18V@4A secondary transformer with less than 100pF between secondary and mains ground.

[Hans Polak]

For a delicate application I need less than 100pF between secondary and mains ground.
I would be glad if someone could steer me in the right direction how to achieve this.

Thx in advance
Hans

 

[euro21]

IMO primary-secondary capacitance is much more important.
Most of noise from the mains spreads through it.

Two bobbin/splitted bobbin transformers has enough low p-s capacitance.

Shielding (tape or one layer wire between primary and secondary) also gives a good result.

 

[jan.didden]

Hans, I have a similar requirement for my eDD NextGen design, but I also need 5kV isolation between prim and sec.
I spoke with Menno van der Veen and it's a tough target. It can easily be more than a nano-Farad.
You could talk to him.
At any rate, it will come with very much reduced efficiency.

Jan

 

[Hans Polak]

IMO primary-secondary capacitance is much more important.
Most of noise from the mains spreads through it.

Two bobbin/splitted bobbin transformers has enough low p-s capacitance.

Shielding (tape or one layer wire between primary and secondary) also gives a good result.

Yes, that’s the capacity I’m referring to.
Do you mean two Transformers one after the other, both with EI cores and each having two separate bobbins ?

Hans

 

[simon7000]

https://catalog.triadmagnetics.com/Asset/VPM18-5560.pdf

 

[Hans Polak]

Thx, this one should be sufficient with ca. 100pF.

Hans

 

[jan.didden]

Hans, where did you see the 100pF spec?

Jan

 

[euro21]

Yes, that’s the capacity I’m referring to.
Do you mean two Transformers one after the other, both with EI cores and each having two separate bobbins ?

Hans

No.
I thought such transformer, when primary part and secondary part are separated:
1.) separated bobbins for pri and sec on the far legs of EI transformer;
2.) or splitted bobbin, when primary and secondary are in separated bobbin part.

I prefer the latter.
Sample:
https://www.reichelt.de/de/en/mains...86a52bb061f180924cf94a811132&LANGUAGE=EN&&r=1

 

[Hans Polak]

Jan, I didn’t but with 10uA in 1K with 264Vac@60Hz this calculates into ca. 100pF.

Hans

 

[Hans Polak]

No.
I thought such transformer, when primary part and secondary part are separated:
1.) separated bobbins for pri and sec on the far legs of EI transformer;
2.) or splitted bobbin, when primary and secondary are in separated bobbin part.

I prefer the latter.
Sample:
https://www.reichelt.de/de/en/mains...86a52bb061f180924cf94a811132&LANGUAGE=EN&&r=1

Fantastic, thx a lot.
I will order this guy and see what it does.

Hans

 

[JensH]

Could an R-core transformer be used?

 

[Hans Polak]

I really have no idea.

Hans

 

[JensH]

The R-core may not help anyway. At least it will depend on how the transformer is made.
I just made a quick measurement on an R-core transformer I have. It measured something like 110pF between primary and secondaries. It was a different transformer with 2 x 15V and 2 x 9V secondaries.
But I think that the primary is split between the two bobbins and the same for the secondaries, probably to get a good coupling/regulation. For lowest capacitance the primary should be on one bobbin and the secondary on the other one. This will probably sacrifice some of the coupling, but I think the capacitance should be lower.