The single-stage high-frequency (HF) isolated ac–dc converter has the advantages of high-power density, long life, and high efficiency. It has a broad application prospect in distributed power generation, ac microgrid, and energy storage system. In this article, its topologies are classified according to the energy storage components. The structure and operating principle of various topologies are introduced. Furthermore, the development of extended multiport single-stage HF isolated ac–dc converter is introduced. On this basis, the advantages and disadvantages, typical characteristics, cost, and power loss of various topologies are analyzed and summarized. Finally, the key issues that need to be solved are pointed out and the development trends are explored. This article aims to provide a reference for the subsequent research on single-stage HF isolated ac–dc converters and the advancement of their practicability.

The current-fed isolated DC-DC converter (CFIDC) possesses the advantages of low current stress, high safety and high efficiency. In this paper, a bidirectional buck-boost CFIDC (B3CFIDC) is proposed to realize the bidirectional buck and boost voltage conversion and accordingly extend the operating range of the CFIDC. The basic modulation is proposed and based on this modulation the operation principle of the proposed BCFIDC is analyzed in detail. Furthermore, the voltage conversion ratio as the functions of the duty cycle ratio of the buck unit and shoot through ratio of the H-bridge converter is derived. The control rule of the two controllable variables is determined based on minimizing the average energy-storage inductor current. In addition, the optimal starting moment of active period of the buck unit in the switching cycle is determined based on minimizing the energy-storage inductor current ripple. The detailed experimental results verify the correctness and feasibility of the proposed topology and modulation.

In this article, a hybrid three-level current-fed series-resonant isolated bidirectional dc–dc converter is proposed, containing an interleaved parallel bidirectional buck/boost converter at the low-voltage side and a half-bridge three-level converter at the high-voltage side. The operation principles and characteristics of the proposed converter are thoroughly analyzed with the duty-cycle plus dual phase-shift control. The relationship between the general case of three independent control variables and the transfer power is derived. Then, a modulation strategy optimization algorithm by taking the minimum root mean square (rms) value of the resonant current as the objective function is presented. Under the optimal operation point, the converter operates efficiently with the minimum rms value of the resonant current and accordingly increases the efficiency. The detailed experimental results verify the correctness and feasibility of the proposed converter and modulation strategy.

In this paper, a modified single-carrier and multi-modulation sine pulse width modulation (MSCMMSPWM) is proposed to simplify the implementation of the single-phase asymmetrical neutral-point clamped gridconnected inverter (ANPC-GCI) for the PV generation system application. Furthermore, the expressions of the total switching loss and grid current total harmonic distortion (THD) of ANPC-GCI controlled by the proposed MSCMM-SPWM are derived. Then taking the grid current THD meet the grid-connected standard in the entire operational range as the constraint, an efficiency enhancement scheme for ANPC-GCI based on online adjusting the carrier frequency and the obtained THD expression is proposed. The control configuration of ANPC-GCI, including the proposed MSCMM-SPWM and online efficiency enhancement scheme is designed. The detailed experimental results verify the correctness and feasibility of the proposed MSCMM-SPWM and the efficiency enhancement scheme.

In the IGBT-based bidirectional isolated dual-active-bridge (DAB) dc-dc converter, it is not necessary that the IGBTs of the secondary-side H-bridge inverter (HB2) retain under on state because in this system, the function of synchronous rectification cannot be realized. This paper firstly analyzes the characteristics of the DAB converter with all of the IGBTs of HB2 keeping under OFF state. Its main drawback is that the flow back current in the primary dc side is caused and the current stress is increased. Furthermore, a hybrid modulation strategy is proposed and the control rule of the two inner phase shifts of the primary and secondary H-bridge inverters is determined to eliminate the flow back currents in the entire power range, enhance the maximum transfer power, and achieve a good inductor current characteristic. The experimental results verify the correctness of the theoretical analysis and the feasibility and practicability of the proposed hybrid modulation strategy.

In this paper, an improved modulation strategy for the single-phase single-stage isolated bidirectional AC-DC converter is proposed. The modulation of AC side cycloconverter is based on the unipolar idea to reduce the switching quantity and the AC side current ripple and provide the bidirectional current flow path. The DC side H-bridge converter is driven by the PWM signals cooperatively to speed up the transformer current and thus to eliminate the voltage spike. Furthermore, the power reversion zone (PRZ) of the converter in an AC voltage period is verified. The action of the proposed modulation strategy in PRZ is analyzed for the first time. It reveals the reverse current overshoot is caused in AC to DC direction and the voltage spike is caused in DC to AC direction. An online switching scheme of the modulation strategy based on the proposed modulation strategy is proposed to eliminate these problems and the system can operate at arbitrary power factor. The detailed experimental results verify the correctness and reliability of the proposed modulation strategy.