DC–DC converters with voltage boost capability are widely used in a large number of power conversion applications, from fraction-of-volt to tens of thousands of volts at power levels from milliwatts to megawatts. The literature has reported on various voltage-boosting techniques, in which fundamental energy storing elements (inductors and capacitors) and/or transformers in conjunction with switch(es) and diode(s) are utilized in the circuit. These techniques include switched capacitor (charge pump), voltage multiplier, switched inductor/voltage lift, magnetic coupling, and multistage/-level, and each has its own merits and demerits depending on application, in terms of cost, complexity, power density, reliability, and efficiency. To meet the growing demand for such applications, new power converter topologies that use the above voltage-boosting techniques, as well as some active and passive components, are continuously being proposed. The permutations and combinations of the various voltage-boosting techniques with additional components in a circuit allow for numerous new topologies and configurations, which are often confusing and difficult to follow. Therefore, to present a clear picture on the general law and framework of the development of next-generation step-up dc–dc converters, this paper aims to comprehensively review and classify various step-up dc–dc converters based on their characteristics and voltage-boosting techniques. In addition, the advantages and disadvantages of these voltage-boosting techniques and associated converters are discussed in detail. Finally, broad applications of dc–dc converters are presented and summarized with comparative study of different voltage-boosting techniques.

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Step-Up DC–DC Converters: A Comprehensive Review of Voltage-Boosting Techniques, Topologies, and Applications

Due to limitations of low power density, high cost, heavy weight, etc., the development and application of battery-powered devices are facing with unprecedented technical challenges. As a novel pattern of energization, the wireless power transfer (WPT) offers a band new way to the energy acquisition for electric-driven devices, thus alleviating the over-dependence on the battery. This paper presents an overview of WPT techniques with emphasis on working mechanisms, technical challenges, metamaterials, and classical applications. Focusing on WPT systems, this paper elaborates on current major research topics and discusses about future development trends. This novel energy transmission mechanism shows significant meanings on the pervasive application of renewable energies in our daily life.

Wireless Power Transfer—An Overview

More than a century-old gasoline internal combustion engine is a major contributor to greenhouse gases. Electric vehicles (EVs) have the potential to achieve eco-friendly transportation. However, the major limitation in achieving this vision is the battery technology. It suffers from drawbacks such as high cost, rare material, low energy density, and large weight. The problems related to battery technology can be addressed by dynamically charging the EV while on the move. In-motion charging can reduce the battery storage requirement, which could significantly extend the driving range of an EV. This paper reviews recent advances in stationary and dynamic wireless charging of EVs. A comprehensive review of charging pad, power electronics configurations, compensation networks, controls, and standards is presented.

Wireless Power Transfer for Vehicular Applications: Overview and Challenges

With a good balance between power transfer distance and efficiency, wireless power transfer (WPT) using magnetic resonant coupling is preferred in many applications. Generally, WPT systems are desired to provide constant output voltage with the highest possible efficiency as power supplies. However, the highest efficiency is not achieved by the reported closed-loop WPT systems that maintain constant output voltage against coupling and load variations. In this paper, an efficiency evaluation method is put forward to evaluate the closed-loop control schemes. Furthermore, a maximum efficiency point tracking control scheme is proposed to maximize the system efficiency while regulating the output voltage. This control scheme is unique and prominent in that it fixes the operating frequency at the receiving-side resonant frequency and converts both the input voltage and the load resistance at the same time. Thus, the maximum efficiency point on the constant output voltage trajectory can be tracked dynamically. Therefore the system's output voltage can be maintained constant and its efficiency is always the highest. The experimental results show that the maximum efficiency point is tracked and a very high overall efficiency is achieved over wide ranges of coupling coefficient and load resistance.

A Maximum Efficiency Point Tracking Control Scheme for Wireless Power Transfer Systems Using Magnetic Resonant Coupling

Wireless power transfer (WPT) is the preferred charging method for electric vehicles (EVs) powered by battery and supercapacitor. In this paper, a novel WPT system with constant current charging capability for sightseeing car with supercapacitor storage is designed. First, an optimized magnetic coupler using ferrite cores and magnetic shielding structure is proposed to ensure stable power transfer and high efficiency. Compared with the traditional planar shape ferrite core coupler, the proposed magnetic coupler requires lesser ferrite material without degrading the performance of the WPT system. Second, the model of supercapacitor is applied to the WPT system and the relationship between equivalent load resistances of supercapacitor and charging time is analyzed in detail. Then, a Buck converter with proportional integral (PI) controller is implemented on the secondary side to maintain constant charging current for the variable load. Finally, the proposed design is verified by experiments. Constant charging current of 31.5 A across transfer distance of 15 cm is achieved. The peak transfer power and system efficiency are 2.86 kW and 88.05%, respectively.

A 3-kW Wireless Power Transfer System for Sightseeing Car Supercapacitor Charge

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.

Analysis and Tracking of Optimal Load in 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.

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

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.

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

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%.

A 13.56 MHz wireless power transfer system without impedance matching networks

Class $E^2$ DC–DC converter composed of Class $E$ power amplifier (PA) and rectifier is a promising candidate for MHz wireless power transfer. It is soft-switching based and able to achieve high efficiency at MHz frequency. However, the converter implemented through traditional static design is sensitive to the variations of operation condition. Its performance gets deteriorated when DC load and coil coupling deviate from their respective optimum values. This paper demonstrates that the degradation of system efficiency is mostly due to the mismatch of PA load, and the efficiency drop can be efficiently improved by adding an L-type impedance matching network (IMN) after PA. A fixed IMN is sufficient to maintain a high efficiency, while a tunable IMN is required to ensure stable output power when operation condition dramatically changes. Key techniques, particularly system-level optimization, are discussed in this paper that ensure high efficiency over a wide range of variation in operation condition and also with reduced capacitor/inductor tuning ranges in the IMN. The 6.78-MHz Class $E^2$ DC–DC converters with and without the fixed/tunable IMN are fabricated and measured for validation purposes. The experimental results show that both high efficiency ( $>$ 66%) and stable output power (around 9 W) are maintained for the tunable converter when there are variations in the DC load and coil coupling.

Xinen Zhu

Papers

Analysis and Tracking of Optimal Load in Wireless Power Transfer Systems

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

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

A 13.56 MHz wireless power transfer system without impedance matching networks

Tunable Class $E^2$ DC–DC Converter With High Efficiency and Stable Output Power for 6.78-MHz Wireless Power Transfer