Switch mode power supply secondaries queries

If you want 2 secondaries of say + and - 20 volts with common zero. Assuming you have the right primary and drive requirement, number of turns and gauge of wire for the secondary voltages and loads , what arrangements are permissible? Obviously a continuous run of insulated magnet wire, tapped in the centre , but what about a doubled up run, bifilar,in the same sleeving, with commoned at one end joined together for zero volts and the other end split off, one to one polarity of rectifier and one to the other. What about a bifilar run again but one end of each commoned,exterior, but the "plus" line coming from one end of one wire and the "negative" line from the other end of the other wire.

The first is out as too much bulk of sleeving and retain proper primary/secondary isolation. I'm aware that for unipolar secondaries say

0,+5V,+10V then it is permissible to run more wires/thicker wire between 0 and 5 than between 5 and 10 to reduce sleeving bulk.

Is there any practical difference, at 40 to 50KHz operation, between say 10 turns of secondary wire with cross-section area A and 10 turns of quadrupled thinner wire each of area A/4. ?

Reply to
N Cook
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Bifilar as a twisted pair works well to make the two secondaries have the same voltage. A twisted quad with pairs hooked in parallel works a little better.

[....]

A seven wire, wire rope works even better. The next stop is Litz wire. Both lower the losses. At the harmonics caused by the sharp edges, you have quite high frequencies. If you have very fast switching in the semiconductors the circuit has lower losses in total but a bigger loss happens in the transformer. Going to multiple thinner wire helps to lower this.

Reply to
MooseFET

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Where does this "sleeving bulk" come from? Transformers are normally wound with enamelled wire, and the enamel is a few microns thick (depending on grade)

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There is a practical difference between 10 turns of secondary wire with a cross-sectional area of A and ten turns of quadifilar wire where each wire has a cross-sectional area of A/4 for high frequency operation.

It arises because of the frequency dependent "skin effect" which forces the current flow towards the surface of the wire. Check out Litz wire.

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The Wikipedia article does propagate the myth that it is worth silver- plating copper wires to minimise skin resistance - bulk silver does have a slighty lower resistance than bulk copper (16 versus 1.7 nano- ohms metre) but electroplated silver doesn't - you'd have to melt the electoplated silver (perhaps with a pulsed laser) before you saw any improvement.

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

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Reply to
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So current going in opposite directions in 2 closely spaced secondary wires makes no difference, compared to separated wires otherwise generating the same voltages/current ?

I would have thought there was some sort of mutual inductance effect.

Reply to
N Cook

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The current in the primary goes clockwise, and in ALL the secondaries the current goes counterclockwise. The 'split supply' secondaries are different only in that end A of the (+) winding is grounded, while end B spits out current, while end B of the (-) winding is grounded and end A sucks current in.

Both suck current at end A, both spit it out at end B. The current goes the same way.

Reply to
whit3rd

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The current in the primary goes clockwise, and in ALL the secondaries the current goes counterclockwise. The 'split supply' secondaries are different only in that end A of the (+) winding is grounded, while end B spits out current, while end B of the (-) winding is grounded and end A sucks current in.

Both suck current at end A, both spit it out at end B. The current goes the same way.

Confirmed in a practical test today. The other configuration of both end 'A' grounded produces a correct rectified voltage in one and approximately zero in the other.

Could I recommend the use of interlayer material on the primaries. I'm rebuilding a small SMPS transformer where there was a gap in the winding (as originally wound or developed over time - don't know) about 1/10 way in the first of 3 layers. No interlayer and about 9/10 along the next return layer had dropped into the first layer , local hot spot and shorting at that point, leaving about 1/3 of intended turns functional, but of course knocking out the driver.

Rebuilding on a 50 KHz SMPS type toroid core rather than the original as it is easier , hence the limitations of secondaries isolation sleeving rather than primary / secondary divider isolation wrap.

Reply to
N Cook

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