Conducted emission analysis and suppression design of a DC/DC semi-isolated power supply

This paper studies the conducted electromagnetic interference (EMI) problems in a DC/DC switched-mode power supply (SMPS) with semi-isolated topology. A high frequency circuit simulation model is firstly established to predict the conducted emissions of this power supply. Then the common mode (CM) and the differential mode (DM) conducted emission mechanisms of this power supply are analyzed in detail according to its working principle and structure. The voltage of the rectifier diodes at the transformer's secondary is founded to be the main CM EMI sources. A DM noise coupling phenomenon from the voltage ripple of output port is discovered and its role on EMI emission is explained too. Based on conduced emission paths analysis, three kinds of EMI filters, including traditional input EMI filter, high frequency input and output port EMI filters and a new DM topology filter are proposed to restrain the conducted emissions respectively. Their effectiveness is verified with experiments and the merits and drawbacks of different designs are also introduced.