Xinen Zhu

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.

Theoretical Analysis of RF-DC Conversion Efficiency for Class-F Rectifiers

In this paper, an analytical model for the Class-F rectifier RF-dc conversion efficiency is presented. This model analyzes each kind of diode power losses in the rectifier due to the diode series resistor, junction capacitor, built-in potential, and breakdown voltage based on the time-domain diode voltage and current waveforms. The model provides a simple calculation routine to determine the threshold input power at which the peak reverse voltage across the diode starts to exceed the diode breakdown voltage and cause the diode efficiency to decrease. Two sets of closed-form equations are derived to calculate the Class-F rectifier efficiency in the input power region where the input power is either smaller or larger than the determined threshold input power, respectively. The implementation of a MATLAB code for the model calculation is also presented. Using this tool, the diode efficiency in Class-F rectifiers of different diode parameters can be easily determined, which provides a useful guideline for the optimal diode selection in various applications. The calculated diode efficiency using this model is compared with the Class-F rectifier ADS simulation at different load conditions, and all the results agree well with the model. To verify the model at different frequencies, two Class-F rectifiers working at 900 MHz and 5.8 GHz, respectively, are designed with the highest measured efficiency of 80.4% for the 900-MHz rectifier and 79.5% for the 5.8-GHz rectifier. Both of the measurement results agree well with the model prediction, which further verified the accuracy of the proposed model.

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

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.

A 13.56 MHz wireless power transfer system without impedance matching networks

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%.

Optimal load analysis for a two-receiver wireless power transfer 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

A high-efficiency power amplifier (PA) is important in a Megahertz wireless power transfer (WPT) system. It is attractive to apply the Class-E PA for its simple structure and high efficiency. However, the conventional design for Class-E PA can only ensure a high efficiency for a fixed load. It is necessary to develop a high-efficiency Class-E PA for a wide-range load in WPT systems. A novel design method for Class-E PA is proposed to achieve this objective in this paper. The PA achieves high efficiency, above 80%, for a load ranging from 10 to 100 Ω at 6.78 MHz in the experiment.

Wireless Charging of a Supercapacitor Model Vehicle Using Magnetic Resonance Coupling

This paper discusses the basic considerations and development of a prototype demo system for the wireless charging of supercapacitor electric vehicles, which uses magnetic resonance coupling. Considering future ubiquitous wireless vehicle stationary and dynamic charging facilities, supercapacitor could be an ideal device to store a reasonable amount of electrical energy for a relatively short period of time. The prototype system includes all the major functional components for an electric vehicle’s powertrain and wireless charging system including coils for energy emitting and receiving, a FPGA PWM input generation board, high frequency DC/AC inverter and AC/DC rectifier circuits, an on-board supercapacitor module, sensors for SOC level measurement and charging position detection, etc. All the components are integrated into a model electric vehicle. The prototype system well demonstrates the idea of the fast and frequent wireless charging of on-board supercapacitors. Promising results from initial experiments are explained; while further investigations, optimized design of components and a system-level optimization are needed.Copyright

Subsystem-level efficiency analysis of a wireless power transfer system

In this paper, a mid-power 13.56 MHz wireless power transfer (WPT) system is used as an example to discuss the relationship between the overall system and each subsystem. There are four subsystems discussed, including a power amplifier, a pair of resonant coils, a rectifier, and an electronic load. The paper evaluates the system performance under different mutual inductances in two cases. One is a simple WPT system without a rectifier, and the other with the rectifier. It is found that the existence of rectifier introduces more power reflection, especially when strong coupling occurs. The analysis in this paper can guide the design of an optimal load and input impedance matching network. Finally, the theoretical analysis is validated by experimental results.

Wireless Charging of Electric Vehicles: A Review and Experiments

In this paper, the technologies for electric vehicle wireless charging are reviewed including the inductive coupling, magnetic resonance coupling and microwave. Among them, the magnetic resonance coupling is promising for vehicle charging mainly due to its high efficiency and relatively long transfer range. The design and configuration of the magnetic resonance coupling based wireless charging system are introduced. A basic experimental setup and a prototype electric vehicle wireless charging system are developed for experimental and research purposes. Especially the prototype system well demonstrates the idea of fast and frequent wireless charging of supercapacitor electric vehicles using magnetic resonance coupling. Though the idea of wireless energy transfer looks sophisticated, it is proved to be a handy technology from the work described in the paper. However, both component and system-level optimization are still very challenging. Intensive investigations and research are expected in this aspect.Copyright