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Resonant wireless power transfer technology & integration roadmap
This work presents a novel approach to generate multiple passbands from an asymmetric two-coil transformer. The details of the asymmetry between the primary and secondary coils controls the passbands’ location and bandwidth (BW). A cylindrical asymmetric connector was designed, and prototyped. The S-parameters were measured with a Vector Network Analyzer (VNA) which shows a very good match with theoretical model up to the second passband frequency (10Mhz~1 GHz). The first passband is used for power transfer since the lower frequency is better for power transmission. The higher frequency of the second passband can be used to transmit high bandwidth data. A carrier signal centered at second passband frequency was used to modulate digital data at the transmitter and demodulated in the receiver side. This connector can be used for transmitting only power, or only data or for both power and data simultaneously. We demonstrate transfer 19 mW of power, and 100 Mbps of data simultaneously in a 5mm diameter and 7mm high cylindrical structure. To the best of our knowledge, it is the highest data rate demonstrated through a Wireless Power Transfer link.
The authors developed a high-efficiency gallium-nitride (GaN) Class-E converter for a 6.78 MHz magnetic resonant wireless power transfer system. A negative-bias gate driver circuit made it possible to use a depletion mode GaN high-electron-mobility transistor (HEMT), and simplified the converter circuit. As the depletion mode GaN HEMT with very small gate–source capacitance provided almost ideal zero-voltage switching, the authors attained a drain efficiency of 98.8% and a total efficiency of 97.7%, including power consumption of a gate driver circuit, at a power output of 33 W. In addition, the authors demonstrated a 6.78 MHz magnetic resonant wireless power transfer system that consisted of the GaN Class-E converter, a pair of magnetic resonant coils 150 mm in diameter with an air-gap distance of 40 mm, and a full-bridge rectifier using Si Schottky barrier diodes. The system achieved a dc–dc efficiency of 82.8% at a power output of 25 W. The efficiencies of coil coupling and the rectifier were estimated to be ∼ 94 and 90%, respectively.
In this study, human exposure to close-range wireless power transfer (WPT) systems operating in the frequency range 0.1–10 MHz with coil diameters up to 150 mm are investigated. Approximation formulae, which include scaling factors derived from numerical simulations that take variations of complex human anatomies into consideration, are proposed to conservatively estimate human exposure with respect to the most authoritative exposure guidelines. The approximation has been verified for two precommercial prototype WPT systems, the first of which, a 5-W system operating at 100 kHz, has been evaluated in this study; the second system been verified was reported in a separate study and operates at 6.78 MHz with a nominal current of 5.4 A$_{rm bf rms}$. Based on the results obtained, the optimal operational frequency range for WPT with respect to compliance with exposure safety guidelines is revealed to be ca. 1–2.5 MHz. In summary, this study provides novel and insightful information for the design of an exposure-compliant close-range magnetic resonant WPT system.
The Alliance for Wireless Power (A4WP) Version 1.0 Baseline System Specification (BSS) is an interoperability specification for loosely-coupled (LC) wireless power transfer (WPT) systems that meet next-generation user experience and industrial design requirements as part of the vision of Ubiquitous Power for portable, hand-held, consumer electronic devices. This survey of the A4WP Version 1.0 BSS reviews an LC WPT reference model, power transmitting and power receiving unit device classification, power system interfaces and key parameters, the power control specification, the signaling specification, and the framework for certification and acceptance testing based on reference power transmitting and power receiving resonators. The selection of ISM band (6.78MHz) as the WPT operating frequency and ISM band (2.4GHz) for WPT management protocol via BluetoothLE is discussed.
In this work, we show experimentally that wireless power transfer (WPT) using strongly coupled magnetic resonance (SCMR) and traditional induction are equivalent. We demonstrate that for a given coil separation, and to within 4%, strongly coupled magnetic resonance and traditional induction produce the same theoretical efficiency of wireless power transfer versus distance. Moreover, we show that the difference between traditional induction and strongly coupled magnetic resonance is in the implementation of the impedance matching network where strongly coupled magnetic resonance uses the mini-loop impedance match. The mini-loop impedance mach provides a low-loss, high-ratio impedance transformation that makes it desirable for longer distance wireless power transfer, where large impedance transformations are needed to maximize power transfer.
