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Winding Switch Mode Power Supply Transformers.

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fireant007

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Hi all,

I have got hold of a program from st.com which will let you roll your own SMPS.

Is does nothing except give you a schematic and I am fully aware that there are many many hurdles to over come when laying out and optimising these circuits.

The program has recommended a transformer.

The program suggests using a EER40 core and winding the following.

39 Primary windings of 3 paralleled Litz 27x0.079mm wire.

There is of course a secondary and in this case an auxiliary.

When they say 38 windings in parallel .. do they mean running 3 lengths of wire round the bobbin at the same time ? And join them all at each end.
Not parallel in the electrical sense of transformers in parallel or series ?

The program does not give you much. So I am going to assume that the windings are just wound over the top of each other with sufficient Mylar tape between windings to insulate and eliminate creepage to the required amount ?
 
Yes, I Would Assume it is 3 of these wires, Wound at the Same time and soldered at the ends to create one Lager Litz Wire.
A Link to that "ST.com Article", might be more Helpful in helping you.
 
run 3 lengths of wire round the bobbin at the same time. And join them all at each end.
Here is a picture, very different, but easy to see. Here 5 windings. This is because 5 small wires are easy to bend while one very large wire is hard to bend around.
toroidal_choke_coil_bifilar_strong_style_color_b82220_winding_strong.jpg

39 Primary windings of 3 paralleled Litz 27x0.079mm wire
I can tell that is is high current. High frequency.
You are using 27 x 3 small wires not one very large wire for two reasons.
1) Mechanically easy
2) At high frequency electrons travel on the outside of wires. The inside of the wire is not used. So many small wires have a large amount of outside and little inside.
 
run 3 lengths of wire round the bobbin at the same time. And join them all at each end.
Here is a picture, very different, but easy to see. Here 5 windings. This is because 5 small wires are easy to bend while one very large wire is hard to bend around.
toroidal_choke_coil_bifilar_strong_style_color_b82220_winding_strong.jpg


I can tell that is is high current. High frequency.
You are using 27 x 3 small wires not one very large wire for two reasons.
1) Mechanically easy
2) At high frequency electrons travel on the outside of wires. The inside of the wire is not used. So many small wires have a large amount of outside and little inside.


Beautiful.
I figured as much but wanted to check with you experts :)
Yes the frequency spec is from 30 to 120kHz... not sure exactly so it's high yes.

Thanks guys
 
One last question if I may.

This bobbin is one big fat one square sided one with no "separators" ....

I am taking primary and secondary separation seriously and creepage as per all the pdf's I have read.

So it's ok to just wind the primary on the bottom and separate the layers with Mylar tape ? Seems to be the way to go. (not too much tape of course).

Apparently
the "fill factor of the windings is
Primary 38%
Secondary 59%
Aux 2%

So a pretty full bobbin.

I can only assume Mylar is the way to go here.
 
I use only one layer of tape between primary layers.
I use 3 layers of tape between P and S.
The real trick is to keep he P & S away form each other. It is possible for a S. wire to fall off the edge and down onto the P. In production there are ways to stop that. If you are building only one then just watch out for that. Maybe put some tape on the corners of the bobbin to keep the S sires where they belong.
 
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