Stability of large power electronics systems is often studied via impedance-based approach. Thereby, admittance passivity criterion is widely used to predict destabilizing interactions between converter and grid. It involves evaluating, in frequency range critical for stability, passivity of converter's admittance at connection port. However, this approach requires assumptions about termination at all other converter's ports, which may not be known, or may change. Consequently, as demonstrated in this article, port-coupling induced instability is difficult to be predicted by standardly used single-input single-output admittance passivity criterion. To overcome this, we propose use of multiple-input multiple-output (MIMO) admittance passivity properties as a novel tool to analyze stability and develop stabilization methods for dc-dc converters. Analytical MIMO admittance frequency responses are validated using experimentally measured ones, obtained for buck converter prototype. Stability implications from resulting MIMO passivity properties are verified by time domain results, demonstrating capability of the proposed method to easily predict port-coupling instability.
