Impact of the Cascaded DC-DC Converter on the D-Q Impedance of a PFC Converter

A single-phase power supply unit (PSU) is composed of a front-end power factor correction (PFC) and a cascaded dc-dc converter load. It is heavily used in data centers, electrical vehicle chargers, etc. Several instability events in data centers have been reported, which is due to the dynamic interactions between the PSU loads and other components in the system. D-Q impedance modeling is an effective method to analyze such system dynamics behavior. By properly shaping the PSU input impedance, a stable system operation might be achieved. To understand the roles of the cascaded dc-dc converter in the system behavior, this paper analyzes the cascaded dc-dc converter's impact on the input d-q impedance of its front-end PFC converter. It is found that the negative resistor characteristic of the cascaded dc-dc converter impacts the PFC's impedance at the low-frequency range. It reduces the gain and phase of the PFC's d-d channel impedance, which could be detrimental to system stability. Whereas its impact in the high-frequency range is negligible. The experimental measurement result validates the analysis.