Design of common mode filter circuit

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Design of common mode filter circuit [Copy link]

The interference on the power line includes differential mode interference and common mode interference.
Differential mode interference transmitted between two pairs of lines is called symmetrical interference;

Common mode interference is transmitted between the wire and the ground, which is called asymmetric interference.

Usually below 50kHa, differential mode interference is the main factor;
above 5MHz, common mode interference is the main factor. In the value range of 500kHz ~5MHz, differential mode interference and common mode interference cannot be ignored.

Differential mode interference has a low frequency and causes less interference; common mode interference has a high frequency and can also radiate through wires, causing greater interference.

If you want to reduce conducted interference and keep it below the limit level specified by the EMC standard, the best way is to add common-mode filters to the input and output circuits of the switching power supply.

For the interference signal generated by the switching power supply, the EMC standard can be met by simply selecting the corresponding common-mode filter.

The method to reduce differential-mode conducted interference is to connect a differential-mode choke in series on the live and neutral wires of the power line, connect a differential-mode capacitor (X capacitor) in parallel between the live and neutral wires, or use capacitors and inductors to form a low-pass filter to reduce high-frequency interference; the way to reduce common-mode conducted interference is to connect a common-mode choke in series on the live and neutral wires of the power line, connect a common-mode suppression capacitor in parallel between the live, neutral and ground wires to form an LC filter to filter and remove noise.