ABSTRACT
The causes of spikes or electromagnetic pulses (EMPs) on the line are lightning, large inductive equipment shutting off, and magnetic pulses created by nuclear activity. Lightning and nuclear activity create common mode pulses between the lines and ground, whereas the equipment type creates a differential mode type of pulse between the lines. On the load side, the switcher parasitic and the diodes are the leading culprits generating noise. Any current pulse, on the load side of the off-line regulator, to ground appears as common mode to the line. Adding a transformer or keeping the power supply isolated reduces these common mode conducted emission pulses. The one exception is the primary to secondary capacitance of the transformer, but this is so small that it can be neglected. Power supply filter capacitors pumping current to ground should be eliminated. To eliminate the common mode, the EMI filter employs common mode chokes and feed-through capacitors to ground. The problem is another specification limiting the leakage current. This is the capacitive reactance current flow through the capacitor between line and ground. This is another misnomer; it is really reactive current.
