Diagnosis of conducted interference with discrimination network

Power supply circuit testing for compliance to conducted emission limits utilizes a line impedance stabilization network (LISN) and is usually carried out during the final stage of the product design. However, the LISN measures only the total contribution of common-mode (CM) and differential-mode (DM) emissions in the live or neutral line. In EMI filter design, it is always desirable to be able to determine whether the differential- or the common-mode component is dominant in order to pin-point the effective element of the filter to be changed to reduce the total emission. This, in essence, minimizes the trial-and-error guess work in selecting the proper components of the filter and cuts down on design time. This paper touches on the conventional way for measuring total conducted noise emissions of a product and how the CM/DM discrimination network (CM/DM DN) can be used as an effective diagnostic tool for the modification of power supply filters to reduce conducted noise emissions.<>