observerms

why: is there a Y capacitor between isolated secondary ground to AC input common ?

why is there  a Y capacitor between isolated secondary  ground to the AC input common ?

I realize that SMPS will generate High frequency ripple "noise"  at the DC output terminals

In order to maintain TOTAL isolation of the DC output terminals , the Y capacitor is eliminated :

Why can't this noise be attenuated by output circuit filters, including ferriter beads ?  

Ceramic capcitors, and mylar capacitors     of the output filter are connected to a well constructed

high frequency output ground plane.  

Thanks for whatever suggestions you have !

observerms

Your nameobserverms

observerms

why: is there a Y capacitor between isolated secondary ground to AC input common ?

why is there  a Y capacitor between isolated secondary  ground to the AC input common ?

I realize that SMPS will generate High frequency ripple "noise"  at the DC output terminals

In order to maintain TOTAL isolation of the DC output terminals , the Y capacitor is eliminated :

Why can't this noise be attenuated by output circuit filters, including ferriter beads ?  

Ceramic capcitors, and mylar capacitors     of the output filter are connected to a well constructed

high frequency output ground plane.  

Thanks for whatever suggestions you have !

observerms

Your nameobserverms

observerms

why: is there a Y capacitor between isolated secondary ground to AC input common ?

why is there  a Y capacitor between isolated secondary  ground to the AC input common ?

I realize that SMPS will generate High frequency ripple "noise"  at the DC output terminals

In order to maintain TOTAL isolation of the DC output terminals , the Y capacitor is eliminated :

Why can't this noise be attenuated by output circuit filters, including ferriter beads ?  

Ceramic capcitors, and mylar capacitors     of the output filter are connected to a well constructed

high frequency output ground plane.  

Thanks for whatever suggestions you have !

observerms

Your nameobserverms

Hi observerms,

Yes you are correct that the ideal isolated system shouldn't have a Y-capacitor across the isolation.  However, due to applications specially for chargers with limited space and cost consideration, Y cap is the best solution for common mode noise filter but it is limited only to 100pF capacitance for safety leakage current limit.  Previously it was 220pF but most of the customers now only allows up to 100pF.

Due to symmetrical imbalances, imperfect transformer construction and stray capacitances, the high freq output filters (ceramic caps, ferrite beads) is usually not enough to suppress common mode noise.  The Y cap is providing the shortest return path so that it won't go out of the AC line.  Hopefully this gives you some understanding.  Thank you.