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This work introduces an improved and simplified method to design electromagnetic interference (EMI) filters for both dc-dc and ac-dc switching power supplies. This method uses the practical approach of measuring the power supply noise spectrum and using the data to calculate the maximum possible magnitude and minimum possible magnitude of the differential mode and common mode noise impedances. The noise impedance magnitude information aids the design of the EMI filter. Phase information for the noise impedance is not required. In addition, information about the topology and control method of the power supply is not needed. This method solves the limitations of existing EMI filter design methods, which are either too complicated to use, or are based on ideal cases that neglect the noise impedance. The analysis and experimental results show that this method can guarantee that the required attenuation can be achieved, especially at low frequencies.
Date of Publication: Nov. 2004