On the Applicability of SISO and MIMO Impedance-Based Stability Assessment of DC-DC Interlinking Converters

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This article presents a formal mathematical correlation between the standardly used port-level (terminated) single-input single-output (SISO), and the recently acknowledged device-level (unterminated) multiple-input multiple-output (MIMO) impedance-based method for the stability assessment of dc-dc interlinking converters. Based on this, the conditions that must be met to ensure the correct stability assessment by the SISO method applied to a single port-pair are derived. It is shown that without prior knowledge on whether these conditions are met, the SISO method must be applied to every port-pair to account for possible port-level hidden dynamics . Alternatively, the MIMO method can be used, which is revealed to inherently account for any port-level hidden dynamics . It is further analyzed which method is advantageous in terms of computational complexity, intuitiveness, and simplicity for applications featuring meshed grids or multi-port interlinking converters, as well as in terms of interpreting the resulting stability margins. Finally, suitability of the MIMO method for termination-independent stability-oriented controller design and stability assessment based on measurements is highlighted. The presented methodology is illustrated for a simplified dc system with a current-controlled buck converter. Analytical stability predictions are validated using hardware-in-the-loop simulations and also experimentally, using a laboratory hardware prototype.