common mode filter design

[mail2vasu2004]

Hi.

I am designing a common mode filter for my battery charger that is rated at 270W and works in flyback configuration.

I read few articles to understand common mode noise and as my understanding goes, the cause for this noise is the stray capacitances developed between primary and secondary of the transformer improper winding. This capacitance tries to carry the di/dt current flowing through the primary due to the switching of the chopper/switch to secondary. This current needs a return path and when such a path is provided by shorting the primary and secondary common mode voltage is devloped which can venture into the mains line.

My questions:

1) Is my understanding of the topic correct?
2) How is this noise present in both the lines?
3) Is there any similar explanation to diffrentail noise and how is it different from common mode noise?
4) Is there a way to know the amount of noise that is being generated in the circuit?
5) On what basis am i suppose to decide on the order of filter to be used( I, II or III)?
6) Is there any book on notes on this topic that i must read?


Cheers

Vasu

[darkfenriz]

dv/dt is more important when it comes about capacitance.
A first order filter made of ferrite coomon mode choke is uually enough.
A good solution is also to wind an extra screening winding between primary and secondary and ground it to primary side ground.

[mail2vasu2004]

Thank you for the reply.

What i wanted to tell was that di/dt current that is flowing in the primary leaks to the secondary though the parasitic capacitances.

Is there any reason to support the first order filter selection?

Cheers
Vasu