winding the primary

[m-tech]

I am winding the transformer for my smps, and need a little help.
I know everything about splitting the primary, but need to know something: I wound 25 turns in first layer, then secondary. The problem is that I need 60 turns for primary, so in the last layer i need 35 turns. Point is that primary isn't halved in two equal parts. Is this OK ?
SMPS is low power ( ~120W ), half-bridge topology, core ETD39.
Thanks.

[m-tech]

Somebody

[N-Channel]

Why the imbalance? Was 25T all that could fit in one layer? But, since you ask, I'm not aware of any adverse operation with uneven windings for the primary.

[18thell]

In order to wind a balance winding, bifiller, trifiller method may employed. bifiller means use two same guage copper wire parallel the two wires and start to wind at the starting point, then after the required turns, then ending. If two different colour of copper used, that would be more easily to define coil 1, and coil 2.

Trifiller use three wires, same way as above.

[luka]

Hi

Every time I saw split primary was like so: 50% of turns, all sec, 50% of pri. This can be one continuous wire or wires (bifilar,...), or split into two, like where first layer stops, is soldered to second half outside of core (may be on its pins). So what this means is that length of outer 50% of winding will be longer, since winnding is more away from core...

[N-Channel]

Yeah, but I'm wondering why he's going with 25T (inside half of primary) + 35T (outside half of primary). Is this because 25T was all he could fit in one layer at a time?

m-tech, I am curious: Can you maybe kick the switching frequency up a bit to accomodate a 50T primary? This could solve your winding area problem. Just a thought.

Luka- You are right on seeing the "center tap", if you will, of the primary on one of the bobbin's pins- this is common, mainly in AT & ATX PSUs.

This gives me an idea - Trying a center-tap push-pull drive for the primary, using this pinned-out center-tap. Let's see..... for a typical primary of 38T (19T + 19T), at ~160V across the primary, this would be ~+80V applied to each half of the "ct" primary. Hmmm....... Maybe an idea for someone DIYing a DC-DC SMPS operating off ~65-90VDC. HMMM......

[m-tech]

Originally posted by N-Channel
Yeah, but I'm wondering why he's going with 25T (inside half of primary) + 35T (outside half of primary). Is this because 25T was all he could fit in one layer at a time?

Because I made mistake in calculations, and don't want to rewind it again. And, yes, only 25 turns fits in one layer.

Originally posted by N-Channel
...This could solve your winding area problem...

I don't have problem with winding area.

[luka]

Hi

Then just remove 10T any you should be still ok if this is regulated design

[AndrewT]

How high a turn to turn voltage can enamelled wire withstand?

Single layer gives no real issue. Multiple layers are a completely different ballgame.

What if one enamel layer develops a crack or is scratched, can the adjacent enamel provide sufficient insulation?

[N-Channel]

Originally posted by m-tech


Because I made mistake in calculations, and don't want to rewind it again. And, yes, only 25 turns fits in one layer.

I don't have problem with winding area.

m-tech- I understand now...... is it possible to just go with 50T (25T + 25T)?

Originally posted by luka
Hi

Then just remove 10T any you should be still ok if this is regulated design

Luka- Yes, if the core material he chose will allow it. Usually, shouldn't be a problem.

Originally posted by AndrewT
How high a turn to turn voltage can enamelled wire withstand?

Single layer gives no real issue. Multiple layers are a completely different ballgame.

What if one enamel layer develops a crack or is scratched, can the adjacent enamel provide sufficient insulation?

Andrew- Most wire should have the dielectric strength of its enamel coating called out in the specs.




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