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.
We all agree that the ti.com app note is correct from a totally idealised and non_real_world situation.However, the people reading it are not professors, and need to be told that its an idealisation which never occurs in the real world...i have lost count of the number of companys where you get told
"we dont need a common mode filter or Y caps because we only have a 2_lead_input DCDC in a plastic enclosure."There is not a conducted EMC test house in the world that wont also test a 2_lead_input DCDC for common mode aswell as diff mode emissions.
I dont know if there is anybody 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
virtually always connect a DCDC SMPS 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 2_lead_input 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 nor 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)
Also, we could just decide to take our 2_lead_input DCDC to a conducted EMC test house where they only
will test it for Differential mode emissions. -Though as you know, we cannot do that. All Conducted EMC test
houses will have a metal ground plane from the DUT to the LISN and so we have our "third conductor", and again
we have a common mode EMC problem. (even without the ground plane, we still have the surrounding earth) -And indeed, the test house will test for these common mode conducted emissions.
So yes, i understand that what ti.com is saying is from an "idealised" point of view...but its not helpful without
them actually declaring that, since some people will try and solve failed conducted tests with only diff mode
filtration, because "they saw it in a ti.com app note"
I have actually been in a co that was doing a LED light for Boeing (they hoped Boeing would buy it, i doubt they ever did)....and they were saying that they didnt
need a common mode filter because they only had a 2_lead_input.
At 3:48 on this video...
..we see that a 2_lead_input DCDC has big common mode problem.
I understand that we all appreciate and agree with this.
And as such, the problem with the ti.com article (and others its not just ti.com), is that referring to totally unrealistic ideal conditions which never occur, (without explaining the real world situation that actually dominates) is very very unhelpful to many of the people who
will end up reading that app note. (app notes dont tend to get read by top consultant level designers and professors)
I also know of a company that designed SMPS for BAE systems......the gaffer of that company told me that a "2_lead_input" SMPS that he had had designed had "no common mode noise"
Obviously as we know, he had been reading stuff like that ti.com app note.
I gaurantee that their are huge numbers of EMC test houses all over the world, giving failure reports to engineers who think they "only have a diff mode problem because theirs is a 2_lead_input DCDC in a plastic case".
At 9:16 of the below we see the 2_lead_input DCDC having its common mode path made by the EMC ground plane in the test lab.....
But typically, most companies always think they dont have a common mode problem, and even if they do, they regard that it will be insignificant, which is deffo not the case.
The problem with Common mode chokes... specially with unisolated DCDC's. is that they put an impedance into the ground wire, when that is not wanted, for whatever reason.
Though another point, is how will any 2_lead_input SMPS with no common mode choke, and no impedance in the ground plane.....how will it ever pass conducted immunity testing?....it won't.