.5% and 84.3%, respectively.

A 15-W Quadruple-Mode Reconfigurable Bidirectional Wireless Power Transceiver With 95% System Efficiency for Wireless Charging Applications

In this article, a 15-W quadruple-mode reconfigurable wireless power-receiving unit with high efficiency for Inductive coupling and magnetic resonance-based standards, such as wireless power consortium (WPC), power matters alliance (PMA), alliance for wireless power (A4WP), and magnetic secure transmission (MST) applications is proposed. Quadruple-mode gate controller(QMGC) is proposed in quadruple-mode synchronous rectifier to manage sizes of core and drivers which results in maximum power conversion efficiency (PCE) for each mode based on conduction and switching losses. In QMGC, switchable zero voltage and current sensing block is proposed for WPC/PMA to maximize PCE regardless of load current variations. For A4WP mode, a digitally controlled delay adjustment block is adapted to minimize power consumption while compensating for delay which results in reverse leakage current and power loss. Finally, for MST mode, low power TX (LPTX) block is implemented to minimize current consumption so that burden on battery can be minimized. A high-resolution current sensor is proposed in low-drop out regulator to sense current for overcurrent limit and overcurrent protection. The measured PCE of proposed system at 15 W are 95.3% and 92.3% for WPC/PMA and A4WP mode, respectively. For MST, power consumption is reduced to 35% courtesy LPTX block.

Output Voltage Analysis of Inductive Wireless Power Ttransfer with Series LC and LLC Resonance Operations Depending on Coupling Condition

This paper analyzes the output voltage of an inductive wireless power transfer (WPT) depending on coupling conditions. When the optimum efficiency and maximum output power are obtained, it is called critical coupling, so the receiving coil and the transmitting coil should be separated by a certain distance. When the distance between the transmitting coil and receiving coil is very short, it is called over coupling, and output power decreases with the optimal operating state of the critical coupling condition. To design the entire circuit system for the inductive WPT depending on the coupling condition, it is beneficial to analyze the output voltage according to a load variation, an input voltage, and an operating frequency. Therefore, the output voltage depending on the coupling condition in the inductive WPT is analyzed in this paper. The output voltage gain in critical coupling condition is greater than one and is not affected by a load variation by a series LC resonant operation. The reduced output power in an over coupling condition can be recovered by a series LLC resonant operation. In addition, the output voltage gain is almost one and is affected by the load variation in the over coupling condition. A 5W prototype is implemented with the wireless power consortium standard coils and experimental results are shown to verify theoretical analysis and operation.

A Triple-Mode Wireless Power-Receiving Unit With 85.5% System Efficiency for A4WP, WPC, and PMA Applications

This paper presents the design of a triple-mode wireless power-receiving unit (TWPRU) for battery charger with high efficiency. The TWPRU is proposed based on Alliance for Wireless Power (A4WP), Wireless Power Consortium (WPC), and Power Matters Alliance (PMA) standards. An adaptive alignment gate controller technique is proposed in the triple-mode active rectifier to block the reverse leakage current and improve the power conversion efficiency (PCE). This technique can compensate for the delays in the gate control signals of the main switching mosfet s at different operating frequencies for A4WP, WPC, and PMA. The dead time of a dc–dc converter is optimally determined depending on the voltage and the temperature variations by phase calibration circuit. This chip with an active area of 5.0 mm × 3.5 mm is implemented in 0.18- μ m BCD technology. The maximum PCEs of the triple-mode active rectifier are 91.7% in the A4WP mode and 92.7% in the WPC/PMA mode, respectively. The maximum PCE of the dc–dc converter is 92.3% at a load current of 500 mA, while the system efficiencies of TWPRU at A4WP and WPC/PMA modes are about 84.5% and 85.5%, respectively.

An Implantable Wireless Neural Interface System for Simultaneous Recording and Stimulation of Peripheral Nerve with a Single Cuff Electrode.

Recently, implantable devices have become widely used in neural prostheses because they eliminate endemic drawbacks of conventional percutaneous neural interface systems. However, there are still several issues to be considered: low-efficiency wireless power transmission; wireless data communication over restricted operating distance with high power consumption; and limited functionality, working either as a neural signal recorder or as a stimulator. To overcome these issues, we suggest a novel implantable wireless neural interface system for simultaneous neural signal recording and stimulation using a single cuff electrode. By using widely available commercial off-the-shelf (COTS) components, an easily reconfigurable implantable wireless neural interface system was implemented into one compact module. The implantable device includes a wireless power consortium (WPC)-compliant power transmission circuit, a medical implant communication service (MICS)-band-based radio link and a cuff-electrode path controller for simultaneous neural signal recording and stimulation. During in vivo experiments with rabbit models, the implantable device successfully recorded and stimulated the tibial and peroneal nerves while communicating with the external device. The proposed system can be modified for various implantable medical devices, especially such as closed-loop control based implantable neural prostheses requiring neural signal recording and stimulation at the same time.

