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Common Mode Choke Theory

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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.

Power Supply / Line Noise Schematic
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

In the example above, the power line voltage, “Vs”, causes current, “Iz”, to flow through the load, “Z”. At any non-zero instance, Current “Iz” flows into “Z” through one power line wire and returns through the other power line wire. EMI voltage, “Vnc1”, causes current “Inc1”, to flow through the load “Z”. Similarly, EMI voltage, Vnc2 causes current “Inc2” to flow through the load “Z”. Because the EMI is generating both “Vnc1” and “Vnc2” – the two voltages tend to be in phase. There is very little current flow between them. Current “Inc1” does not flow through both power line wires. It flows through one power line wire and through the ground path. Similarly, current “Inc2” does not flow through both power line wires. It flows through one power line wire and through the ground path. In this example only “Vnc1” produces electrical noise across load “Z” because the “Vnc2” end of “Z” is grounded. In practice, the effective ground point could occur somewhere between the two ends of load “Z”.

Figure 3 below illustrates the same application with a Common Mode Choke.

The common mode choke has two windings. Each winding of the common mode choke is inserted between the end of a power line wire and the load.

Power Supply / Line Noise Schematic

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.

Differential & Common Mode Noise Filtering Application

The inductance of Winding A restricts (reduces) the flow of current “Inc1” (when compared to Figure 1), thereby reducing the noise voltage across “Z”. Similarly the inductance of winding B restricts (hence reduces) the flow of current “Inc2”. Windings A and B have the same number of turns. The ampere-turns created by Current “Iz” (but excluding any “Inc1” current component) flowing through winding A is cancelled by the opposing ampere-turns created by current “Iz” flowing through winding B. Ideally, the cancellation results in zero inductance and no restriction (no reduction) of current “Iz”. “Iz” produces the same voltage across load “Z” as it does in Figure 1. In practice this will not be true. The common mode choke will have some leakage flux between windings A and B hence incomplete cancellation. Windings A and B will have some winding resistance. Both of these will have some effect on “Iz” (reduces “Iz”).

In contrast, the load current “Iz” flowing through both Winding A and Winding B of the differential choke shown in Figure 1 do not cancel, hence “Iz” will be restricted (reduced). Differential chokes are useful when the electrical noise frequencies are much higher than the operating frequencies. The higher choke impedance at the high frequencies block the electrical noise while having a tolerable effect at the operating frequencies.

Some common mode chokes are intentionally designed to have significant leakage inductance. The leakage inductance acts in series with the load hence the leakage inductance provides differential noise filtering. One common mode choke functions like the combined chokes shown in Figure 1 but may differ in levels.