This time, we will introduce you to the common mode choke coil.
"Common mode choke coils are distinguished according to interference and signal transmission methods"
The previously used chip ferrite beads and chip three-terminal capacitors use the principle that the interference frequency is slightly higher than the signal frequency, and are used as a low-pass filter to selectively cancel the interference. Although the common mode choke coil is also an interference filter, the principle is to distinguish between interference and signal, not frequency, depending on the conduction mode. Therefore, it is necessary to understand the two modes of conduction, common mode and differential mode.
Common Mode and Differential Mode
Usually, the current flowing from somewhere in the electrical circuit on the substrate will flow through the load to other circuits and flow back through other routes on the substrate. (Many times the return route is the ground plane of the substrate) This is the differential mode flow.
Figure 1 Conduction route of differential mode
On the other hand, although there is no clear wiring, there are other conductive lines. Since a weak parasitic capacitance is generated between each wiring on the substrate and the ground as a reference, a conduction path is formed which flows out from the ground and is folded back from the opposite direction to the ground. This is called common mode.
Figure 2 Common mode conduction line
Although the parasitic capacitance between the ground and the ground is very weak, when the signal frequency increases, the weak parasitic capacitance decreases, and the common mode current easily passes. Generally, the common mode current does not actively pass through the electronic circuit, but when the ground of the power supply circuit or the driver IC is loose, the circuit driven by the whole is unstable, thereby forming common mode interference. In this circuit, if there is a cable connected to the outside, a common mode current will pass through the cable, and since the potential is relatively unstable, an interference wave is formed.
"Common mode choke coil is a squelch filter that only acts on common mode current"
The common mode choke coil is a squelch filter that distinguishes noise and signals by the above-described common mode and differential mode conduction modes. In a word, it is a filter that acts only on the common mode.
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