Common Mode Choke Theory
A common mode choke may be used to reduce a type of electrical noise known as common mode noise.
Electro-Magnetic Interference (EMI)
EMI in the circuit’s environment is one source of electrical noise. EMI induces or couples unwanted electrical signals into the circuit. It is desirable to filter out the unwanted noise signals without significantly affecting the desired signal. Environmental sources of EMI often create an independent return path (ground path) for the electrical noise signals. The return path of the desired signal is a different path. Because there are two different return paths, a Common Mode Choke can be used to significantly reduce or block the unwanted noise signal at the load without significant reduction in the desired signal.
A.C. Power Lines
A.C. Power Lines provide a good example. They are known to carry significant levels of electrical noise. Their long length gives environmental EMI ample opportunity to generate unwanted electrical noise into the power lines.
Figure 2 below illustrates an application without a Common Mode Choke.

Vnd, Ind = Differential Noise Voltage, Current from S.M.P.S
Vnc 1, Inc 1 = Common Mode Noise Voltage, Current Created in Line 1
Vnc 2, Inc 2 = Common Mode Noise Voltage, Current Created in Line 2
Vs, Is = Source Voltage, Current
Iso = S.M.P.S Current, includes the noise Current
Vz, Iz = Load Voltage, Current
S.M.P.S = Switching Mode Power Supply

As in Figure 1 below, current “Iz” flows through both power line wires and currents “Inc1” and “Inc2” each flow through one power line wire and return through the ground path. Observe that current “Iz” flows through both windings but in opposing winding directions, while currents “Inc1″ and Inc2” each flow through only one winding and in the same winding direction. The ground path does not flow through a winding.
