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Winding a Flyback SMPS for best EMC...Dots of coils at same end of former

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treez

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Hi,
As you know, a flyback SMPS transformer must be wound with dotted ends at the same end of the bobbin for every coil. This is for EMC reasons.
But if you get the transformer manufactured outside, how do you best check/ensure that they did actually put dotted ends at same end?

You can check which pin is "dot" and which is "no dot"...but you cant see "inside"
 

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It might not be possible to determine the winding direction of a single part, but it should be easy to determine the difference between two otherwise identical parts with windings in opposite directions (if the winding direction actually impacts performance, that is). I'm guessing the SRF of each winding would be the best indicator.

For this design, telling them to always start winding from the same end of the bobbin, and always spin the bobbin the same direction, should be sufficient. Though you show a few layers having turns pushed towards the ends of the bobbin, not sure if they can actually do that without splitting the layer into two separate winding operations.
 
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Thanks,
I also hope and pray its ok to reality check my flyback smps shield wind polarity method here?....

When in a flyback smps transformer you are working out where the dot of the shield is, you think of the primary conducting current, lets say with current flowing into primary “dot”. At this time, if the shield could flow current, then it would flow it in a direction which would put the field in the opposite way in the core to the primary. And when current flows into primary “dot”, then it would flow out of shield “dot”…..this your way too?
(obviously its hypothetic as shield cant flow current)
 

A contractor has done a offline flyback transformer where secondary dot is at the same end of the bobbin as primary no-dot and bias no-dot.
Surely there could be no reason for this? It just makes dv/dt and EMC worse?
Surely this must be a mistake?
 

on the build sheet - the start pins for the windings dictates the dot placement - given the same rotation of the bobbin.

As you have shield wdgs, it would be prudent to have the connected part of the shield wdg close to the noisy end of the pri wdg ...
--- Updated ---

you can further connect the core to a static pri side potential - and add a shield wdg around the finished pri
 
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As you have shield wdgs, it would be prudent to have the connected part of the shield wdg close to the noisy end of the pri wdg ...
Thanks, the top post is an exerpt from a power integrations PI Exeprt design for an offline flyback. As can be seen, the connected part of the shield winding is not at the same end of the bobbin of the noisy part of the primary winding.

All power integrations flybacks designs show all dotted ends of all coils at the same end of the bobbin.
 

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