Characterization of RF noise source impedance for switched mode power supply

A novel method to characterize the RF common-mode (CM) and differential-mode (DM) noise source impedance of any switched mode power supply (SMPS) under loading condition has been developed using a two current probes approach. The proposed method allows measurement of noise source impedance of the SMPS without interrupting its normal operation. With proper setup calibration, it is capable of deriving noise source impedance of either very high or very low value, in terms of equivalent resistive and reactive elements, with which the most effective filter configuration and correct component values for the built-in power line EMI filter of the SMPS could be designed with ease.