What's the Common mode choke?
A common-mode (CM) choke is a special application of chokes where it is used to act upon a common-mode signal. These chokes are useful for suppression of electromagnetic interference (EMI) and radio frequency interference (RFI) frequently introduced on high current wires such as on power supply lines, which may cause unwanted operation. Reducing this noise is frequently done by using a common mode choke - two parallel coil windings on a single core. Common mode chokes allow differential currents to pass while blocking signals that are affecting both wires. Because the magnetic flux produced by differential-mode currents in the core of a common mode choke tend to cancel each other out, the choke presents little impedance to differential mode currents. It achieves this by the placement of windings such that they generate equal but opposite fields that cancel each other out for differential mode signals. Normally this also means that the core will not saturate for large differential mode currents, and the maximum current rating is instead determined by the heating effect of the winding resistance. On the other hand, common mode currents see a high impedance path due to the combined inductance of the windings that reinforce each other.
CM chokes are commonly used in industrial, electrical and telecommunications applications to remove or decrease noise and related electromagnetic interference.
When the CM choke is conducting CM current, most of the magnetic flux generated by the windings is confined with the inductor core because of its high permeability. In this case, the leakage flux, which is also the near magnetic field emission of the CM choke is low. However, the DM current flowing through the windings will generate high emitted near magnetic field since the windings are negative coupled in this case. To reduce the near magnetic field emission, a twisted winding structure can be applied to the CM choke.