This paper demonstrates that SiC and GaN diode rectifiers are used in a magnetic resonant coupling (MRC) for wireless power transfer system. The size of the resonance coils, which are used in the wireless power transfer using a MRC, depends on the transmission frequency. So, the MRC is desired to operate at a high frequency in the Industry Science Medical (ISM) band such as 13.56 MHz. In the receiving side, a rectifier which converts to the DC voltage from high frequency AC voltage is necessary to supply the power to the applications such as a battery charger for EV and home appliances. The experimental results show the maximum efficiency from a Radio Frequency (RF) power supply to DC outputs is 75.2% when the transmission distance is 150 mm. In addition, A power loss separation method of the wireless power transfer system is discussed in this paper. The experimental results verify the reflected power of the resonance coil which dominates the largest amount of the loss in the total loss. Therefore, the suppression of the reflected power is important for the wireless power transfer system using a MRC.
Wireless power transfer is demonstrated mathematically and experimentally for M primary coils coupled to N secondary coils. Using multiple primary coils in parallel has advantages over a single primary coil. First, the reduced inductance of the transmitting coils makes the amplifier less sensitive to component variations. Second, with multiple receiving coils, the power delivery to an individual receiver is less sensitive to changes in the loads attached to other coils. By using a 16 cm by 18 cm primary and a 6 cm by 8 cm secondary coil, going from a 1:2 coupling to a 2:2 coupling, we show an increase in received power from 1.8 to 9.5 W, with only a small change in coupling efficiency. The advantages of the multiple primary coil topology increase the feasibility of charging multiple wireless portable devices simultaneously.
The equations governing the operation of an idealized class E RF power amplifier have been derived. These equations were first used to determine the circuit elements required to obtain an efficiency of 100 percent, and then used to determine the optimum set of parameters for 100-percent efficient operation. The harmonic structure of the collector waveform was then determined, and analogies for related circuit configurations were presented. A brief comparison of the optimum class E amplifier to other amplifiers is given.
In this paper we present an analysis of resonant wireless power transfer in systems with multiple receivers. We show that maximum power transfer can be achieved when the source is impedance matched to the set of receivers, i.e. matched to their equivalent impedance as seen by the source. The interaction of the receivers, or coupled modes, simply represent an interdependence of impedances that can be modeled and impedance matched. We explore three methods to achieve impedance matching: frequency tuning, impedance transformation and resonant tuning and show that the later two can achieve the maximum theoretical power transfer for a wide range of coupling between receivers.
In wireless power systems for charging battery-operated devices, the selection of component values guaranteeing certain desired performance characteristics can be a tedious trial-and-error process, either sweeping component values in circuit simulations or changing components by hand. This difficulty is compounded by the variable nature of the load resistance presented by a device under charge. This brief considers component selection for a specific wireless power system architecture, which is an open-loop class-e inverter using a series-parallel arrangement for load impedance transformation. Formulas for the optimal receiver, transmitter, and class-e components are derived given a set of constraints on the resistance, phase, quality factor, and drain voltage waveform. Using a 16 cm times 18 cm primary and a 4 cm times 5 cm secondary coil, the derived formulas are used to build a wireless power system. We show that the system has desirable performance characteristics, including a power delivery of over 3.7 W, peak efficiency of over 66%, and decreasing power delivery with increasing load resistance.
A method to determine various operating modes of a high-efficiency inductive wireless power transfer system which is capable of supporting more than one receiver is proposed. The three operating modes are no-load, safe, and fault modes. The detection scheme probes the transmitter circuitry periodically to determine the operating mode. For power saving, the transmitter is powered down when there is no valid receiver placed on the transmitting coil. If any conductive or magnetic object that can affect the total effective inductance of the transmitting coil is located nearby, the system will enter the fault mode and shut down the transmitter so that it will not be damaged. The safe mode is the nominal operation mode when the power transmission efficiency is high with minimum power loss and zero-voltage switching operation of the class-E transmitter is achieved. The determination of the operating mode is achieved by analyzing the transmitting coil voltage and supply current space, requiring no communication link between the transmitter and receiver. The linear relationship between the power delivery and the supply current can be used to calculate the power delivered to the load(s).
