Selection Process for Common Mode Transformers for smps

If you know the amplitude of the lowest relevant harmonic currents being pumped into the environment, and the maximum common mode capacitance permitted (safety leakage current implications) , you can establish a minimum L value required to meet intended interference levels (by an acceptable margin) in a 50 ohm LISN receiver.

This L value will be more difficult to achieve as the power requirement of the load (and the line current) increases. Heavier gauge wires take up more room, larger cores are more expensive and higher frequency performance will be more easily compromised. At higher peak line currents, the high permeability core may saturate on it's leakage flux. At higher temperatures the core saturates at lower leakage flux levels.

At some stage it may become more economical and effective to use two inductors in a two-stage filter.

Physical iterations involving realistic physical representation of the final product, at the earliest stages possible, are highly recommended. Models don't necessarily suck, they are just first-order approximations resulting from an inevitably inaccurate schematic.

RL

Thanks... I do some googling on LISN and max CM C.. Also I'll think about the all magnetics parameters when selecting a CM xformer. D from BC

you can also get some CM cores that are designed to have high DM inductance as well - I used one to fix a customers design that failed EMI.

If you dont know how much CM L you need, fit the biggest one you can, and hope for the best :)

Ideally you calculate the CM and DM noise at the design stage, and select an appropriate part (as Legg said), then (using a DMRN or similar) measure the CM noise at the prototype stage, and adjust to suit. If you include SRF in your calculations the results can be quite reasonable - until, of course, layout parasitics rear their ugly heads!

I have a couple of MeanWell SMPS here that have two DM/CM chokes. And a layout designed to maximise EMI....

Cheers Terry

Yes!! :) If I stay under the OEM CM xformer Irating and then I can go for max L until I can't tolerate the physical size..

I was fishing for 2 solutions... The do it fast solution: Use biggest L as real estate allows, cost not important, gamble on noise rejection. The do it right solution: Calculate, measure, select a part and test.

Thanks.. I'll look out for the CM/DM xformers as well..

D from BC