As key component for the flexible dc distributed power system, the dual active bridge (DAB)-converter-based solid-state transformer (SST) with high efficiency for a wide operating range is essential. However, with the traditional phase-shift control, high backflow power and current stress will significantly affect the conversion efficiency. In this paper, the backflow power characteristics in both sides of DAB-based SST converters are comprehensively analyzed. On this basis, complete transmission power, backflow power, and peak current mathematical models are established. Then, a minimum-backflow-power-based extended-phase-shift control strategy is proposed with the determination of optimal phase-shift pairs by using the Karush-Kuhn-Tucker function for various scenarios. The backflow power and current stress curves with different algorithms are compared. It shows the proposed control can improve the output power regulation flexibility, minimize the backflow power, and improve the efficiency in wide operating range. Finally, a DAB-based SST prototype was developed and the experimental results verified the effectiveness of the proposed control strategy.

This paper deals with an optimized 3-level modulated phase shift control using particle swarm optimization (PSO) strategy based on the unified phasor analysis with aim to improve the efficiency of the bidirectional dual active bridge (DAB) converter for the whole operation range. A unified mathematical model based on Fourier transform is built for the DAB converter. All possible operation states under 3-level modulated phase shift control are covered. Accurate complex mathematic expressions for the inductor current, the transmission power and the reactive power are obtained. Both modulus and angle variables are illustrated with respect to the inner and outer phase shift angle with the phasor diagram. The proposed method is able to achieve the minimum reactive power under 3-level modulated phase shift control by obtaining the optimal phase-shift angles directly. The cumbersome process of the optimal operation mode selection for different voltage conversion ratio and load conditions in conventional methods is overcomes successfully, thus greatly simplified the theoretical calculation and implementation difficulty. Simulation and experimental results in terms of the reactive power, soft-switching range, and efficiency are provided to verify the practical feasibility of the proposed method for the bidirectional DAB converters.

Dual-active-bridge (DAB) converters have been widely recognized as one promising topology for high-frequency-link power conversion applications. However, one critical issue concerning DAB converters is the transient dc-bias current due to the voltage-second imbalance, which may occur frequently in typical dynamic operations of DAB converters such as the start-up, sudden load or output voltage change, and power flow direction change. Due to the high-frequency operation, the dc-bias current of DAB converters will impose more harm to magnetic components because of the limited range of operating flux density and possible magnetic saturation compared with the counterpart low-frequency dc-bias current. Over the past several years, extensive researches have been conducted to address the high-frequency transient dc-bias current of DAB converters under different phase-shift modulation strategies such as single phase shift, extended phase shift, and triple phase shift. In this article, a comparative review of typical dc-bias current mitigation strategies under typical phase-shift strategies for DAB converters is presented. An experimental evaluation of selected strategies under various scenarios is conducted in terms of the settling time, inductor current stress, efficiency, switching number, and implementation complexity. This article ends up with an overview of high-frequency dc-bias mitigation strategies and a discussion for the further optimization.

Efficiency and reliability are essential for isolated bidirectional dc-dc converters in electric vehicle (EV) battery charging systems. As one promising power interface between energy storage components and power bus, dual-active-bridge (DAB) dc-dc converters have gained wide applications due to the advantages of galvanic isolation, symmetric circuit structure, high power density, and easy control implementation. In this article, based on the triple-phase shift (TPS) control, a fast and effective fault diagnosis strategy is proposed for DAB converters to improve both the power conversion efficiency and reliability. First, the operation principle of DAB converters with TPS control is presented. Then, a fast and effective open-circuit fault (OCF) analysis under different operation modes of TPS control is presented. Based on the resonant phenomenon during the fault conditions, an OCF diagnosis strategy that uses midpoint voltage as diagnosis criteria is proposed for DAB converters under the advanced TPS control to identify the faulty devices quickly and avoid the system collapse. After the failure switches are located, fault-tolerant operation is adopted to maintain the power transmission ability of DAB converters and ensure safe operation. The experimental results are provided to validate the correctness and effectiveness of the OCF performance analysis, fault diagnosis, and tolerant method.

Asymmetric duty modulation (ADM), as a recently proposed modulation scheme, provides new chance to improve the performance of dual active bridge (DAB) converter. On this basis, an optimal asymmetric duty modulation (OADM) scheme is proposed in this paper to minimize the inductor peak-to-peak current over the whole voltage and power range. Firstly, the DAB converter adopting ADM scheme is analyzed in frequency-domain and time-domain, and four operating modes are established. Then, the minimum peak-to-peak inductor current for each operating mode is obtained under Karush—Kuhn—Tucker (KKT) condition, and the corresponding optimal closed-loop control with low calculation complexity is derived. Then, the key values and features of the OADM scheme are compared with recent state-of-the-art, mainly including the inductor root mean square (RMS) current and soft-switching performance. Finally, an experimental prototype was built to validate the efficiency enhancement of the converter by adopting the proposed OADM scheme.

This paper presents an improved linear quadratic regulator (LQR) control based on linear matrix inequalities (LMI) to optimize the key parameters for the closed-loop control of DAB converters with high system uncertainty. Firstly, the polytopic model of an uncertain DAB converter is introduced according to the simplified DAB equivalent circuit. LMI are then used in the improved LQR control to derive the optimized control parameters under the given constraints. An improved LMI-LQR hybrid closed-loop control is adopted with the output current introduced in the control loop to enhance the dynamic performance. The performance of the proposed LMI-LQR is compared with the conventional LQR in terms of transient responses under various load and line disturbances. Both the simulation and experimental results are provided to validate the advantages of the proposed control.