Stability Issue of Cascaded Systems With Consideration of Switching Ripple Interaction

Impedance-based criteria are often used for assessing the stability of cascaded converter systems. In this paper, it is pointed out that the switching ripple interaction (SRI) between the source converter and the load converter in a cascaded system would change the gain of the pulsewidth modulation of the source converter, causing the cut-off frequency and the phase margin of the source converter to change accordingly. Thus, even if a cascaded system is shown to be stable under an impedance-based criterion, it can, in fact, be unstable due to the SRI. This paper proposes an adaptive modulation sample-and-hold method for eliminating the effect of the SRI. A 48 V-12 V-5 V cascaded system consisting of two buck converters is tested for verification.