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Line filter – The last barrier in the switch mode power supply


Switch mode power supplies result in conducted interference because they generate radio interference voltage on the mains side. This can interfere with other equipment supplied with mains power. Line filters help to suppress the generated radio interference voltage. These can easily be designed from passive components such as current-compensated line chokes and X / Y capacitors. This article concerns the design of a single-phase line filter.


In conclusion, a line filter for switch mode power supplies without a common mode choke is not sufficient. Individual capacitors are not sufficient to completely suppress the interference emission. Additional longitudinal coils before the line filter help if further suppression of the differential mode interference is required. Using a line filter, all interference levels are below the permissible limit value and the switch mode power supply can pass an EMC test.

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