Modeling and measurement of the impedance of common mode noise source of switching converters

The EMI noise source impedance of an SMPS is essential in its power line EMI filter design, and when ignored, either over design, or system requirements not being met would occur. Because DM noise is easy to identify but CM noise is still hard to characterize, more concerns are with CM noise. This paper deals with the modeling of generation and propagation of CM noise and introduces a simple way to measure the CM noise source impedance of switching mode power converters. Experiments and EMI test results are presented to validate the model and the method of CM noise source impedance measurement.