Reverse Winding Incorrect
I started with the attached TNY3-24V-800mA.uds design.
In this design, the secondary is a single layer, starts on pin 7, and is wound in the same direction as the primary.
The Transformer Construction page, and the Schematic page, agree that pin 7 should connect to the output diode.
I then increased the gauge of the secondary winding (from #25 to #24), so that it no longer fits on a single layer. (See attached TNY3-24V-800mA-reversed.uds design.)
The Transformer Construction page now says that the secondary should start on pin 6, and be wound in the reverse direction (opposite of the primary).
Is this correct? Is this really the same thing as starting on pin7 , and winding in the same direction as the primary?
More significantly, the Transformer Construction page shows that pin 7 should connect to the secondary diode, but the Schematic page shows that pin 6 should connect to the scondary diode. Obviously, one of these is wrong.
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In a case like this, where the secondary takes 1.2 layers, how should it be wound?
a) Fill the first layer completely, and spread the remaining turns (may only be 1 or 2) along the second layer.
or
b) Wind 1/2 of the turns on the first layer - spread evenly, and 1/2 of the turms on the second layer - spread evenly.
When you reverse wind you change the electrical polarity - so yes electrically, its the same as before.
However if you look at the main schematic it shows the dot on the pin 6. This should infact be next to pin 7. We'll fix this in a future revision. But the transformer Construction figures, instructions are accurate.
On a separate note - generally you yould try to keep the secondary to 1 layer whenever possible for better coupling and more effective shielding...
When you use 1.2 layers you are wasting 0.8 lkayers... Better to either use 1 size thicker wire or parallel thinner wires to get to cover more of the 2nd layer.
Your copper losses go down, and also you get better consistency between transformers. Once you do this, then, you can spread out the layers across winding area
Just spoke with the PI Expert team and it looks like you helped us find a bug we didn't know about.
With the shield winding feature turned on, the increase in secondary layers to a value greater than 1 generates the need for a reverse wound secondary (for the shield windings to work correctly). Apparantly, this information isn't getting passed back into the schematic portion of the application correctly. I'm forwarding all this to the PI Expert team so they can fix it in the next PI Expert update.
In the meantime, the information on the transformer diagram is correct. Pin 6 is the *mechanical* start of the secondary but pin 7 remains as the dotted terminal of the secondary winding. The pin values in the schematic are getting reversed in this situation.
-The Traveler