Design of a Patterned Soft Magnetic Structure to Reduce Magnetic Flux Leakage of Magnetic Induction Wireless Power Transfer Systems

A soft-magnetic-metal-based shield structure was designed to reduce magnetic flux leakage in magnetic-induction wireless power transfer systems. Soft magnetic metals have the advantages of high permeability and low magnetic loss, but have the disadvantage of high power loss owing to eddy current that is induced on the surface as a result of low-insulating characteristics. In order to solve this problem, a patterned soft magnetic metal was used to cut the route of the induced current. This decreases the power loss and reduces the leakage of magnetic field. A soft-magnetic-metal-based structure that has various patterns was designed to find the optimal structure for reducing the leaking magnetic field. By applying this structure to Wireless Power Consortium commercial A10 coil, the inductance, transfer efficiency, and magnetic flux leakage of the coil according to the material and the structure of the soft-magnetic-metal-based structure were observed. Fabrication and measurement tests were performed to verify the proposed structure, and it was found that the test results corresponded to the simulation results. It was confirmed that the proposed structure had a 84% thinner thickness compared with that of a conventional ferrite shield, an equivalent transfer efficiency of 74.5%, and a reduction in magnetic flux leakage of 20.9%.

A Dynamic EV Charging System for Slow Moving Traffic Applications

Inductive power transfer (IPT) is by far the most popular method to transfer energy wirelessly and has attracted considerable attention in recent times. The Wireless Power Consortium has developed a standard (Qi) for low power consumer electronics, whereas the Society of Automotive Engineers (SAE) is working on a standard (J2954) to charge electric vehicles (EVs) wirelessly. SAE’s current efforts are focused only on transferring power to the vehicles at rest (static), whereas no work has been done so far on developing the standards for transferring power to the vehicles on the move (dynamic). This paper presents the magnetic design of an IPT system for a dynamic EV charging application, to continuously deliver a power of 15 kW to an EV, along the direction of travel within the lateral misalignment of ±200 mm. The experimental validation of system operation, however, was conducted at 5 kW. The design aims at distributing the cost and complexity of the system between the primary and secondary sides, while achieving a smooth power transfer profile. In addition, the system is designed to exploit the shielding effect provided by the vehicle, as the field generating components of the system are covered by the vehicle body under all operating conditions.

Improving Wireless Power Transfer Efficiency Using Hollow Windings With Track-Width-Ratio

With the widespread growth of slim portable electronics, research into planar spiral windings for wireless power transfer (WPT) battery charging systems has gained a lot of attention due to their low profile, reproducibility, and manufacturability. The major design goal for windings in WPT systems is high-quality factor (Q) for a given inductance. This study presents a new method to improve Q by using a nonunity track-width-ratio (TWR) geometrical arrangement in conjunction with increasing the inner radius of winding to make an improved hollow planar spiral winding. The inductance regulation of this technique combined with its exceptional improvements in Q make it a superior choice for resonant WPT systems. Inductance and resistance models are provided in order to assess the Q improvement. A 5-W, 110–200-kHz WPT system was built and tested based on the wireless power consortium Qi standard to confirm the advantages of the improved windings on the transmission efficiency of the system. An increase from 70% to 90% are observed from employing the proposed winding technique designed using the provided models.

Development and Operation of Dual-Mode Analyzers for Wireless Power Consortium/Power Matters Alliance Wireless Power Systems.

We have designed a protocol analyzer to be used in wireless power systems and analyzed the operation of wireless chargers defined by standards of Qi of Wireless Power Consortium (WPC) and Power Matters Alliance (PMA) protocols. The integrated circuit (IC, or microchip) developed so far for wireless power transmission is not easily adopted by chargers for specific purposes. A device for measuring the performance of test equipment currently available is required to transform and expand the types of protocol. Since a protocol analyzer with these functions is required, we have developed a device that can analyze the two protocols of WPC and PMA at the same time. As a result of our research, we present a dual-mode system that can analyze the protocols of both WPC and PMA.

