With the number of electric vehicles (EVs) on the rise, there is a need for an adequate charging infrastructure to serve these vehicles. The emerging extreme fast-charging (XFC) technology has the potential to provide a refueling experience similar to that of gasoline vehicles. In this article, we review the state-of-the-art EV charging infrastructure and focus on the XFC technology, which will be necessary to support the current and future EV refueling needs. We present the design considerations of the XFC stations and review the typical power electronics converter topologies suitable to deliver XFC. We consider the benefits of using the solid-state transformers (SSTs) in the XFC stations to replace the conventional line-frequency transformers and further provide a comprehensive review of the medium-voltage SST designs for the XFC application.

Extreme Fast Charging of Electric Vehicles: A Technology Overview

This paper provides a comprehensive review and analyses on the state-of-the-art and future trends for high-power conductive on-board chargers (OBCs) for electric vehicles. To provide a global context, a summary of global charging standards and electric vehicle (EV) related trends are presented, which demonstrates momentum toward the OBCs with higher power rating. High-power OBCs are either unidirectional or bidirectional, and they have either an integrated or non-integrated system architecture. Non-integrated high-power OBCs are studied both from industry and academia, and the former are used to illustrate the current state of the art. The latter are classified on the basis of the converter design approach, studied for their principle of operation, and compared over power density, weight, efficiency, and other metrics. In addition to non-integrated OBCs, recent advancements in propulsion-machine integrated OBC solutions are also presented. Other integrated OBC techniques, such as system integration with the EV's auxiliary power module and wireless charging systems, are also discussed. Finally, future charging strategies and functionalities in charging infrastructures are addressed, and global OBC trends are summarized.

Global Trends in High-Power On-Board Chargers for Electric Vehicles

The power supply chain of data centers from the medium voltage (MV) utility grid down to the chip-level voltage consists of many series connected power conversion stages and accordingly shows a relatively low efficiency. Solid-state transformers (SSTs) could improve the efficiency by substantially reducing the number of power conversion stages and/or directly interfacing the MV ac grid to a 400 V dc bus, from where server racks with a power consumption of several tens of kilowatts could be supplied by individual SSTs. The recent development of SiC MOSFETs with a blocking voltage of 10 kV enables the realization of a simple and, hence, highly reliable two-stage SST topology, consisting of an ac/dc power factor correction rectifier and a subsequent isolated dc/dc converter. In this context, an isolated 25 kW, 48 kHz, 7 kV to 400 V series resonant dc/dc converter based on 10 kV SiC MOSFETs is realized and tested in this paper. To achieve zero voltage switching of all MOSFETs, a special modulation scheme to actively control the amount of the switched magnetizing current on the MV- and low voltage-sides is implemented. Furthermore, the design of all main components and, especially, the electrical insulation of the employed medium-frequency transformer are discussed in detail. Calorimetric efficiency measurements show that a full-load efficiency of 99.0% is achieved, while the power density reaches 3.8 kW/L ( $63~\text {W}/\mathrm {in^{3}}$ ).

/pdf/99-efficient-10-kv-sic-based-7-kv-400-v-dc-transformer-for-o9ceafiknv.pdf

99% Efficient 10 kV SiC-Based 7 kV/400 V DC Transformer for Future Data Centers

This article reviews the design and evaluation of different DC-DC converter topologies for Battery Electric Vehicles (BEVs) and Plug-in Hybrid Electric Vehicles (PHEVs). The design and evaluation of these converter topologies are presented, analyzed and compared in terms of output power, component count, switching frequency, electromagnetic interference (EMI), losses, effectiveness, reliability and cost. This paper also evaluates the architecture, merits and demerits of converter topologies (AC-DC and DC-DC) for Fast Charging Stations (FCHARs). On the basis of this analysis, it has found that the Multidevice Interleaved DC-DC Bidirectional Converter (MDIBC) is the most suitable topology for high-power BEVs and PHEVs (> 10kW), thanks to its low input current ripples, low output voltage ripples, low electromagnetic interference, bidirectionality, high efficiency and high reliability. In contrast, for low-power electric vehicles (<10 kW), it is tough to recommend a single candidate that is the best in all possible aspects. However, the Sinusoidal Amplitude Converter, the Z-Source DC-DC converter and the boost DC-DC converter with resonant circuit are more suitable for low-power BEVs and PHEVs because of their soft switching, noise-free operation, low switching loss and high efficiency. Finally, this paper explores the opportunity of using wide band gap semiconductors (WBGSs) in DC-DC converters for BEVs, PHEVs and converters for FCHARs. Specifically, the future roadmap of research for WBGSs, modeling of emerging topologies and design techniques of the control system for BEV and PHEV powertrains are also presented in detail, which will certainly help researchers and solution engineers of automotive industries to select the suitable converter topology to achieve the growth of projected power density.

DC-DC Converter Topologies for Electric Vehicles, Plug-in Hybrid Electric Vehicles and Fast Charging Stations: State of the Art and Future Trends

To reduce the dependence on oil and environmental pollution, the development of electric vehicles has been accelerated in many countries. The implementation of EVs, especially battery electric vehicles, is considered a solution to the energy crisis and environmental issues. This paper provides a comprehensive review of the technical development of EVs and emerging technologies for their future application. Key technologies regarding batteries, charging technology, electric motors and control, and charging infrastructure of EVs are summarized. This paper also highlights the technical challenges and emerging technologies for the improvement of efficiency, reliability, and safety of EVs in the coming stages as another contribution.

