Wide bandgap (WBG) devices offer the advantages of high frequency, high efficiency and high power density to power converters due to their excellent performance. However, their low parasitic capacitance and fast switching speed also make them more susceptible to switching oscillations. The switching oscillations can cause voltage and current overshoots, shootthrough, electromagnetic interference (EMI), additional power loss, and even device damage, which can seriously affect the performance of power converters and systems. However, a comprehensive and in-depth overview is lacking on this topic. This paper reviews the types, the causes and negative effects, the effects of parasitic parameters and suppression methods of these switching oscillations, which is helpful for practical engineering. Firstly, the switching oscillations are divided into different types, and their causes and negative effects are reviewed. Then, the effects of different parasitic parameters on the switching oscillations are overviewed. It is found that due to the different physical structures of silicon carbide metal-oxide-semiconductor field-effect transistors (SiC MOSFETs), enhancement-mode gallium nitride high-electron mobility transistors (eGaN HEMTs) and cascode GaN HEMTs, the effects are also different. Finally, the main methods of suppressing the switching oscillations are summarized, and the advantages and disadvantages of these methods are presented.

In order to realize power flow control among photovoltaic (PV) panel port, battery port, and load port in the distributed photovoltaic power generation system, integrated three-port DC/DC converters (TPCs) are widely adopted. The objective of this paper is to provide a comprehensive, comparative, and insightful analysis and overview of these TPCs. Firstly, based on the power transmission and control requirements of the system, five topological structures of a three-port DC/DC conversion system (TPCS) composed of two or three two-port DC/DC converters are presented. And the five structures are comparatively analyzed. Secondly, based on these topological structures, and combing with the basic two-port DC/DC converters, three groups of typical TPCs with different galvanic isolation requirements are derived respectively by sharing switches, combining branches, etc. And comprehensive analysis, comparisons, and reviews of these TPCs with different structures of each isolation type are presented with tables and figures. Finally, the comparison results of the three isolation groups are summarized, which leads to a guidance for the selection of TPCs in different applications, and then some suggestions for the future research on TPCs are given.

A five-degree-of-freedom (5-DOF) modulation scheme is proposed in this paper for dual active bridge (DAB) converter to improve its efficiency in a wide operation range. Firstly, the possible operation modes of the converter under the 5-DOF modulation scheme is analyzed. As a result, eleven effective modes are obtained and their steady-state characteristics are analyzed, including the inductor root-mean-square (RMS) current, inductor peak-to-peak current, and transmission power, etc. It is worth mentioning that the inductor peak-to-peak current expression is simple and it can be regarded as an indicator for RMS current. Thus, the peak-to-peak current is treated as the objective to obtain the optimal solutions of each mode, and the global minimum peak-to-peak (GMPP) modulation scheme is obtained. However, there are only two switches that can realize soft-switching operation in the low power range under GMPP modulation scheme, which results in significant switching loss. Therefore, to further expand the soft-switching operation range in the low power range, the soft-switching conditions are also taken into consideration, and an optimized five-degree-of-freedom (O5-DOF) modulation scheme is proposed. With the proposed O5-DOF modulation scheme, almost all switches can realize soft-switching operation during the whole load range, and the inductor RMS current is minimized simultaneously.

In order to further improve the efficiency of dual-active-bridge (DAB) converter, a hybrid five-variable modulation scheme with minimal root mean square (rms) current is proposed in this article, which combines the traditional phase-shift and pulsewidth modulation schemes. By adopting the proposed optimal scheme, a wider zero-voltage switching (ZVS) operation range and lower rms current level are achieved simultaneously when compared with other modulation schemes, which significantly improves the efficiency of the DAB converter. Moreover, for the complexity of the proposed modulation scheme with five variables, the Fourier decomposition based analytical expressions are derived for the converter and a generalized performance analysis results are presented. Besides, in order to avoid the analysis error, especially at light-load condition, the optimization strategy takes the rms current as the optimal objective, which includes the eight harmonic component of the inductor current rather than just considers the fundamental component. And an offline particle swarm optimization algorithm is introduced to acquire the optimal operating points for the whole power range. To substantiate the effectiveness of the proposed modulation scheme, a comprehensive comparison among related works in terms of the rms value and ZVS range were carried out under various operation condition. Finally, a 1.2 kW experimental platform was built and the highest experimental efficiency under the proposed modulation scheme reaches 96.8%.

Grid-connected current quality is seriously affected by the low-order background harmonics in the gird-inverter dynamic system. The PCC voltage feedforward impedance reshaping method of the grid-connected inverter is generally applied to solve this problem, however about 90° phase lag of the inverter output impedance is brought in, which would deteriorate the adaptability of the inverter to the weak grid. To solve this problem, this paper proposes a novel frequency-division virtual impedance shaping method, which shapes the output impedance of the grid-connected inverter with different coefficients in different frequency bands respectively. The basic principles, parameters design and application analysis are studied in detail. With the proposed scheme, the grid-connected inverter can be equipped with strong adaptability to a wide range of grid impedance and suppression ability to the background voltage harmonics simultaneously. Theoretical analysis and experimental results confirm the effectiveness of the proposed method.