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I was reading a page on the Recom site (https://recom-power.com/en/rec-n-very-low-noise-filter-for-isolated-dc!sdc-converters-46.html?3) where they devised a filter setup to achieve very low noise on on of their DC-DC converters. The final circuit is as follows:

enter image description here

But I am trying to understand how the common mode chokes L1 and L2 are actually blocking common mode voltages since in either case there is no connection back to the "ground" to which the common mode voltage is referenced to. I thought C3 might be the path but it is before the choke.

Would these still be operating as common mode chokes in this configuration? (ie. L1 blocks common mode voltages on Vin terminal, L2 blocks common mode voltages on Vout terminals)

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    \$\begingroup\$ I would say that the design effectiveness is compromised without caps to ground. The article seems to be missing the point. \$\endgroup\$
    – Andy aka
    Aug 15, 2020 at 15:58

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There's always a connection to GND, even if it's an AC only connection. For example, that C3 of yours is explicitly drawn in. But it's always there, even if there was no discrete capacitor there, by virtue of the fact the transformer inside has two charged surfaces with a gap in between (i.e. the primary and secondary coil).

Same thing with input to GND and output to GND.

Also note that if you connected one of the coils on the common mode choke in reverse it turns into a differential mode choke. For common mode choke, you hook up the two windings such that they oppose each other when current flows from left to right in both of them. FOr differential mode choke, you hook up the two windings so that they oppose each other when current flows left to right in one, and right to left in the other (which is basically just adding an inductor onto one of the lines, except it's spread out amongst two different locations).

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