Y cap in offline Flyback SMPS

[treez]

Hello,
Given that an offline , isolated flyback only has live and neutral supply, then would you put the Y cap from primary ground to secondary ground, or from primary high voltage (DC Bus) to secondary ground?

 

[ahsan_i_h]

See the thread titled " https://www.edaboard.com/threads/230724/ "

 

[treez]

thanks but it does not say whether the y cap should best connect to the primary ground, or the primary DC bus?.
I aays connect it pri_gnd to sec_gnd via the y cap, but a colleague has done it with pri_dc_bus to sec_gnd. Shouldn't make much difference but I wondered if I overlooked something.

 

[FvM]

I believe the linked thread has a different focus.

I see both connection variants. I have e.g. a small 5V/2A mdoule at hand that connects 2.2n between 2 flyback pins (DC+ and secondary ground), apparently to short influenced currents as direct as possible. But connecting the Y capacitor to the mains input is also common.

 

[mtwieg]

IMO if you don't have an earth ground, then you can't even call it a Y cap....

But anyways, since only high frequency switching noise is going to matter to that capacitor, the difference between the DC input bus terminals shouldn't be significant. Connecting it back to neutral is probably also acceptable, but I wonder if putting the bridge rectifier in its path would impede that path during certain parts of the AC line cycle.

 

[FvM]

IMO if you don't have an earth ground, then you can't even call it a Y cap....

Functionally it's not an usual Y cap, but a capacitor with Y cap specification (safe insulation) is required in this place.