Common mode choke coils are used to suppress common mode
noise. This type of coil is produced by winding the signal or supply
wires one ferrite core.
Since magnetic flux flows inside the ferrite core, common mode
choke coils work as an inductor against common mode current.
Accordingly, using a common mode choke coil provides larger
impedance against common mode current and is more effective
for common mode noise suppression than using several normal
inductors.