Stabilization of a cascaded DC system via adding a virtual impedance in series with the load converter

Interactions between the individually designed power subsystems in a cascaded system may cause system instability. This paper proposes the concept of a virtual impedance to be connected in series with the input impedance of the load converter, and makes the magnitude or phase of the load converter´s input impedance modified only in a small range of frequency. As a result, the instability problem of the cascaded system is solved, and the load converter keeps a good dynamic performance. The expressions of the required series virtual impedance (SVI) are derived, and the control-based realization of the SVI is presented. Finally, considering the worst stability problem that often occurs at the system whose source converter is an LC filter, a 100 W cascaded system consisting of an LC input filter and a 48V-12V buck converter is fabricated and tested to validate the effectiveness of the proposed control methods¹.