EMC / EMI :
It is a ability to control the leakage of radiation from machines that prevent interference with other normal equipments.
Differential mode& Common Mode:
There are common mode and differential mode interference in the two incoming lines of switching power supply. The interference signal and direction on the two incoming lines are same in common mode, like figure 1. But the differential mode interference is opposite, like figure 2.
Power line filter:
As there are two kinds of interference signals on the two incoming lines of switching power supply, so the power line filter must be able to suppress both the common mode and differential mode. Figure 3 is a common circuit of power filter. L1、L2、C1 can remove differential mode interference, L3、C2、C3 can remove common mode interference.