Research On Switched Mode Power Supply With Metal Enclosure Radiated Emission

The EMI problems in the Switched Mode Power Supply have received moreand more attention today. As a part of the SMPS, metal enclosure plays the roleof supporting and shielding. However, a wrong connection not only do nothing toshielding, but also make the radiated emission rose. In order to study theinfluence of metal enclosure in far-field radiation emission, a Boost converterhas been built as a research object. Analysis on the radiated emission mechanism,establishment of high frequency equivalent models, research on the effect of theenclosure to PCB parasitic parameters, prediction of the far-field radiation havebeen done, and the suppression methods of far-field radiation have been given.The mechanism of electromagnetic radiation of a Boost converter isanalyzed, which can identify the major sources of interference. A Boostconverter has been designed and its PCB is made, based on which, the highfrequency equivalent circuit model will be established and obtain theinterference sources. Then, the interference sources can be used as excitationsource in the radiated model. Afterwards, a full-wave electromagnetic simulationwill be done and the far-field radiation is predicted, then the validity of thesemodels is verified by using the test results. The effect of the Boost converter withlong input/output cables on radiation is also considered.Shielding effect of metal enclosure is analyzed and the shieldingeffectiveness is calculated with experiments. The most important to establish themodel with metal enclosure are the parasitic parameters of the main circuit loopinductances and the capacitances between enclosure and the drain of MOSFET,etc. After calculating these parasitic parameters, the new high-frequency circuitmodel and radiation model can be created. These models can been verified byexperiments and predict the radiation of Boost converter with metal enclosure.According to the features of common-mode current on the cables, twomethods are presented to suppress radiated emission. One is putting ferrite ringsaround the cables to increase the common mode impedance. Another is to installlow impedance components between the PCB and the metal enclosure, thismethod can decrease the common mode impedance of SMPS. All of thesemeasures can reduce the radiated emissions, especially in the second one. What’smore, they can cooperate together and suppress the radiation better.