ABSTRACT
There is a wide-ranging set of opinions about the definition, cause, and elimination of both common-mode and differential-mode noise. This chapter aims to cover most of these concepts. It considers the common-mode effects of a DC-DC converter, or a PWM switching supply. Common-mode transients are most often the cause for concern, particularly so with sensitive analog and digital circuits, as this noise often leads to susceptibility issues where circuits can operate erratically, leading to adverse functional behavior or even failure. Differential-mode noise is most often attributed to power supply or load switching. Other causes are transients, surges, or interrupts that occur on any line with respect to a ground reference. The primary-to-secondary capacitance of the transformer carries the common-mode current. However, the secondary has many paths to ground through the winding capacitance to center taps at the transformer. A transformer was placed between the lightning strike and the filter. For the most part, this creates differentialmode “energy” at the filter.
