Swiss company Schurter offers EMC filters, common mode chokes, as well as filters integrated with inlet, switch and fuse. Typical EMC filter contains common mode choke and X and Y class capacitors. But how does it work?
Interference Sources
Typical sources of interference are, for example, IGBT inverters for motor control and switched power supplies. Both devices generate voltages and currents with steep edges in their operation. The interference spectrum covers the entire range of 0.15 to 30MHz where the conducted emissions is measured and 30 to 1000MHz where radiated emission is measured.
Differential (Symmetrical) Interference Mode
For low frequencies, in hundreds of kHz, the interference is spread the same way as the power supply voltage. Current flows in the loop formed by the L and N conductors.
Common (Asymmetrical) Interference Mode
For higher frequencies, above 1MHz, the parasitic capacities in the source of interference and the disturbed equipment also produce an interference current in the earth circuit. This common-mode interference current flows towards the disturbed equipment along both connecting lines and returns to the interference source through earth.
