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It is known that megahertz (MHz) wireless power transfer (WPT) is particularly beneficial in terms of spatial freedom of power transfer and circuit compactness. Meanwhile, design complexity is a main obstacle to the spread of MHz WPT. In this article, a new modular design concept is proposed to conveniently reconfigure a MHz WPT system for different applications. Multiple predesigned functional unit modules can easily be combined to form a complete new WPT system with a different power level and transfer distance. A vertically stacked architecture is especially chosen to save space in actual applications. Detail analytical derivations are conducted to analyze the modular design, including the influence of different combinations of the unit modules. A new interleaved coil design is developed to reduce the proximity effect and cross coupling among the vertically stacked modules. Finally, the modular design concept is validated by experiments and demonstrates the desired scalability in both power levels and transfer distances. Through 15 combinations of the unit modules, the experimental WPT system can be reconfigured to provide an output power from 4 to 116 W. The effective transfer distance can be also extended from 30 to 90 mm and 80% system dc–dc efficiency is achieved within a 70-mm transfer distance using 100-mm-diameter printed circuit board coils. [ABSTRACT FROM AUTHOR]

This article develops explicit design of an impedance matching network (IMN) to enhance the robustness of megahertz (MHz) wireless power transfer (WPT) systems against variations in their final load and coil coupling. Unlike the existing trial-and-error-based solutions, analytical derivations are conducted to calculate the parameters of a two-port T- or $\bf \Pi$ -IMN. The load-pull technique is first applied to determine a target impedance ray that represents a desired loading condition of the power amplifier. Based on the analytical derivations, the three degrees of the design freedom of the IMN are then fully utilized to accurately match the starting point and direction of the target impedance ray. Different features of a final MHz WPT system, such as load-independent output voltage and high tolerance to coil misalignment, are further achieved by having different target impedance rays. Performance enhancement of an experimental 6.78-MHz WPT system is investigated under largely varied final load (10–50 $\mathbf{\Omega }$) and coil coupling (0–53%). With an explicitly designed IMN, the maximum efficiency drops are reduced from 18% to 6% under the changing final load and 21% to 3.9% under the changing coil coupling (0–40%), respectively. An actual application of wireless drone charging is also included to demonstrate the practicability of the proposed IMN design [ABSTRACT FROM AUTHOR]

Wireless power transfer (WPT) systems operating at several MHz are known to be advantageous for realizing high spatial freedom of power transfer and compact and lightweight designs. The combined use of multiple transmitting (Tx) coils for magnetic field shaping is expected to further increase the spatial freedom. This article studies an extendable planar Tx-coil array architecture. This architecture provides new conveniences for the MHz WPT systems to efficiently charge devices in 3D space. Time-domain and phasor-domain modelings are carried out in turn to analyze the magnetic field shaping effect under the current phase shift modulation of the Tx coils. An overlap design of the Tx-coil layout is also developed to minimize the cross coupling between the coils, which reduces the interference between Tx coils. Finally, the above concepts are experimentally implemented. The results clearly demonstrate the new advantages of the planar Tx-coil array-based magnetic field shaping, in terms of spatial freedom of the power transfer, efficiency, and output power capability, when charging receivers with six-degrees-of-freedom positions and orientations in 3D space. For instance, a perpendicular receiver in the center of the Tx-coil array can be charged with 82% dc–dc efficiency and 45 W, which is difficult for a conventional single Tx-coil solution. [ABSTRACT FROM AUTHOR]

In megahertz (MHz) wireless power transfer (WPT) systems, a varying reflected reactance due to a changing final load deteriorates the overall system performance, such as efficiency and output voltage stability. This article aims to achieve a compact and robust MHz WPT system working over a wide range of load. A novel concept of reactance compression design is proposed to compress the variation of the reflected reactance through positioning of a “reactance window.” Efficient receiving-side parameter design procedures are then developed to best compress the reactance variation and satisfy maximum output power requirement in a wide load range but without adding any other hardware. This makes it possible to directly apply the existing voltage-source design using a modified Class E power amplifier (PA) and current-source design using an LCC transformation network. An edge inductor also becomes worthy to replace the bulky air-core infinite inductor required by the modified Class E PA, thereby further improving circuit compactness. Finally, the receiving-side design concept and the edge inductor are experimentally implemented to verify their performance over a 5–50- $\Omega$ load and with the maximum final output power of 20 W. The results well validate the improved reactance compression, output voltage stability, efficiency, and harmonic distortions over the wide load range. [ABSTRACT FROM AUTHOR]

