Analysis of input filter interactions in cascade buck converters

Many applications of switch-mode dc-dc converters require high conversion ratios (V_out/V_in), thus encouraging the use of n-stages connected in cascade. However, when an input filter is added to an existing cascade converter ldquoblack boxrdquo, it can cause system instability and performance degradation due to the negative dynamic input resistance characteristic of the converter. In this paper, we investigate this interaction of input filters in cascaded buck converters. The problem is treated by using a small-signal averaged model of the converter in which natural parasitic resistances of the circuit elements are also taken into account. Based on the open-loop transfer function of the system, a design criterion is proposed for the dimensioning of input filter damping that ensures closed-loop stability of the filter-converter system without over sizing or under sizing the filter. Theoretical results of this paper are validated experimentally.