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Why has our Flyback SMPS transformer got so many strangely connected shield windings?

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Flyback

Well-Known Member
Hello,
Please could you advise why there are two strangely connected shield windings in our Offline Flyback transformer?
There are two shield windings in the transformer as shown in the transformer diagram attached to this post.
This Flyback SMPS is 67W and VAC input is 100VAC to 265VAC.
The output charges up to 340V, so to make the voltage reflected to the primary less, the designer used three separate secondaries (N3, N4 and N5) and “stacked” their outputs. (as shown in the schematic attached).
It actually appears that the two shield windings are connected in a manner which we would not want. I thought that shield windings were just supposed to connect to a single place, eg primary or secondary ground or power?
What is the purpose of doing the two shield windings as in the attached transformer diagram?

(LTspice simulation also attached though not needed)
 

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ronsimpson

Well-Known Member
Most Helpful Member
stacked windings are common. (or at least to me)

I use shields in a transformer for several reasons. One is to prove the Primary and Secondary can not get together.
Some times there is a shield over the primary that connects back to the power line ground. AND There can be a secondary shield that connects to the output ground. This way primary AC noise gets shorted back to the primary side. The secondary shield keep secondary noise from getting to the primary side and back on the power line.
 

Flyback

Well-Known Member
Thanks, but the two “shield” windings shown in the transformer diagram of the top post are not connected in the normal way for shield windings. It looks to me that this transformer is a transformer that got used previously for a 5 output secondary, and they have just thrown it into our design “willy nilly”.
Do you agree?
This Flyback SMPS was designed in China for us.
 
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