Common mode inductor filter circuit, La and Lb is the common mode inductor coil. The two coils are wound on the same core, with the same number of turns and the same phase. So, when the normal current in the circuit through common mode inductance, current winding inductance coil system in the same phase generated in the reverse magnetic field and offset each other, influenced by the normal signal current is mainly controlled by the coil resistance (and a small amount of leakage caused due to damping; when there is common mode)

Common mode inductance flow through a coil, due to the same common mode current in the coil, will produce the same magnetic field to increase the inductance of the coil, the coil impedance is high, have strong damping effect, the attenuation of common mode current, to achieve the purpose of filtering.