Conducted EMI Suppression and Stability Issues in Switch-mode DC-DC Converters

Electronic power converters are sources of electromagnetic interference at their input power source as well as in other neighbouring electronic equipment, and suppression of EMI is a major issue in switch-mode power converter design. By their very nature, switching converters have fast voltage and current changes that generate conducted noise due to reflected ripples in the input current. Fast switching logic loads also produce conducted noise that reflects back onto the input of the converter. In order to meet conducted emission and susceptibility standards, an LC input filter is often employed between a dc-dc converter and its power source. However, its presence, in turn, often results in degradation of dynamic performance and instability of the system even if the filter attenuation is brilliantly designed. The detrimental interaction between an input filter and a switched mode converter is a function of input filter parameters as well as supply voltage. In this paper, some design practices for EMI noise suppression are discussed for dc-dc converter applications. Then the instability problem is addressed that results from the complex interaction of dc-dc power converter with its input EMI filter. Its causes are discussed and a design procedure is suggested for its treatment by damping the input filter without over sizing or under sizing. Results corresponding to boost converter are presented to illustrate the procedure.