Current Limiting in Overload Conditions of an LLC-Converter-Based DC Transformer

The dc transformers based on an LLC converter operated in open loop near resonant frequency have several crucial advantages compared to other topologies. These advantages are the stiff voltage ratio with a natural power flow, the dc grid isolation via medium-frequency transformer, and the high efficiency due to the resonant converter nature. Despite these advantages, the open-loop operation does not provide any options to limit the transferred current during overload conditions in the dc grid. This is critical, since it prevents the grid from recognizing that either some local primary regulation or load shedding is required. This paper proposes a closed-loop control method that utilizes variable duty cycles to limit the current once the overload situation is detected. An applicable control scheme is derived and implemented. Since the duty-cycle modulation increases the losses of the converter, an online thermal model is implemented to protect the dc transformer during the overload conditions. The proposed current-limiting mode with the thermal model are validated using an exemplary low-voltage dc transformer prototype, demonstrating the general feasibility and overall good performance.