Novel Measurement Procedure for Switched-Mode Power Supply Modal Impedances

The measurement of modal impedances of switching-mode power supplies (SMPSs) is of paramount importance in electromagnetic compatibility applications, especially when designing electromagnetic interference filters to suppress the conducted and radiated electromagnetic emissions is the target. A novel measurement technique that makes use of an impedance analyzer and a coupling transformer and allows extracting the complex value of common-and differential-mode noise source impedance of an SMPS is proposed. The measurement procedure is first studied from the theoretical point of view, then experimentally validated with passive components, and eventually implemented in a CISPR-25 conducted emission test setup to measure the magnitude and phase of modal impedances of an automotive SMPS under different operation conditions.