On EMI-filter interactions in a regulated converter - stability and load-transient performance

The EMI-filter design rules based on certain impedance parameters are developed in the late 1970´s by means of a small-signal equivalent circuit valid only for the converters operating under direct-duty-ratio control in continuous conduction mode. Basically the design rules are valid for an arbitrary switched-mode converter but the implicit nature of some of the critical parameters has prevented the full understanding of the design process. The practical design rules usually dictate that the EMI-filter output impedance has to be designed less than the open and closed-loop input impedances of the converter. The paper shows consistently based on theory and practical evidence that the critical impedances are the closed-loop and short-circuit input impedances of which the short-circuit input impedance usually dominates in the interactions. The voltage and peak-current-mode buck converters are used as the source of practical evidence.