Reducing EMI in switch mode power supplies

In general, as the operating frequency of a switch-mode converter increases, the task of EMI (electromagnetic interference) suppression becomes more difficult. Many times, a reduction in power supply size is negated by bulky input and output filters. Additional mechanical and printed circuit board design iterations have resulted due to EMI difficulties. With these difficulties in mind, the author examines the types of EMI, their origins, and methods of measurements. He considers the minimization of the required `after the fact´ filtering. If this is done early in the design phase, some of the extra design iterations can hopefully be eliminated. To accomplish this, various components and circuit configurations are examined. A 100 W forward converter was used to verify most of the methods discussed. Specifically, it was shown that, by controlling parasitic capacitance, significant reductions can be made in the conducted EMI. A byproduct of controlling these parasitics is the fact that the conducting-loop area is reduced and therefore the radiated EMI is also reduced