EMI Filter Design, 3rd Edition

10Common-Mode Components

The causes of spikes or electromagnetic pulses (EMPs) on a power line or conductor are lightning, large inductive equipment shutting off, and magnetic pulses created by nuclear activity. Lightning and nuclear activity create common-mode pulses between power lines or between conductors and ground, whereas the equipment-type disturbances create a differential-mode type of pulse between the lines. On the load side, PWM switching and switching diodes are the leading culprits for generating noise. Any current pulse seen between the load side of the off-line regulator and ground appears as common mode to the line. Adding a transformer or keeping the power supply isolated reduces these common-mode-conducted emission pulses. ...