Stability Enhancement of Cascaded DC Systems via Source Output-Impedance Reshaping Using Model-Free Control
Cascaded DC systems may become unstable through source-load impedance interactions even when their constituent converters are locally stable. This paper proposes a source-side model-free control (MFC) method that reuses bus-voltage feedback to reshape the source output impedance while preserving nominal reference-to-output dynamics. The controller uses only nominal source-loop information and requires neither a load-converter model nor load-side measurements. A current-controlled buck source supplying a regulated buck-resistor load that exhibits constant-power load (CPL) behaviour at low frequencies is used as the case study. Impedance analysis predicts that the proposed MFC increases the maximum stable load power by approximately 50%, thereby improving the stability margin of the cascaded system. Switched-model simulations confirm preservation of the nominal reference-to-output response and show that the MFC-controlled system remains stable at load levels for which the baseline system becomes unstable.