Proposal of a Figure of Merit for the Characterization of Soft-Magnetic Shielding Material Used in Inductive Wireless Power Transmission Systems

In inductive wireless power transmission systems, often soft-magnetic shielding is used to avoid lossy eddy currents being induced in electrically conducting components like batteries or ground layers of electronic circuits. Datasheet information on such shielding materials are often limited to magnetic permeability and sometimes exemplary loss information. For designing inductive wireless power systems, e.g., at variable frequency, detailed loss information are of interest. Therefore, it is proposed to measure the impact of these materials on the power transmission in a standardized setup, which is closely related to the real application. This consists of two coils, a transmitting and a receiving coil. Here, a configuration as described in the Qi standard for wireless charging of mobile devices published by the wireless power consortium is used as reference. A figure of merit, i.e., the product of the coupling factor $k$ and the geometric average of the coil’s quality factors Q , is proposed to qualify materials concerning both: 1) shielding against conducting components on the backside of the receiving coil and 2) establishing a high mutual inductance of the transformer coils, resulting in higher system efficiencies. Furthermore, considerations on the measurement setup as well as qualifications of shielding materials are presented.

History, Current Status and Future of the Wireless Power Consortium and the Qi Interface Specification

In November 2008, eight companies founded an open industry group, the Wireless Power Consortium, because these companies concluded that wireless charging solutions for mobile phones could not be introduced successfully unless wireless chargers were compatible across brands. By jointly developing a wireless power interface specification,the members of the Wireless Power Consortium share the risks and the investments needed to create a market for their products. At the same time, the members of the Wireless Power Consortium compete with each other in the market. They compete for market share by providing implementations of the standard that have unique properties. The competition between the members makes sure that performance increases, cost go down, and that more applications are served with solutions tuned to specific market needs. The Wireless Power Consortium published the Qi interface specification in August 2010, together with a test specification and a certification procedure. More than 200 companies are now a members of the Wireless Power Consortium, and more than 700 different products have been certified to be compliant with the Qi interface specification. In the coming years the Wireless Power Consortium will extend the Qi interface specification by providing more choice in power levels, more choice in transfer distance, more choice in charging area, more choice in low-cost solutions, more choice in multidevice chargers, more choice in systems for automotive use, and more choice in systems for public space charging.

Qi Standard Wireless Power Transfer Technology Development Toward Spatial Freedom

?Qi?, the wireless power transfer interface specification published by the Wireless Power Consortium (WPC), has been regarded as the wireless charging standard and widely adopted in industry since its first release in August 2010. Larger spatial freedom between the power transmitter and power receiver provides significant convenience for users and device manufacturers, and is one of the critical factors for the success of wireless charging. In this paper, the recent progress of Qi technology development towards spatial freedom is reported. The achieved larger X/Y/Z placement shows that Qi standard is also capable to fulfill the spatial freedom requirement.

Analysis of in situ electric field and specific absorption rate in human models for wireless power transfer system with induction coupling

This study investigates the specific absorption rate (SAR) and the in situ electric field in anatomically based human models for the magnetic field from an inductive wireless power transfer system developed on the basis of the specifications of the wireless power consortium. The transfer system consists of two induction coils covered by magnetic sheets. Both the waiting and charging conditions are considered. The transfer frequency considered in this study is 140 kHz, which is within the range where the magneto-quasi-static approximation is valid. The SAR and in situ electric field in the chest and arm of the models are calculated by numerically solving the scalar potential finite difference equation. The electromagnetic modelling of the coils in the wireless power transfer system is verified by comparing the computed and measured magnetic field distributions. The results indicate that the peak value of the SAR averaged over a 10 g of tissue and that of the in situ electric field are 72 nW kg−1 and 91 mV m−1 for a transmitted power of 1 W, Consequently, the maximum allowable transmitted powers satisfying the exposure limits of the SAR (2 W kg−1) and the in situ electric field (18.9 V m−1) are found to be 28 MW and 43 kW. The computational results show that the in situ electric field in the chest is the most restrictive factor when compliance with the wireless power transfer system is evaluated according to international guidelines.

Planar Wireless Charging Technology for Portable Electronic Products and Qi

Starting from the basic principles of Tesla's wireless power transfer experiment in the 1890s, this review article addresses the key historical developments of wireless power and its modern applications up to formation of the world's first international wireless power standard “Qi” launched in 2010 for portable electronics. The scientific principles laid down by Nicolas Tesla for wireless power transfer, which still remain valid today, are first explained. Then, modern wireless power applications based on nonradiative (near-field) magnetic coupling for short-range applications are described. Some industrial application examples emerging since the 1960s are highlighted. The article then focuses on the comparison of the horizontal and vertical magnetic flux approaches developed in the early 1990s for low-power planar wireless charging pads. Several critical features such as localized charging, load identification, and freedom of positioning that are essential to wireless charging of portable electronic devices are explained. The core technologies adopted by the Wireless Power Consortium (WPC) for the “Qi” Standard in 2010 are summarized. Finally, the latest research and developments of wireless power transfer for midrange applications based on the domino-resonator concept and their future application potential are described.