What does FG mean in this part? (VTX-214-030-115)

[riccardo]

Hi, I am considering using this part in a project...
**broken link removed**

One pin is labelled as FG but not described in the datasheet. Is it supposed to be the mains earth connection? What does FG stand for?

 

[Nigel Goodwin]

A quick google shows it as 'frame ground'

 

[riccardo]

Thanks, I also saw a definition as it could be Floating Ground.

I suppose it makes little difference but should I connect this to mains earth for good practice?

 

[Nigel Goodwin]

I presume that's where it's supposed to be connected, although I doubt it will make any difference (and isn't shown in the suggested connections).

Are you earthing the output?.

 

[riccardo]

No, I will have the DC- common with the negative output of a bridge rectifier on the same mains input to serve as the circuit GND.

 

[JimB]

FG is generally Frame Ground in these types of PSU.
It is a bit of a stretch to think of "frame", ie some metalwork, on a potted power supply.

I am guessing that an Y capacitors in the input mains filter will be connected to FG, also the inter-winding screen (if there is one) in the transformer.

JimB

 

[Tony Stewart]

Correct, the Y-rated caps are responsible for EMI C-L-C or Pi filter bypass currents to suppress noise in each direction, by lowering impedance to FG but but too big a cap that it exceeds the IEC, UL safety leakage specs.

This one is rated at 47-63Hz 0.3mA.
As I recall the limit per PSU is 0.5mA and 2.5mA per commercial unit, which ay have multiple PSU's.

i should warn you that the downside of earthing secondary DC in SMPS is increased susceptibility damage in HIPOT tests. Factories will test unground secondary and pass and I discovered % catastropic failure in one source after they transferred production from SD to mexico . The causes were obvious to me with greater HiPOT stress before switching transformer, since the isolation in the transformer is now shunted. It took me a 1 lot reject to get the QA 's attention with Root Causes illustrated. This a product I designed and being assembled with 1k units/mo for Avaya/lucent.

 

[Tony Stewart]

This means, if this is a commercial design, ensure source changes their test procedure to ground secondary during outgoing HIPOT test.

 

[riccardo]

Thanks everyone. Great info.