Add an EMI ground or chassis ground and connect a Y cap, it's no longer a two-lead supply.
Thanks, and if a metal chassis is added to a DCDC, (ie its enclosure or heatsink) then its a very good idea to always connect Y caps from DCDC circuit to that chassis anyway, for noise mitigation issues.
I dont know if there is anyway that would say "i have not connected my metal enclosure to the DCDC inside it and therefore i dont need a common mode filter"
....this wouldnt happen, you would always connect DCDC circuit (usually its ground) to chassis with y caps.
Now if the DCDC's enclosure was entirely all plastic, then it would be a different story. However, the problem is that a DCDC does not "know" whether it has an enclosure or not. If it exists somewhere on the planet earth, then the DCDC "thinks" that the surrounding earth is its enclosure.....and due to the earths enormous mass, it has low effective conductivity, and so even if inside a plastic enclosure, the DCDC needs a common mode choke in its 2 lead input. The Y caps will be provided by the stray Y caps.
Even if we have a 2 lead input DCDC, in a plastic case, and fed from a battery......neither battery or DCDC "knows" if it has a conductive enclosure or not.....they both think that the surrounding earth is their conductive enclosure, and so again, we have stray capacitance to the earth, (our "third connection" and so we need the common mode filter.)
Here we see a common mode filter being designed for a DCDC
(3:10 onwards)