The impedance of an inductor is proportional to its inductance and the frequency of the signal passing through it. Inductors, therefore, can act like low-pass filters, that is they allow low-frequency signals to pass through them, while they block higher frequency signals. A common mode choke inductor is a special type of filter with two inductor windings on the same core. In order to understand common mode choke windings, it is important to first understand what “common mode” means.
Normal Mode: During normal operation of any circuit (such as a switch-mode power supply) connected to the AC Mains, AC current flows in through the AC Line and out through the AC neutral. This is called Normal Mode Current (or Differential Mode Current.)
Common Mode: There are other noise currents generated in most circuits that tend to be higher in frequency. These currents can be conducted out of the AC Mains in the form of noise. This is called Common Mode Noise. The current flows out on both the AC Line and AC neutral, hence the term Common Mode. It is this noise that a Common Mode Choke seeks to suppress.
How do common mode choke windings work?
A common mode choke includes a magnetic core and two windings, making it a four-terminal device. The two common mode choke windings have the same number of turns and are wound in opposite directions in a mirror configuration.
In this configuration, normal mode current flows into the circuit through one of the windings, and out through the other. The direction of current in each common mode choke windings combined with the mirror winding configuration means the flux generated by each winding cancels out the flux generated by the other winding. Thus the common mode choke presents a very low impedance to normal mode currents.
For common-mode noise, however, the flux generated by the current in each winding is additive, causing the choke to present a very high impedance to common mode currents.
As stated above, the impedance of the common mode choke is proportional to the frequency of the current. So determining the required inductance of the choke depends on the lowest frequency of common-mode noise the designer is trying to suppress.
The wire for a common mode choke must be larger enough in diameter to handle the Normal Mode current without overheating.
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