Soft Startup Strategies for DAB-Based DCX in Composite Converters

In a composite dc-dc converter, regulation and dual active bridge (DAB)-based fixed-ratio (DCX) modules are arranged to minimize stresses and losses over wide ranges of input and output voltages. The system relies on smooth mode transitions in which a DCX must be quickly powered up from zero output to its nominal fixed-ratio operation. In the case a DCX precedes a regulation stage, the DCX is exposed to a high input voltage, leading to a large inrush current during startup. In the case a DCX follows a regulation stage, the DCX needs to charge the output dc capacitor while simultaneously supplying a high load current. To address these challenges, a generalized DCX modulation scheme is proposed in this paper to maximize the average DCX output current and to enable soft startup transitions in the two configurations considered. The approach is verified by simulations and by experimental results on a 350 V, 25 kW SiC-based prototype.

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