A simulation of conducted EMI in flyback converters

The measure of conducted EMI needs strict environment and expensive instruments, most of the designers don´t have this condition. So, there are demands for economic forecast of conducted EMI before the product being tested by the government. This paper tries a simulation method to forecast the conducted EMI of a flyback converter. First, this paper analyses the mechanism of conducted EMI which shows the transformer and MOSFET is key point of conducted EMI. The voltage pulsating of primary MOSFET is a source of CM noise (common mode EMI). The transformer and its´ parasitic parameters are the coupling channels for conducted EMI from primary to secondary. The DM noise (differential mode EMI) is introduced by current pulsating of the diode in the secondary, which refluxes through the circuit of converter. Considering the structure of the flyback transformers, especially the effects of shielding layer on conducted EMI, this paper proposes a module of transformer with shielding layer and a formula of coupling capacitance between shielding layer and the windings. After discussing the parasitic capacitance of heat sink, the module of power MOSFET is built. Use Q3D software to extract the parasitic parameters of PCB board and put the whole module into SABER to simulate the conducted EMI. Finally, the simulated result is compared with a measured result which shows the simulation result is credible, and proves the simulation method is a economic and fast way to forecast conducted EMI.