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This paper develops a 200 W two-stage rail grade dc–dc module based on gallium-nitride devices. It converts a wide input voltage (64–160 V) to a constant output voltage (24 V). Different devices and topologies are evaluated for the target application, based on which a two-stage configuration is proposed. The first stage is a two-phase regulated buck converter working under critical mode ($>$ 400 kHz), which is followed by an unregulated 48-V/24-V LLC converter (2 MHz) for 3000-V dc isolation. The voltage regulation is discussed for the buck converter when the inductors are negatively coupled, and then the zero voltage switching (ZVS) extension is explored. However, due to the limitation of the digital controller, a tradeoff is made between the dynamic response and the footprint. The compromise solution is to use a small and slow microcontroller for high frequency control (including voltage regulation, critical mode operation, phase interleaving) and various demands (including soft start-up, external communication, and various protections). Finally, all the required functions are verified in a standalone quarter-brick prototype module. It can achieve 95.1% peak efficiency and 130 W/in $^3$ power density. [ABSTRACT FROM AUTHOR]

This paper develops a 200-W wide-input-range (64–160-to-24-V) rail grade dc–dc converter based on gallium nitride devices. A two-stage configuration is proposed. The first regulated stage is a two-phase interleaved buck converter ($>$ 400 kHz), and the second unregulated stage is an LLC (2-MHz) dc transformer. In order to achieve high frequency and high efficiency, the critical-mode operation is applied for the buck converter, and the negative coupled inductors are used to reduce the frequency and the conduction losses. Then, a systematical methodology is proposed to optimize the planar-coupled inductors. For the unregulated LLC converter, it can always work at its most efficient point, and an analytical model is used to optimize the planar transformer. Finally, the proposed dc–dc converter, built in a quarter brick form factor, is demonstrated with a peak efficiency of 95.8% and a power density of 195 W/in $^3$. [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 proposes a new battery cell voltage equalization approach using multiple-receiver wireless power transfer (WPT) working at megahertz (MHz). Compared with existing multiwinding transformer, the MHz multiple-receiver WPT system is advantageous in terms of saved weight and space, ease of implementation, and improved safety. In this paper, the unique operating principle of the WPT-based equalization is first explained, through which equalization currents are naturally determined by the battery cell voltage distribution. The currents are also analytically derived. This facilitates the discussion on power amplifier (PA) design. Performance analysis is then provided to investigate the influences of system parameters on the efficiency and equalization ability of the proposed WPT-based equalization system, and thus guide following design and implementation. Considering the uncertainty in PA load due to the random cell voltage distribution, a current-model Class $E$ PA is designed that enables approximately constant PA output current. Experimental results show that the proposed multiple-receiver WPT-based battery cell equalization system can achieve high overall system efficiency (above 71%) when equalizing six lithium-ion battery cells under loosely coupling ( $k$ = 0.065). A good match between the experimental and calculation results also validates the correctness of the theoretical discussion. [ABSTRACT FROM PUBLISHER]

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]

Wireless power transfer (WPT) working at megahertz (MHz) is now being widely considered a promising candidate for the midrange transfer of a medium amount of power. Efforts have been made to build high-efficiency MHz WPT systems via both component- and system-level approaches. However, so far there have been few discussions on high-frequency rectifier for MHz WPT applications. The soft-switching-based rectifiers, such as the Class E rectifiers, are one of the promising candidates for MHz rectification. This paper investigates the application of a Class E full-wave current-driven rectifier, for the first time, in WPT systems. A procedure is also developed to optimize the design of the rectifier and the MHz WPT system. For comparison purposes, the performances of both the Class E rectifier and the conventional full-bridge rectifier are investigated in terms of total harmonic distortion (THD), efficiency, power factor, voltage/current stresses, and voltage/current transfer functions, when being applied in an example 6.78-MHz WPT system. The simulation and experimental results show that the input voltage THD of the Class E full-wave rectifier is reduced to one-fourth of the THD of the full-bridge rectifier. In the optimally designed MHz WPT system, efficiencies of both the rectification (over 91%) and the overall system (around 80%) are obviously improved compared to the system using the conventional full-bridge rectifier. [ABSTRACT FROM PUBLISHER]

In this paper, the loading effect of a Class E power amplifier (PA) driven 6.78 megahertz (MHz) wireless power transfer (WPT) system is analyzed at both circuit and system levels. A buck converter is introduced and controlled to track an optimal equivalent load that maximizes the system efficiency under uncertainties in the relative position of coils and the final load. For power control, an additional degree of freedom is provided by adding an ultracapacitor bank. A control strategy is proposed to track the maximum efficiency and charge/discharge the ultracapacitor bank through the on/off control of the Class E PA. Thus, high system efficiency can be maintained under various uncertainties and load power demands. Finally, the theoretical analysis and the control scheme are validated in experiments. The results show that the proposed Class E PA-driven MHz WPT system can stably achieve a high efficiency under different coil distances and various constant/pulsed power profiles. The measured highest system efficiency can reach 72.1% at a load power level of 10 W. [ABSTRACT FROM AUTHOR]

