Common mode choke for Offline SMPS

[eem2am]

Hello,
In the attached AC filter we can see that L1 is a common mode choke.
We are trying to reduce the component count of this filter. Do you agree that we can get away with omitting the Cy1 capacitors , but that we must keep the Cy2 capacitors?
In fact, why would the Cy1 capacitors be needed at all?.....i mean, any noise that has escaped out out to earth, would always want to take the least resistance path, and that would definitely be through the Cy2 capacitors….because then the noise would be going back through the common mode choke and thus would not see the high common mode impedance of the common mode choke.
So why are the Cy1 capacitors included? If anything the Cy1 capacitors should just be used as extra capacitors at the same position as the Cy2 capacitors. Do you agree?

 

[Silvio]

The type of filter that is illustrated is a 2 stage filter. If you want to lessen the component count and also the efficiency of the filter opt to do half of that it is illustrated. The cy1 caps are there to filter out the very high frequency noise that is not filtered by the first stage

Hope that helps

 

[eem2am]

Thankyou Silvio.
When you say high frequency may i ask what kind of level you refer to?...do you mean those frequencies that are so high that they go through the parasitic capaictance of the common mode choke?

 

[Silvio]

Hi eem2am Every frequency has its harmonics The filter being a 2 stage in your illustration has 1 common mode choke L1 which is Bifilar wound and also CX1 and CX2. L2 and L3 are probably 2 small ferrite rod chokes with a few turns on them making the second filter the latter being tuned for higher frequencies. The combinations in each filter stage is also combined with CY1 caps to complete the filter. Omitting any part of this filter will make the filter less effective. It all depends what you are using it for and what type of EMI (Electro magnetic interference) suppression are you looking for.

I hope that will help understand more clearly.

Silvio

 

[eem2am]

Thanks Silvio,
I agree, and we wonder what roughly is the frequency above which the Cy1 capacitors are definetely needed?