Detailed explanation of common mode filter noise reduction
Time:2022-12-15
Views:1360
In order to use one of the noise reduction countermeasures of inductance, this paper will introduce the content of noise reduction of common mode filter. Strictly speaking, the common mode filter is not an inductor, but a magnetic device, which is an important part of the noise reduction strategy.
Common mode filter
The structure of the common mode filter is that two windings are wound on a magnetic core, which is equivalent to the combination of two inductors (see the figure below).
When the current flows through the winding, the magnetic core generates magnetic flux, which can make the current difficult to flow (choke) against sharp current changes. This is the same as the self inductance of the inductor.
Common mode filter basically plays the role of common mode current not flowing and differential mode current flowing. The key is that the two wires are wound on a magnetic core in the same direction.
As shown in the figure, the differential mode current flows back and forth on two wires, so the magnetic flux generated by the magnetic core is in the opposite direction, and the magnetic flux is offset, so it can not act as a choke, but directly passes through.
In contrast, the flow direction of common mode current is the same, so the magnetic flux is enhanced, and the current is not easy to flow. In other words, common mode current=common mode noise is difficult to pass through and is filtered (Figure 1).
Figure 1: Current flow when magnetic flux increases
Countermeasures for Reducing Noise Using Common Mode Filters
Since the noise of the switching power supply is mentioned here, an example of the use of the input filter as the power supply is given below (Fig. 2).
Figure 2: Typical power supply input filter
Insert a common mode filter into the input line of the power supply as shown in the figure. Compared with common mode filters for signal lines, common mode filters for power lines use a split winding structure with large differential mode impedance. These products are generally introduced as common mode filters for power lines, and the effect of differential mode noise reduction is also expected. However, due to the very low differential mode impedance of several hundred k~several MHz, it is generally used together with π filter and other filters for differential mode noise.
Key points
Common mode filter is used to eliminate common mode noise.
Common mode filter is a filter that uses self induction to prevent common mode current from passing through.
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