Hello guys, i am designing a pwm controlled led lamp for my girlfriend and the last step for me is to design a CM choke for the ac line to supress the emi emission. I read a lot about it while sailing the web and i have a few questions to ask.


This is the image i am using to get my design for the CM choke and a simple DM choke. And i am aware of how to wind up the toroid and the ferrites i will be using to make the inductors.

My questions are the following:

1) To supress EMI does it matter the cutoff frequency of the Capacitor + Inductor of each wire in the CM choke part or even the DM choke part? It needs to be like the frequency of the AC line? I am a little confused about this.

2) I do not intend to make those double caps with a ground between them, will this make a big difference?

3) What would be a typical value for an CM mode choke for a 220 or 120 VAC line? With a 50hz or 60hz frequency ?

4) Is there an optimal material for the caps ? Like ceramic.. film...

5) Best wire thickness to achieve the typical value ?

I thinks my doubts are just these. Thanks in advance. Sorry for any theorical mistakes i made.

 

1) The filter cutoff frequency is arbitrarily low to suppress high frequency noise. That frequency is unrelated to the power line frequency.

2) Yes. Those capacitors are there to help suppress the CM noise.

3) Typical values are a few mH up to perhaps a 100mH. (Here's a discussion on CM chokes that may help you.)

4) Either ceramic or film type caps should work equally well so long as they have a sufficient voltage rating above the peak line voltage.

5) The minimum wire thickness is determined by the current the choke has to carry.

 

Understood, even if i rise the inductance on the CM choke to compensate for not having those caps?

Edit: i could not acess the link you put there

 

Understood, even if i rise the inductance on the CM choke to compensate for not having those caps?

Edit: i could not acess the link you put there

Raising the inductance is more expensive then adding the caps. And it still may not filter as well.
What's the problem with adding the caps?

Try this link.

 

The energy in my house doesnt have a third connection to earth, also the ferrite core i have here is a toroidal with 2400nH per turn AL

 

You may already know this, but for the capacitors that go across the power line and the capacitors between each line and chassis you should use capacitors marked to show that they are specifically designed for these applications and they should also bear the markings of the relevant safety agency. The capacitors that go across the line are marked as "X" capacitors and those from line-to-chassis are marked as "Y" capacitors. Each type is designed, manufactured, and tested for safety concerns associated with its particular place in the circuit.

About the common mode choke -it is difficult to know what type of choke (inductance, resistance, core type, etc.) will work best for a particular application without a lot of experience on the particular type of power supply under consideration.

If you are interested in doing this "properly" you will need some specialized test equipment, but you can probably come very close with a scope that supports an FFT function and an impedance stabilization network. The frequencies, allowable levels and the specification of the impedance stabilization network (usually some passive components) is individual EMI standards.

For home projects, I just try to make things "quiet" rather than shooting for a particular specification. If interference crops up, it is attended to.

 

Yes i'm aware that this isn't a "properly" CM choke or emi filter, but i think it's better than having nothing to supress this kind of EMI, right ?

 

Agreed!

 

You seen like a very wise guy, when i am done with the schematic i'll post so you could take a look, then i will ask a few things about the positioning of the components...