Radiated Emission Due to Common Mode Current in Smps

The high frequency switching of semiconductor switches in Switched Mode Power Supplies (SMPS) cause high dV/dt and dI/dt resulting in differential mode (DM) and common mode (CM) conducted and radiated Electromagnetic Interference (EMI). The CM noise current circulating through the ground path is the major contributor for radiated EMI in the frequency range of 30 MHz to 1 GHz which will usually be above the stipulated international standards and are addressed here. The high dV/dt and dI/dt are major sources of EMI producing noise currents which will get coupled to ground through parasitic capacitances. The prominent parasitic capacitors are present in high frequency transformer and the semiconductor’s coupling to ground. They provide path for both DM and CM noise currents. The CM currents flowing in the different prominent parasitic capacitors are obtained by simulation for the four different topologies namely, non-isolated Buck, non-isolated Boost, Flyback and Forward converters. The radiated Emissions are calculated for each of the topologies and are presented. All the four converters are operated at same switching frequencies with same values of parasitic capacitances. The non-isolated Boost converter is found to generate higher radiated emissions due to CM current than the non-isolated buck converter and Forward converter has higher radiated emissions than Flyback converter. The results presented here can be used to decide on the topology of SMPS for a given application when EMI mitigation is a priority.