Analysis and improvements of isolated DC/DC converters for dc microgrids

The increasing adoption of DC microgrids in applications such as residential buildings, data centers, and electric vehicle charging stations calls for isolated DC/DC converters that combine efficiency, bidirectional capability, and modular scalability. This thesis focuses on the analysis and improvement of three isolated converter families: the dual-active bridge (DAB), the asymmetrical bidirectional DC/DC (AB-DC/DC) converter, and the series resonant converter operated as a DC transformer (SRC-DCX). The research follows a three-stage methodology of analytical modeling, numerical simulations in PLECS, and experimental validation through hardware prototypes. For the DAB, the work derives closed-form zero-voltage switching (ZVS) boundaries and investigates auxiliary inductors as a means to extend ZVS at light load and wide voltage ratios. The analysis is further extended to the current-fed DAB (CF-DAB), providing new insights into unipolar and bipolar DC grid operation. In the AB-DC/DC converter, passive power sharing between the active and diode bridges is shown to be highly sensitive to magnetic tolerances, which can compromise thermal balance in the semiconductor devices. A probabilistic framework is developed to quantify this effect, and novel active power sharing control and hybrid modulation strategies are introduced to enable controllable power distribution between the parallel-connected bridges. Large-scale simulations and an experimental prototype confirm the feasibility of these methods. Finally, the thesis addresses the SRC-DCX, a topology well-suited for high-efficiency fixed-ratio conversion but lacking inherent power regulation. A sigma–delta cycle-skipping modulation is proposed to recover fine-grained power control without sacrificing soft-switching. The method is validated on a 1 kW prototype, and also verifies the effective current sharing between the input-parallel and output-parallel modular configuration. Overall, the thesis provides analytical, control, and experimental evidence that improve the efficiency, and controllability of isolated DC/DC converters in DC microgrids. The results are expected to support the development of scalable and robust converter for next-generation energy systems.