Measuring the Noise Impedances of Switched-Mode Power Supplies

In this paper, a novel approach to complex impedance measurement is proposed, using an Impedance Analyzer (IA) connected to the system under investigation through a transformer, which allows direct measurement of the magnitude and phase of the common mode (CM) and differential mode (DM) impedance of a Switched-Mode Power Supply (SMPS) during working conditions. The theoretical analysis of the measurement circuit both for CM and DM impedances is provided as the first step. Then, a fine calibration process is explained and validated with passive components. Eventually, the proposed method is exploited for experimental evaluation of the CM and DM noise impedances of an automotive SMPS in a test set-up for conducted emission verification.