Minfan Fu

A Cascaded Boost–Buck Converter for High-Efficiency Wireless Power Transfer Systems

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.

Analysis and Tracking of Optimal Load in Wireless Power Transfer Systems

All the wireless power transfer (WPT) systems share a similar configuration including a power source, a coupling system, a rectifying circuit, a power regulating, and charging management circuit and a load. For such a system, both a circuit- and a system-level analyses are important to derive requirements for a high overall system efficiency. Besides, unavoidable uncertainties in a real WPT system require a feedback mechanism to improve the robustness of the performance. Based on the above basic considerations, this paper first provides a detailed analysis on the efficiency of a WPT system at both circuit and system levels. Under a specific mutual inductance between the emitting and receiving coils, an optimal load resistance is shown to exist for a maximum overall system efficiency. Then, a perturbation-and-observation-based tracking system is developed through additional hardware such as a cascaded boost-buck dc-dc converter, an efficiency sensing system, and a controller. Finally, a 13.56-MHz WPT system is demonstrated experimentally to validate the efficiency analysis and the tracking of the optimal load resistances. At a power level of 40 W, the overall efficiency from the power source to the final load is maintained about 70% under various load resistances and relative positions of coils.

Parameter Design for a 6.78-MHz Wireless Power Transfer System Based on Analytical Derivation of Class E Current-Driven Rectifier

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

Compensation of Cross Coupling in Multiple-Receiver Wireless Power Transfer Systems

k

A 13.56 MHz wireless power transfer system without impedance matching networks

=0.1327), the experimental 6.78-MHz WPT system can achieve 84% efficiency at a power level of 20 Watts.

Optimal load analysis for a two-receiver wireless power transfer system

Simultaneous wireless charging of multiple devices is a unique advantage of wireless power transfer (WPT). Meanwhile, the multiple-receiver configuration makes it more challenging to analyze and optimize the operation of the system. This paper aims at providing a general analysis on the multiple-receiver WPT systems and compensation for the influence of the cross coupling. A two-receiver WPT system is first investigated as an example. It shows that theoretically by having derived optimal load reactances, the important system characteristics can be preserved, such as the original system efficiency, input impedance, and power distribution when there is no cross coupling between receivers. The discussion is then extended to general multiple-receiver WPT systems with more than two receivers. Similar results are obtained that show the possibility of compensating the cross coupling by having the derived optimal load reactances. Finally, the theoretical analysis is validated by model-based calculation and final experiments using real two- and three-receiver systems.

Low-Harmonic-Contents and High-Efficiency Class E Full-Wave Current-Driven Rectifier for Megahertz Wireless Power Transfer Systems

A 13.56 MHz wireless power transfer (WPT) system is analyzed and implemented in this paper. This system consists of five subsystems: a class-D power amplifier, a pair of resonant coils, a rectifier, a DC/DC converter and various loads. By analyzing the transfer characteristics of the resonant coils, an optimum impedance is derived in order to minimize the power reflection. This impedance requirement is fulfilled by designing the rectifier and the DC/DC converter properly. The power reflection due to impedance mismatch can be controlled at 5%. When the load is a resistor, the system efficiency can reach 73% . When the loads are batteries or supercapacitors, the system efficiency can reach 66%.

Loading and Power Control for a High-Efficiency Class E PA-Driven Megahertz WPT System

Wireless power transfer to multiple devices is of great importance to simultaneous charging mobile devices, where coupling system is crucial component determining system efficiency. In a multiple-receiver system, external loads values would affect the coupling efficiency, especially for low quality factor coupling system. In this paper, a two-receiver model is proposed to analysis the optimal external loads that would maximize the coupling efficiency. Optimal loads formulas for a two-receiver system are derived using circuit model and validated using a 13.56MHz printed circuit board coupling system.

A high-efficiency Class-E power amplifier with wide-range load in WPT systems

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.

Megahertz Multiple-Receiver Wireless Power Transfer Systems With Power Flow Management and Maximum Efficiency Point Tracking

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.