Magnetic resonance coupling working at megahertz (MHz) is widely considered as a promising technology for the mid-range transfer of a medium amount of power. It is known that the soft-switching-based Class E rectifiers are suitable for high-frequency rectification, and thus potentially improve the overall efficiency of MHz wireless power transfer (WPT) systems. This paper reports new results on optimized parameter design of a MHz WPT system based on the analytical derivation of a Class E current-driven rectifier. The input impedance of the Class E rectifier is accurately derived, for the first time, considering the on-resistance of the diode and the equivalent series resistance of the filter inductor. This derived input impedance is then used to develop and guide design procedures that determine the optimal parameters of the rectifier, coupling coils, and a Class E PA in an example 6.78-MHz WPT system. Furthermore, the efficiencies of these three components and the overall WPT system are also analytically derived for design and evaluation purposes. In the final experiments, the analytical results are found to well match the experimental results. With loosely coupled coils (mutual inductance coefficient $k$ =0.1327), the experimental 6.78-MHz WPT system can achieve 84% efficiency at a power level of 20 Watts. [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]

A multitransmitter (multi-TX) inductive power transfer system would not only offer better spatial freedom but also improve the overall output capability. Under heavy-load condition, it is desirable to fully utilize the output power capability, and the overall input power needs to be balanced among the multiple TXs. This article proposes a simple and low-cost current sharing method to balance the input power. The current distribution is analytically studied for an example two-TX system, and the sharing path is proposed to balance the power under coupling variation and coil inductance mismatch. It also explores the influence of sharing path on the efficiency and zero-phase operation. Besides, the proposed method is valid for other compensations and can be extended for N-TX (N $>$ 2) systems. Finally, a two-TX system is implemented to verify the sharing effect. The current imbalance factor is effectively reduced from 37% to 2.7% under the worst coupling cases. [ABSTRACT FROM AUTHOR]

In conventional electromagnetic energy harvesting systems, a two-port ac–dc rectifier is utilized to process the harvested power. However, it is difficult to achieve voltage regulation and maximum-power-point-tracking (MPPT) simultaneously. To resolve this issue, this article proposes a three-port power-electronic-interface (PEI) dedicated to this application. A battery port is introduced to buffer the redundant energy. A bidirectional dc–dc converter is utilized to tightly regulate the load side voltage. To capture the maximum power for the harvester, a simple MPPT method is proposed for this PEI. The power tracking of ac source can be simplified by dealing with the power of the battery port. The regulations of power and output voltage are fully decoupled with different control degree-of-freedoms. Moreover, when the harvester is in idle mode, the load can be powered by the battery via the bidirectional dc–dc converter independently. A 24-mW rated laboratory prototype, which processes the power flow among a 0.6 V, 100 Hz ac source, a 1.2 V battery, and a 3.3 V constant voltage load is developed and tested. The proposed concept is validated by experimental results. [ABSTRACT FROM AUTHOR]

Capacitive power transfer is becoming an attractive alternate for wireless chargers. A coupler with large self- and mutual capacitance is desirable from the power and efficiency perspective. However, state-of-the-art couplers suffer from the tradeoff design issue between these two types of capacitance. In order to improve the required capacitance, an interleaved capacitive coupler is proposed and evaluated based on the Maxwell-based simulation. Using more plates, it can effectively use the coupling space to maintain the required self- and mutual capacitance, while minimize the cross coupling effects. Meanwhile, the new coupler is also helpful to reduce the radioactive electric field, and the high-radiation region is reduced dramatically. In the experiment, three types of couplers are built with the same coupling space, compensations, and output power. The proposed interleaved structure can achieve the highest efficiency and minimum radiation. [ABSTRACT FROM AUTHOR]

Capacitive power transfer is a promising mid-range wireless charging technique. The existing induced current source model suffers from the coupling variation issue, which causes troubles in resonance design and compensation development. This article explores an induced voltage source model for the capacitive coupler. Using this model, the six coupling-dependent capacitors between each pair of plates are equivalently represented by the self-capacitors and induced voltage sources. The self-capacitance is verified to be coupling independent. Based on this model, the power transfer capability, the efficiency, and the influence of adding external shunt capacitors are easily analyzed. This model is very helpful in investigating the existing topology and developing new compensations, such as the CLL-L compensation. Finally, a capacitive coupler is built to verify the model, based on which a CLL-L compensation is implemented for constant output voltage application. The achieved system peak efficiency is 88%. [ABSTRACT FROM AUTHOR]

High-order compensation provides more design freedom for inductive power transfer systems and can help improve output voltage/current controllability. This paper develops a simplified decomposition and synthesis method for high-order-compensated inductive power transfer systems. It can achieve load-independent (LI) output under coupling variation, and easily fulfill the various charging requirements, such as constant voltage or constant current. The coupling independent resonance is ensured by using the induced source model, and the whole resonant tank is effectively decomposed as three parts. The power transfer characteristics are discussed for each part, and the requirements for LI output and zero phase angle (ZPA) operation are combined to generate the compensation candidates for both sides. Two families of topologies are synthesized for four kinds of conversions, i.e., voltage to voltage, voltage to current, current to voltage, and current to current. Meanwhile, the proposed method dramatically simplifies the evaluation for the influence of the coil equivalent series resistors on the transfer function and efficiency. These resistor-caused effects are quite different for two families of topologies. Finally, a 6.78-MHz system with 10-W output is designed to verify the difference between two-family topologies. [ABSTRACT FROM AUTHOR]

