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I was wandering how the main voltage coming from main in SMPS is not attenuated by the EMI filters. This is the input stage of a AC/DC SMPS by TI (https://www.ti.com/tool/PMP11064): enter image description here

Maybe I'm wrong but the 230VAC voltage coming from the mains is differential by nature so why does this 1mH choke not attenuate this voltage?

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Maybe I'm wrong but the 230VAC voltage coming from the mains is differential by nature so why does this 1mH choke not attenuate this voltage?

Because both L802 and L803 are common-mode chokes i.e. they attenuate common-mode noise in both directions and leave differential AC signals (such as the mains) largely unaffected. That is their purpose --> it's all about the dots on each winding: -

enter image description here

If there dots were at opposite ends then the choke would become a differential mode choke.

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  • \$\begingroup\$ Sorry should have specified how the X-capacitor C805 filters the differential noise but why does it not suppress the input voltage signal? \$\endgroup\$
    – KMN
    Commented Feb 5, 2023 at 15:38
  • \$\begingroup\$ why does it not suppress the input voltage signal <-- why does what not suppress? \$\endgroup\$
    – Andy aka
    Commented Feb 5, 2023 at 15:47
  • \$\begingroup\$ the X-capacitor \$\endgroup\$
    – KMN
    Commented Feb 5, 2023 at 15:55
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    \$\begingroup\$ At 50 Hz it has an impedance of nearly 7 kohm. The AC supply has probably got an effective impedance of around a few ohms hence, the capacitor barely loads the AC source at all and barely creates any measurable attenuation. It's not meant to either. \$\endgroup\$
    – Andy aka
    Commented Feb 5, 2023 at 15:58

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