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

The nonresonant single-phase dual-active-bridge (NSDAB) dc–dc converter has been increasingly adopted for isolated dc–dc power conversion systems. Over the past few years, significant research has been carried out to address the technical challenges associated with modulations and controls of the NSDAB dc–dc converter. The aim of this paper is to review and compare these recent state-of-the-art modulation and control strategies. First, the modulation strategies for the NSDAB dc–dc converter are analyzed. All possible phase-shift patterns are demonstrated, and the correlation analysis of the typical phases-shift modulation methods for the NSDAB dc–dc converter is presented. Then, an overview of steady-state efficiency-optimization strategies is discussed for the NSDAB dc–dc converter. Moreover, a review of optimized techniques for dynamic responses is also provided. For both the efficiency and dynamic optimizations, thorough comparisons and recommendations are provided in this paper. Finally, to improve both steady-state and transient performances, a combination approach to optimize both the efficiency and dynamics for an NSDAB dc–dc converter based on the reviewed methods is presented in this paper.

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Overview and Comparison of Modulation and Control Strategies for a Nonresonant Single-Phase Dual-Active-Bridge DC–DC Converter

The triple phase shift (TPS) modulation scheme, which provides three control freedoms, is of great importance for the optimized operation of a dual active bridge (DAB) isolated bidirectional dc/dc converter. First of all, this paper introduces an accurate, universal model to describe the analytic expressions of the DAB converter under TPS control. Based on this, six operating modes of the DAB converter are further discussed. Afterwards, the concept of global optimal condition (GOC) equations is proposed to derive the closed form of analytic expressions of an optimal modulation scheme that makes the DAB converter operate with minimized root-mean-square (rms) current during whole power range with different operating modes. According to the GOC equations, the physical explanation of the proposed modulation scheme is further given in details, and the complex interaction among the control variables, the transferred power, and rms current is revealed. The real-time optimization process of the proposed method is also specified. Finally, the proposed methods are applied to a laboratory prototype. The experimental results confirm the theoretical analysis and practical feasibility of the proposed strategies.

Modeling and Analysis of a Dual-Active-Bridge-Isolated Bidirectional DC/DC Converter to Minimize RMS Current With Whole Operating Range

This paper proposed a modulation scheme for the dual-active-bridge (DAB) converter to reduce rms current in wide-range operating conditions. The operating principle of the proposed fundamental duty modulation (FDM) is formulated based on the fundamental component analysis of the DAB converter. By modulating the PWM signals in the fundamental component domain, the optimal operation is implemented with a simple controller structure not requiring an operating mode classification, offline calculation, or current information. Operating characteristics including rms current level and ZVS characteristics are analyzed to compare loss breakdowns of the proposed scheme to those of recent related works. The proposed FDM achieves high efficiency under wide operation conditions due to reduced conduction level and wide ZVS range. Experimental results are obtained under various voltage gain and load conditions to confirm the operation of the proposed modulation scheme. A thorough experimental comparison with other sophisticated modulation schemes has verified the efficiency improvement of FDM.

Fundamental Duty Modulation of Dual-Active-Bridge Converter for Wide-Range Operation

Bidirectional DC-DC power converters are increasingly employed in diverse applications whereby power flow in both forward and reverse directions are required. These include but not limited to energy storage systems, uninterruptable power supplies, electric vehicles, and renewable energy systems, to name a few. This paper aims to review these converters from the point of view of topology as well as control schemes. From the point of view of topology, these converters are divided into two main categories, namely non-isolated and isolated configurations. Each category is divided into eight groups along with their respective schematics and a table of summary. Furthermore, the common control schemes and switching strategies for these converters are also reviewed. Some of the control schemes are typically applied to all DC-DC power converters such as PID, sliding mode, fuzzy, model predictive, digital control, etc. In this context, it should be noted that some switching strategies were designed specifically for isolated bidirectional DC-DC converters in order to improve their performance such as single phase shift, dual phase shift, triple phase shift, etc. The features of each topology and control scheme along with their typical applications are discussed, in order to provide a ground of comparison for realizing new configurations or finding the appropriate converter for the specific application.

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Topologies and Control Schemes of Bidirectional DC–DC Power Converters: An Overview

This paper presents a comprehensive analysis of the current-stress optimization and soft-switching operation of the isolated bidirectional dual active bridge (DAB) dc–dc converter with the unified triple-phase-shift (UTPS) control. On this basis, the current-stress-optimized modulation scheme is proposed for DAB, which leads to the minimum current stress with the required transmission power and voltage conversion ratio in the whole load range. Moreover, the full soft-switching operation is achieved for the converter simultaneously. Distinct from the previous modulation schemes, the proposed optimized modulation scheme is deduced from a unified analysis of TPS where all effective switching modes are investigated. A novel algorithm based on the Karush–Kuhn–Tucker (KKT) conditions is originally proposed to derive closed-form solutions for the global optimal control parameters. This paper also presents a typical closed-loop control strategy for DAB with TPS control and detailed descriptions about the closed-loop operation. A laboratory prototype is applied, and the experimental results validate that the current-stress optimization and efficiency improvement are realized by applying the optimized modulation scheme. The experimental results also verify the effectiveness of the closed-loop control strategy for DAB with TPS control.