A 20 cm by 20 cm transmitting coil is designed for an inductively-coupled power transfer system. The coil design is a spiral, whose geometry is optimized to ensure an even magnetic field distribution. This guarantees uniform power delivery regardless of recieving coil position. The transmitting coil is tested using Litz wire, a switchmode power amplifier, and a 6 cm by 8 cm recieving coil connected to a rectifier and a variable load. The system achieves a maximum efficiency of 80.9% and a maximum power delivery of 11.8 W. At a fixed load, the power delivery has a coefficient of variation of 2.2% as the recieving coil's position is varied on the transmitter. In general, the system efficiency is high and insensitive to receiver placement as well as loading conditions.
In this paper, a high-power high-efficiency wireless-power-transfer system using the class-E operation for transmitter via inductive coupling has been designed and fabricated using the proposed design approach. The system requires no complex external control system but relies on its natural impedance response to achieve the desired power-delivery profile across a wide range of load resistances while maintaining high efficiency to prevent any heating issues. The proposed system consists of multichannels with independent gate drive to control power delivery. The fabricated system is compact and capable of 295 W of power delivery at 75.7% efficiency with forced air cooling and of 69 W of power delivery at 74.2% efficiency with convection cooling. This is the highest power and efficiency of a loosely coupled planar wireless-power-transfer system reported to date.
A wireless power system via magnetic induction that can deliver 32 W to a laptop is designed and fabricated. A 60% peak end-to-end regulated efficiency is achieved. A load detection scheme is also implemented, which detects when a device is placed on the transmitter and can shut down the system if faults are detected. The system eliminates the need to plug the laptop into AC power when running or charging.
An analysis of a modified series- L / parallel-tuned Class-E power amplifier is presented, which includes the effects that a shunt capacitance placed across the switching device will have on Class-E behaviour. In the original series L /parallel-tuned topology in which the output transistor capacitance is not inherently included in the circuit, zero-current switching (ZCS) and zero-current derivative switching (ZCDS) conditions should be applied to obtain optimum Class-E operation. On the other hand, when the output transistor capacitance is incorporated in the circuit, i.e. in the modified series- L /parallel-tuned topology, the ZCS and ZCDS would not give optimum operation and therefore zero-voltage-switching (ZVS) and zero-voltage-derivative switching (ZVDS) conditions should be applied instead. In the modified series- L /parallel-tuned Class-E configuration, the output-device inductance and the output-device output capacitance, both of which can significantly affect the amplifier's performance at microwave frequencies, furnish part, if not all, of the series inductance L and the shunt capacitance C <sub>OUT</sub>, respectively. Further, when compared with the classic shunt- C /series-tuned topology, the proposed Class-E configuration offers some advantages in terms of 44% higher maximum operating frequency ( f <sub>MAX</sub>) and 4% higher power output capability ( P <sub>MAX</sub>). As in the classic topology, the f <sub>MAX</sub> of the proposed amplifier circuit is reached when the output-device output capacitance furnishes all of the capacitance C <sub>OUT</sub>, for a given combination of frequency, output power and DC supply voltage. It is also shown that numerical simulations agree well with theoretical predictions.
The operation of the class E tuned power amplifier may be described by a set of equations based on Fourier component analysis. Previous publications have derived an optimum operating mode in which the collector efficiency of an idealized circuit is 100 percent. Since real amplifiers are made from nonideal components and are subject to nonideal loads, it is necessary to determine the effects of deviations from the ideal. The effects of variations in component values and duty cycle are determined from the basic equations. Numerical results of variations in load reactance, shunt capacitance, load resistance, frequency, and duty cycle are presented. The amplifier was found to be quite tolerant of reasonable circuit variations. With proper output filtering, the amplifier can be operated over nearly an octave bandwidth with less than a 5 percent reduction in efficiency.
Operating Frequency Selection for Loosely Coupled Wireless Power Transfer Systems with Respect to RF Emissions and RF Exposure Requirements
- Nadakuduti Jagadish
Design and Test of a High-Power High-Efficiency Loosely Coupled Planar Wireless Power Transfer System
- Low Zhen
- Ning