/pdf/technology-development-of-electric-vehicles-a-review-41lypxy17x.pdf

Technology development of electric vehicles: A review

A widebandgap-based bidirectional on-board charger (OBC) has been proposed with a variable dc-link voltage, which tracks the battery voltage fluctuation. Although this approach is deemed most efficient for the resonant dc/dc stage, it posts significant challenges for the rectifier/inverter stage, which operates in the critical mode to realize zero-voltage switching (ZVS), while subjecting to the large variations of input and output voltages. Design considerations of the ac/dc stage are presented in this paper, including: the evaluation of 1.2-kV SiC MOSFETs; the ZVS extension techniques to realize ZVS under all input/output variations; and a novel universal control strategy for both the rectifier mode and the inverter mode. A prototype is built which achieves 98.5% efficiency at a switching frequency higher than 300 kHz. Furthermore, a 6.6-kW OBC system is demonstrated, using both SiC and GaN devices with 37-W/in3 power density and above 96% efficiency.

https://ieeexplore.ieee.org/ielaam/41/8062949/7951033-aam.pdf

High-Efficiency High-Density Critical Mode Rectifier/Inverter for WBG-Device-Based On-Board Charger

This paper presents a high-frequency modular medium-voltage AC (4160 VAC or 13.8 kVAC) to low-voltage DC (400 VDC) system that is scalable in order to be used for different scale microgrids. A 15 kW, 500 kHz DC/DC converter is demonstrated as the most important stage of the system overall, which can be scalable to a 225 kW 4160 VAC to 400 VDC system. Motivation of a CLLC resonant converter and its design parameters determination is carefully illustrated. The high-frequency transformer is the key element for the DC/DC converter. Then, the paper focuses on high-frequency transformer design to realize high-voltage insulation, high efficiency, and high density at the same time. Based on a split winding UU core transformer structure, transformer insulation materials and dimension parameters are determined referring to the IEEE insulation standard. The transformer magnetic loss model is reviewed based on which loss design tradeoff is carefully analyzed. Different working frequency impact on transformer design over three different core materials is also presented. Finally, a 500 kHz transformer prototype has been developed and demonstrated with the IEEE standard required insulation tests. A whole CLLC resonant converter is also present with experiments results. The converter holds outstanding 98% efficiency and 2.9 kW/L power density.

High-Frequency Transformer Design for Modular Power Conversion From Medium-Voltage AC to 400 VDC

This paper proposes a novel two-stage topology for a 6.6-kW on-board charger. The first stage, employing an interleaved bridgeless totem-pole ac/dc in critical conduction mode to realize zero-voltage switching, is operated at over 300 kHz. A bidirectional CLLC resonant converter operating at 500 kHz is chosen for the second stage. A variable dc-link voltage is adopted to track the wide battery voltage range, so that the CLLC resonant converter can always operate at its most efficient point. The 1.2-kV SiC devices are adopted for the ac/dc stage and the primary side of dc/dc stage, while 650-V GaN devices are used for the secondary side of dc/dc stage. In addition, PCB winding coupled inductors and integrated transformer are implemented in ac/dc stage and dc/dc stage, respectively, for the purpose of high density and manufacture automation. The proposed structure is demonstrated to have 37-W/in3 power density and above 96% efficiency over the entire battery voltage range, which far exceeds the current practice.

A High-Efficiency High-Density Wide-Bandgap Device-Based Bidirectional On-Board Charger

The momentum toward high power density high-efficiency power converters continues unabated. The key to reducing the size of power converters is high-frequency operation and the bottleneck is the magnetic components. With the emerging widebandgap devices, the switching frequency of power converters increases significantly, to hundreds of kilohertz, which provides us the opportunity to adopt printed circuit board (PCB) winding planar magnetics. Compared with the conventional litz-wire-based magnetics, planar magnetics can not only effectively reduce the converter size, but also offer improved reliability through automated manufacturing process with repeatable parasitics. Another way to reduce the number of magnetic components and shrink the size of power converters is through the magnetic integration. In this paper, a novel PCB winding based magnetic structure is proposed to integrate both inductor and transformer into one component. In this structure, the inductor value can be easily controlled by changing the cross-sectional area of the core or the length of the air gap. A 6.6-kW 500-kHz CLLC resonant converter prototype with 98% efficiency and 130-W/in3 (8 kW/L) power density is built to verify the feasibility of the proposed PCB winding based magnetic structure.

High-Frequency PCB Winding Transformer With Integrated Inductors for a Bi-Directional Resonant Converter

In this paper, the analysis of space vector modulation of a three-phase/wire/level Vienna rectifier is conducted, according to which the implementation of the equivalent carrier-based pulsewidth modulation is deduced theoretically within each separated sector in the diagram of vectors. The voltage balancing ability of dc-link neutral point, which depends on the uneven distribution of short vectors, is analyzed as well. An adaptive and robust controller to balance the output voltage under the unbalanced load limit for different modulation indices is proposed. The proposed controller can work at wide range of unbalanced load condition as well. Furthermore, the maximum unbalanced load is deduced versus the modulation index m when the converter works in unity power factor. An experimental prototype of 2.5 kW was built to verify the effectiveness of the theoretical analysis. Finally, the tested unbalanced limit of outputs for the experimental platform was given under different modulation indices. The output voltages for dual bus are balanced, and the theoretical analysis is verified.

Bin Li

Papers

High-Efficiency High-Density Critical Mode Rectifier/Inverter for WBG-Device-Based On-Board Charger

High-Frequency Transformer Design for Modular Power Conversion From Medium-Voltage AC to 400 VDC

A High-Efficiency High-Density Wide-Bandgap Device-Based Bidirectional On-Board Charger

High-Frequency PCB Winding Transformer With Integrated Inductors for a Bi-Directional Resonant Converter

Equivalence of SVM and Carrier-Based PWM in Three-Phase/Wire/Level Vienna Rectifier and Capability of Unbalanced-Load Control