SMPS flyback Y cap placement

[eem2am]

Hello, i have two SMPS schematics with the Y cap in different places. do yuo know why the Y cap is in different place.

1. here it is from "no_dot" to "no_dot":-

2. here it is from "no_dot" to bus_negative:-

do you know what is the difference caused by different Y cap placement?

 

[ace1903]

Hi,
I am not sure about this but here is my opinion:
In most cases placement is chosen in laboratory during EMI tests. Goal is to achieve lowest EMI emissions with selected capacitor. Sometimes placement is determined by components layout and line of isolation barrier.

 

[max0412]

They are usually placed right next to the transformer,but not always. If you don't have any emi problems don't bother, Y caps are an unnecessary expense.

 

[VVV]

Basically, the two connections are equivalent in AC. That is, if the input cap is really a short-circuit at the frequencies of interest. In many cases it isn't, so the two approaches can yield different results. I prefer the input ground to output ground location. But it has to be validated during EMI testing anyway.