EMC foundation, countermeasures for reducing noise using common mode filters
Time:2024-01-01
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As one of the noise reduction strategies using inductors, this article will introduce the content of using common mode filters for noise reduction. Strictly speaking, a common mode filter is not an inductor, but a magnetic device and an important component in noise reduction strategies.
Common mode filter
The structure of a common mode filter consists of two windings wound around a magnetic core, equivalent to two inductors combined together (see figure below). When current flows through the winding, the magnetic core generates magnetic flux, which plays a role in preventing the flow of current (choking) in response to rapid current changes. This is similar to the self inductance effect of inductance.
The common mode filter basically plays the role of non circulation of common mode current and circulation of differential mode current. The key is that these two wires are wound in the same direction around a magnetic core.
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, which cancels out the magnetic flux. Therefore, it cannot act as a choke, but directly passes through.
In contrast, the direction of common mode current is the same, so the magnetic flux increases and the current is less likely to flow. That is to say, common mode current=common mode noise is difficult to pass through and is filtered out.
Countermeasures for using common mode filters to reduce noise
Due to the mention of noise in switching power supplies, an example of using an input filter as a power supply is provided below.
This diagram is a one that has been used in the input filter of a switching power supply, where a common mode filter is inserted into the input line of the power supply as shown in the diagram. Compared to common mode filters used for signal lines, common mode filters used for power lines use split winding structures with higher differential impedance. These products are generally launched as common mode filters for power lines, and their differential mode noise reduction effect is also worth looking forward to. However, due to the very low differential impedance of several hundred k to a few MHz, it is generally used in conjunction with filters for differential noise such as π - type filters.
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