When doing an EMC test the PCB failed the conducted emission test on AC lines. (it is the external AC/DC converter that is the problem we discovered using dummy load with AC/DC, but I can not change that AC/DC converter because of different reasons)
When we tried to put a common mode choke the design passed the test (even thou we put choke between PCB and AC/DC converter. which feels like the wrong "side" for me since the measure is on AC line... But it worked..I guess some stray capacitive coupling or something..)
but here is my problem: the common mode choke we used (732-1448-ND digikey) for the test that it passed with, did not have enough current rating (we need 5 amp).
So my question: now when I need to implement this improvement how do I pick a choke that will work?
what is the important value to look at? (except for current rating?)
inductance? DC resistance? Attenuation?
Do I need a freewheel diode when I am using a common mode choke together with a fuse?
why are they not using standardized sizes for the chokes so one can make a design and then just swop and try different ones when EMC-testing?!..that they have all different footprint makes it quite hard..
I feel like it a bit of a chicken and the egg situation: I need a PCB to test in EMC lab to see if it is ok. But to make the PCB I need to know what choke to use..and for that I need to test to see what choke to use...
sorry for maybe some stupid questions, but my first time in EMC-test and first time using CM chokes..
it is just that we tried some different common mode chokes in the lab. and the result was quite different for all. some even much worse than with no choke. for me (as a beginner in this field) it feels a bit like black magic =)
conducted emission on AC lines:
old desin used in emc test
new design. tried to implement improvements. what you think?