In real applications, it is usually desirable to have a regulated dc output voltage from a wireless power transfer (WPT) system. This voltage regulation is especially necessary because of possible variation in coupling coil relative position and thus a changing mutual inductance. For WPT systems working at megahertz (MHz), Class E rectifiers are known to be advantageous, thanks to their simple configuration and soft-switching operation. In this paper, we propose a new Class-E-based active rectifier. This rectifier is capable to simultaneously perform high-efficiency rectification and output voltage regulation, but without the need to have an additional regulating circuit. Analytical derivations and analysis are provided to explain the operation principle in detail. Guidance is also developed for a balanced design of parameters, which compromises among different criteria. Final experiments validate the analytical analysis and dc output voltage regulation via the active rectifier. With the duty cycle control of the rectifier, a constant output voltage (10 V) can be maintained under a varying mutual inductance coefficient and with more than 92% rectifying efficiency. [ABSTRACT FROM AUTHOR]

This paper proposes and implements autonomous control of a multiple-receiver wireless charging system. The charging control problem is challenging due to the decentralized nature of the system, possible changing numbers and types of energy storage devices as loads of the receivers, and complexity in wireless power distribution mechanism. The game-theory-based control is developed that fully respects the unique characteristics of the transmitter (i.e., charger) and receivers. The preferences of the individual devices are first quantified using utility functions. Then, the charging control problem is formulated as a generalized Stackelberg game considering the leader–follower relationship between the transmitter and receivers, and the limited total charging power. At each control instant, the generalized Nash equilibrium among the receivers, i.e., charging power distribution here, is reached by searching the Lagrange multiplier while the total charging power from the transmitter is updated in a step-by-step manner. Both simulation and experimental results show that the proposed charging control autonomously manages and updates the power distribution in the cases where the receivers with different energy storage devices quit or join the wireless charging. [ABSTRACT FROM PUBLISHER]

This paper develops a novel voltage equalizer by combining wireless power transfer (WPT) and a voltage multiplier (VM) for series-connected energy storage cells. The physical isolation achieved by WPT can offer several unique benefits that traditional equalizers can hardly provide. In this paper, the characteristics of a multiple-output VM are analytically discussed and used to develop an equivalent one-output VM model. Based on this model, parameters of the WPT system are properly designed. The proposed design methodology can achieve a single-switch voltage equalizer without feedback control, which dramatically decreases the circuit complexity and cost. Moreover, this methodology can naturally ensure a robust system under small coil misalignment. In the experiment, the proposed system is built to balance four series-connected ultracapacitor modules. The system efficiency is 72.8% at a load power level of 10 W. [ABSTRACT FROM PUBLISHER]

This paper develops a 6.78 MHz multiple-receiver wireless power transfer system driven by a Class E power amplifier. Constant output voltage is achieved for each receiver with optimized overall efficiency. A novel one-receiver model is built to analyze the overall power-efficiency characteristics. The loads and input voltage are then designed as two control variables. Through tuning the loads, constant output voltage is achieved by independent controllers at the receiver side. Then, the efficiency is optimized by tuning the input voltage at the transmitter side. Finally, the theoretical analysis and control scheme are validated using a three-receiver system. It shows that different constant output voltages, 5, 9, and 12 V can be achieved independently for different receivers. When the load resistance, real coupling, and number of receivers change, the voltage can be quickly regulated, and the overall optimum system efficiency is 66.6%. [ABSTRACT FROM AUTHOR]

This paper presents systematic analysis and design of a megahertz multiple-receiver (RX) wireless power transfer (WPT) system driven by a Class E power amplifier. Both circuit-level and system-level analyses are conducted to discuss the characteristics of the WPT system. In order to simultaneously manage the power flow and maximize the overall system efficiency, a new control scheme is proposed that combines maximum efficiency point tracking and time-division multiplexing. Through the proposed scheme, RXs with different loading and coupling conditions can be charged by the same transmitter with simplified design and control strategy. Finally, the proposed control scheme is validated using an example 6.78-MHz three-RX WPT system. The experimental results show that various power requirements can be quickly and stably satisfied, and the overall average system efficiency can reach as high as 71.7%. [ABSTRACT FROM PUBLISHER]

