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When I search for differential mode input EMI filters (for power supplies), I get two different filter types:

schematic

simulate this circuit – Schematic created using CircuitLab

What's the difference between these two differential mode filters? (The values are random in the schematics.)

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    \$\begingroup\$ The low-side filter seems to be damped by the \$R_1C_4\$ network. This is to avoid resonance and interaction with the downstream switching converter. As you know, the incremental resistance of a closed-loop-operated switching converter is negative and can potentially interact with the \$LC\$ network if not properly damped. \$\endgroup\$
    – Verbal Kint
    Mar 3, 2021 at 6:35
  • \$\begingroup\$ @VerbalKint thank you for the answer, but how about the C1 on high side filter? It's not used in low side filter. Do you have an opinion about it? \$\endgroup\$
    – Das D.
    Mar 3, 2021 at 6:41
  • \$\begingroup\$ It will interact with the mains output impedance and provide additional filtering for differential mode. As shown here it is often added to work with the leakage inductance of the common-mode choke and provides differential filtering. \$\endgroup\$
    – Verbal Kint
    Mar 3, 2021 at 6:45
  • \$\begingroup\$ C1 is ineffective in the top circuit because it connects directly across a voltage source. \$\endgroup\$
    – Andy aka
    Mar 3, 2021 at 8:53

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