When you say screen you mean a magnetic screen
I was talking about a layer of copper between primary and secondary. Usually 1.25 turns. Not connected so current flows. Usually connected to P or S ground. It is a shield against electrostatic noise. Not magnetic noise.
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What noise:
Fly back switcher, one primary, one MOSFET, One end of primary at supply (350V), Other end at MOSFET.
Place scope close to the primary of the transformer. One end of the primary is quite. The other end is radiating noise. (sharp edges, 600V in 20nS)
Transformer:
The primary and secondary are two pieces of copper spaced very close together. (definition of a capacitor)
The switching noise cap. couples to the secondary.
If the P to S cap. is 10pF and you add a 10nF capacitor on the PCB from Primary to Secondary (grounds) you then just made a 1000:1 divider that reduces the switching noise on the secondary. (more complicated but....)
cancellation winding:
Start at the cold end of the primary and wind turns. This winding is 180 out of phase with the primary. It will make not-noise. LOL
Noise + (180 Noise) = zero.
For one supply I made a small cancellation winding that way only 1/10 of the primary. Then I added a external capacitor over to the secondary. So with the cancellation winding being 1/10 the external cap. was 10x the internal cap. of the transformer.
There are other ways!
Also the noise is coupling to the core of the transformer, so the fix is a little complicated.
More details if you need it.