Unified Triple-Phase-Shift Control to Minimize Current Stress and Achieve Full Soft-Switching of Isolated Bidirectional DC–DC Converter

Dual active bridge (DAB) converters have been widely used in distributed power systems and energy storage equipment. However, the inherent nonlinearity of the DAB converters can cause stability problem, such as output voltage oscillation. In this paper, the dynamic behavior and stability of a digitally controlled DAB converter with a closed-loop controller are studied. First, to accurately study the nonlinear dynamics and stability in a DAB converter, a bilinear discrete-time model considering the output capacitor equivalent series resistance (ESR) and the digital control delay in circuit is established. Based on the model, the nonlinear dynamic characteristic and stability of the DAB converter versus the control parameter are studied. Furthermore, extensive analyses are performed to study the effect of the transformer leakage inductance and the output capacitor ESR on the stability boundaries of the control parameter. The accuracy of the model and the theoretical analyses are validated by simulation and experimental results. The proposed model of the digitally controlled DAB converter can accurately predict the stability boundaries, which can be effectively applied to the design of the system parameters and guarantee stable operation of the converter.

Bilinear Discrete-Time Modeling and Stability Analysis of the Digitally Controlled Dual Active Bridge Converter

The three-phase dual active bridge (3p-DAB) topology has been widely applied in high power dc-dc conversion, which has advantages of bidirectional power transfer, inherent soft switching capability and small filter size. The phase shift control is the conventional modulation strategy for three-phase DAB. However, the conventional control strategy can't operate the converter under soft switching at low loads, especially when the voltage conversion ratio is far from the unit ratio. The aim of this paper is to propose a novel simultaneous pulse-widthmodulation (PWM) control strategy for the three-phase DAB to realize zero-voltage-switching (ZVS) in the whole load range. Seven switching modes are confirmed based on relative relationships of control parameters. The transmission power is calculated and soft switching operation is analyzed in each switching mode. Comparative analysis of ZVS operation with the conventional phase shift control and the proposed control strategy is then presented. A three-phase DAB converter is modeled in Saber and improvements by applying the proposed control strategy are verified by the simulation results.

Simultaneous PWM control to operate the three-phase dual active bridge converter under soft switching in the whole load range

This paper proposes an optimized modulation strategy for three-phase dual-active-bridge (3p-DAB) DC-DC converters, which can minimize RMS inductor current and enhance the efficiency in the whole load range, especially under low load situations. Three control variables, two duty cycles and a phase-shift ratio, are introduced in the proposed modulation strategy. A universal analysis for the operating characterization of a 3p-DAB converter is presented using the multi-frequency approximation method. Moreover, the analytical expressions of RMS inductor current and transmission power are deduced. In order to obtain the optimal situation with minimization RMS inductor current, a constrained nonlinear programming problem is established. On this basis, an optimization algorithm, the interior-point method, is applied to solve the problem and derive control variables of the modulation strategy. An experimental prototype is established and the experimental results validate that the RMS inductor current optimization and efficiency improvement are realized by using the proposed modulation scheme.

Optimized modulation strategy for three-phase dual-active-bridge DC-DC converters to minimize RMS inductor current in the whole load range

The dual active bridge (DAB) converter has been widely used in the distributed power systems and the energy storage devices. However, the inherent nonlinearity of the DAB converter can cause serious stability problem. In this paper, the dynamics of a digital controlled DAB converter with output voltage closed loop control is studied. Firstly, to accurately study the nonlinear dynamics and stability in a DAB converter, a full discrete-time model for the DAB converter is established. The model considers the ESR of the output capacitor and also considers the digital control delay and sample-and-hold process. Using this model, the stability of the DAB converter versus the parameter of the proportional controller is analyzed and the stability boundary is accurately predicted. Further, this paper also points out that the ESR of the output capacitor can bring a great effect to the stability of the system. Finally, simulation and experimental results verify the theoretical analysis.

Jun Huang

Papers

Unified Triple-Phase-Shift Control to Minimize Current Stress and Achieve Full Soft-Switching of Isolated Bidirectional DC–DC Converter

Bilinear Discrete-Time Modeling and Stability Analysis of the Digitally Controlled Dual Active Bridge Converter

Simultaneous PWM control to operate the three-phase dual active bridge converter under soft switching in the whole load range

Optimized modulation strategy for three-phase dual-active-bridge DC-DC converters to minimize RMS inductor current in the whole load range

Full discrete-time modeling and stability analysis of the digital controlled dual active bridge converter