The high-efficiency current-mode (CM) and voltage-mode (VM) Class-E power amplifiers (PAs) for MHz wireless power transfer (WPT) systems are first proposed in this paper and the design methodology for them is presented. The CM/VM Class-E PA is able to deliver the increasing/decreasing power with the increasing load and the efficiency maintains high even when the load varies in a wide range. The high efficiency and certain operation mode are realized by introducing an impedance transformation network with fixed components. The efficiency, output power, circuit tolerance, and robustness are all taken into consideration in the design procedure, which makes the CM and the VM Class-E PAs especially practical and efficient to real WPT systems. 6.78-MHz WPT systems with the CM and the VM Class-E PAs are fabricated and compared to that with the classical Class-E PA. The measurement results show that the output power is proportional to the load for the CM Class-E PA and is inversely proportional to the load for the VM Class-E PA. The efficiency for them maintains high, over 83%, when the load of PA varies from 10 to 100  $\Omega$, while the efficiency of the classical Class-E is about 60% in the worst case. The experiment results validate the feasibility of the proposed design methodology and show that the CM and the VM Class-E PAs present superior performance in WPT systems compared to the traditional Class-E PA. [ABSTRACT FROM AUTHOR]

Wireless power transfer (WPT) working at megahertz (MHz) is widely considered a promising technology for midrange and low-power applications. A Class $ E^2$ dc–dc converter is composed of a Class E power amplifier (PA) and a Class E rectifier. It is attractive for applications in MHz WPT due to the soft-switching properties of both the PA and the rectifier. Using the existing design, the Class $E^2$ dc–dc converter can only achieve optimal performance such as a high efficiency under a fixed operating condition. Meanwhile, in real applications variations in the coil relative position and the final load are common. The purpose of this paper is to analyze and develop a general design methodology for a robust Class $E^2$ dc–dc converter in MHz WPT applications. Component and system efficiencies are analytically derived, which serve as the basis for the determination of the design parameters. The classical matching network of the Class E PA is also improved that provides the required impedance compression capability. Then, a robust parameter design procedure is developed. Both the experimental and calculated results show that proposed design approach can significantly improve the robustness of the efficiency of the Class $E^2$ dc–dc converter against variations in coil relative position and final load. Finally, the experiments show that the range of variation of the system efficiency is narrowed from 47.5%–85.0% to 73.3%–83.7% using the proposed robust design. [ABSTRACT FROM AUTHOR]

Wireless power transfer has shown revolutionary potential in challenging the conventional charging method for consumer electronic devices. Through magnetic resonance coupling, it is possible to supply power from one transmitter to multiple receivers simultaneously. In a multiple-receiver system, the overall system efficiency highly depends on the loads’ and receivers' positions. This paper extends the conventional circuit-model-based analysis for the one-receiver system to investigate a general one-transmitter multiple-receiver system. It discusses the influence of the loads and mutual inductance on the system efficiency. The optimal loads, the input impedance, and power distribution are analyzed and used to discuss the proper system design and control methods. Finally, systems with a different number of receivers are designed, fabricated, and measured to validate the proposed method. Under the optimal loading conditions, an efficiency above 80% can be achieved for a system with three different receivers working at 13.56 MHz. [ABSTRACT FROM PUBLISHER]

Wireless power transfer (WPT) has attracted an ever increasing interest from both industry and academics over the past few years. Its applications vary from small power devices such as mobile phones and tablets to high power electric vehicles and from small transfer distance of centimeters to large distance of tens of centimeters. In order to achieve a high-efficiency WPT system, each circuit should function at a high efficiency along with the proper impedance matching techniques to minimize the power reflection due to the impedance mismatch. This paper proposes an analysis on the system efficiency to determine the optimal impedance requirement for coils, rectifier, and dc–dc converter. A novel cascaded boost–buck dc–dc converter is designed to provide the optimal impedance matching in WPT system for various loads including resistive load, ultracapacitors, and batteries. The proposed 13.56-MHz WPT system can achieve a total system efficiency over 70% in experiment. [ABSTRACT FROM PUBLISHER]