I. Introduction
The development of the renewable energy generation, uninterruptable power supplies, and energy storage systems has been the major driving force to advance the dc–dc converter technology [1], [2]. In various applications, dc–dc converters generally require simple control, high power density, and high efficiency. More specifically, dual-active-bridge (DAB) dc–dc converters, proposed around 1990s [3], have been applied widely in recent years. Besides their high power density and efficiency, DAB converters have the advantages of inherent soft-switching capability and galvanic isolation [4], [5], [6]. After decades of research, the DAB converters have developed a variety of structures for different applications, such as half-bridge DAB, three-phase DAB, and multilevel DAB converters [7], [8], [9]. Among them, a two-to-three (2/3)-level DAB converter has been proposed to increase the voltage blocking capability of the high-voltage (HV) side [10], [11], [12]. As shown in Fig. 1, the 2/3-level DAB converter is composed of a two-level H-bridge in the low-voltage (LV) side, an isolated transformer, and a three-level neutral-point-clamped (NPC) bridge in the HV side. This structure is suitable for the applications where the rated voltages of the input and output sides are considerably different, e.g., the medium-voltage dc photovoltaic (PV) system, as shown in Fig. 2. Compared with the two-level DAB converters, the 2/3-level DAB converters can achieve higher voltage blocking capability and higher step-up ratio and allow the utilization of lower voltage rating power components, saving the cost to some extent. Furthermore, an increased number of switches provide more degrees of control freedom to further improve the performance of the converters.