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In the AC input power supplies; input EMI filters(common + differential) are connected between mains and the rectifier. In some low power power supplies; only differential mode filter is used and this filter is used after the rectifier. What's the reson behind that?

Examples: enter image description here

enter image description here

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    \$\begingroup\$ The purpose of those filters is somewhat different. The top schematic shows a common mode filter which is needed to prevent the high frequency switching noise (generated by fast switching inside the flyback converter) from "escaping" into the mains connection as that would result in large EMI emissions. The bottom schematic is a filter that is ground referenced and used to supply a clean supply voltage to the VIPER011LS chip. \$\endgroup\$
    – Bimpelrekkie
    Jun 3, 2021 at 9:46
  • \$\begingroup\$ The two pictures cannot be compared. The first picture seems to be schematics of an actual product. The second picture looks like block diagram from chip marketing material to approximately show kind of building blocks the chip needs around it, so it is certainly not a finished product you could manufacture and sell. \$\endgroup\$
    – Justme
    Jun 3, 2021 at 9:50

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