An inductive power transfer system is able to charge multiple receivers (RXs) through a single transmitter. In order to develop suitable control methods, this article explores and compares the influence of driving current and loading resistance on the efficiency and power distribution. It starts from a simple system with equal coupling and then extends the discussion to a general one. With proper simplification, an optimization problem is defined to derive several important factors, including the optimal driving current, optimal loading resistance, and the corresponding optimized efficiency under various application constraints. In the final experiments, the dynamic tuning of driving current is able to maximize the system efficiency to 92.1% for a general three-RX system. In another customized system with equal coupling, the maximum error between the achieved efficiency and the real maximum efficiency is 0.4% through the calculation-based current control. [ABSTRACT FROM AUTHOR]

The LCC–LCC compensation is widely used in the inductive power transfer systems. However, there is no simple and accurate small-signal equivalent circuit model for this resonant converter. Based on the extended describing function method, this article proposed a full-order (17th-order) equivalent circuit model to accurately predict the small-signal behaviors. For simplification, the full-order model is gradually simplified to a ninth-order model and finally to a fifth-order model. The input to output transfer function are analytically derived for the first time. Both simulation and experimental results are presented to justify the effectiveness of the model, and the model is accurate up to one-fifth of the switching frequency. [ABSTRACT FROM AUTHOR]

This article explores the potential of relay coils (ReXs) to extend the horizontal charging area for planar inductive couplers. It focuses on the coupling mechanism and derives the equivalent coupling coefficient for model simplification. Based on the new model, a one-ReX coupler is proposed to enhance the coupling under large misalignment. Besides, the influence of coupling polarity change is evaluated and well compensated. In order to further enhance the coupling, a multiple-ReX technique is proposed and justified by a two-ReX coupler and a three-ReX coupler. The experimental result shows that the enhanced coupling coefficient helps maintain the efficiency to be above 92.5% under large misalignment when the one-ReX coupler is used. With three ReXs, the achieved efficiency is further improved by 2.3%. [ABSTRACT FROM AUTHOR]

Due to the high cost and shortage of rare earth materials, electrically excited synchronous machines (EESMs) become a potential alternative for electric vehicles. With the help of inductive power transfer (IPT), the EESM would easily realize the brushless excitation to improve its reliability. However, the dependence of IPT and EESM leads to significant control challenge, especially considering the inductive load characteristics. When the traditional modeling approaches are applied to the whole system, a complicated 13th-order model is built, which is unfavorable for dynamic analysis. For simplification, this article proposes a systematic and universal reduced-order model approach based on the local linear approximation of resonant voltage/current. A reduced ninth-order model of system is then obtained based on the approach. The frequency domain characteristic of primary-side control is analyzed, and the reduced-order model has high accuracy compared with the full-order one. Finally, the experimental results are given out to verify the effectiveness of the proposed modeling method. [ABSTRACT FROM AUTHOR]

Recently, omnidirectional wireless power transfer (WPT) systems have been studied intensely, due to their improved flexibility as compared to their planar counterparts. The LCCL-LC resonant converter topology is selected due to its current source characteristics in this article. The system frequency is pushed to megahertz (MHz) to increase the spatial charging freedom. In a megahertz WPT system, the reactance of the full bridge rectifier can no longer be neglected; therefore, an analytical model of the full bridge rectifier input impedance is built. Furthermore, zero-voltage switching (ZVS) of the switching devices is essential in reducing the switching loss and noise in a megahertz system. A design methodology of the LCCL-LC circuit is proposed to achieve the ZVS operation in the case of one transmitter and one receiver. Then, the ZVS analysis is extended to the scenario of multiple transmitter coils and one receiver coil. Finally, a 6.78-MHz wireless charging system is built according to the proposed design methodology. Experimental results validate the accuracy of the ZVS analysis, and the ZVS operation is well achieved under different coupling and load conditions. The peak system efficiency of 82% at 5-W output power is achieved. [ABSTRACT FROM AUTHOR]

This article presents a novel phasor domain approach to determine the location of faults in non-homogeneous transmission lines. Best implementation requires synchronized voltage and current phasor measurements at all terminals of the transmission line. The overall non-homogeneous transmission line model with fault is established by systematically combining the generalized compact models of all homogeneous line sections. The location of the fault is introduced as an additional state of the overall model. The derivation procedure of the model is without further assumptions, resulting in full consideration of three phase line asymmetry as well as distributed parameters of non-homogeneous transmission lines. Afterwards, the location of the fault is identified using the state estimation algorithm. Extensive numerical experiments in a two-terminal and a multi-terminal non-homogeneous transmission line demonstrate that the method has higher accuracy than existing fault location methods, independent of fault types, locations and impedances. [ABSTRACT FROM AUTHOR]