Common mode noise ??

[forever074]

Hello everyone!
I saw many common mode filters in switch mode power supply and I also know that common mode noise
conducted on two lines (L1, L2) in the same direction and amplitude to ground. But I don't know :
1. How common mode noise affect to the device ?. (Because I think that common mode noise V1=V2 so we use differential supply--> differential noise =V1-V2=0V. So common mode noise don't affect to the device)
2. How we test common mode noise from SMPS by osscilloscope.
3. How to design common mode filter.
Please answer my question.
Thank you!

 

[OLIVE2222]

here a start about the measurement

 

[Arouse1973]

Common mode noise is one of the most common forms of EMI. Most people dont actually understand it and it can be very tricky to get rid off. I once asked one of my friends Keith Armstrong one of the world leading experts in EMC what all this common mode noise was about. He told me it took him 10 years to become comfortable with CM noise. So to explain everthing here and its effects would take some time. But basically common mode noise uses stray capacitance and the common rail of your circuit to get about. So just putting a CM choke on the input does not mean problem solved, other wiring will also be effected and cause issues.
Adam

 

[shrtrnd]

It'll be hit and miss. You have to identify the source, and then work to reduce it. Shielding your components while testing will be your best hunting tool.