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Steve Conner

Thu Mar 15 2012, 07:50AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I don't know if anyone mentioned this already, but putting one half of the centre tapped primary on each core limb is a recipe for MOSFET destruction. The two halves need to be tightly coupled, and when placed on separate limbs they're not coupled at all.

Sulaiman

Thu Mar 15 2012, 01:09PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

For a zvs (etc.) transformer the primary is best wound bifilar for good coupling
ALSO
due to the resonant current in the primary,
the total primary winding copper cross-sectional area should be at least 3x the total secondary winding copper cross-sectional area,
although the secondary winding area with inter-layer insulation may have to be increased if it's several kV.

teravolt

Thu Mar 15 2012, 01:41PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

Steve Conner wrote ...

I don't know if anyone mentioned this already, but putting one half of the centre tapped primary on each core limb is a recipe for MOSFET destruction. The two halves need to be tightly coupled, and when placed on separate limbs they're not coupled at all.

if you put the primary in the center and a secondaries on the outside lims I dont see any problem. the windings will be inside the magnetic loop. however winding everything wraped in the center will giv you the best coupling

klugesmith

Thu Mar 15 2012, 03:02PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

teravolt wrote ...
If you put the primary in the center and a secondaries on the outside lims I dont see any problem. The windings will be inside the magnetic loop. however winding everything wraped in the center will giv you the best coupling

TV, I respectfully reject the first part of your second sentence.

You say "the" magnetic loop, but there are two connected loops. The single-loop simplification only applies if the outer leg windings are absent, or are identical and connected in parallel and in phase.

As steve and forty and I have said, the coupling between one outer leg and the other is not just a bit less than unity. It is around 1/3, and negative! That matters in circuits where the two halves ever have different voltages or currents. Also, the 1/3 is nonlinear because each magnetic loop has nonlinear reluctance, even at ordinary transformer |B| levels well under saturation.

Steve Conner

Thu Mar 15 2012, 03:28PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes! Teravolt, I reject your transformer design and substitute my own.

The two outer limbs are in parallel magnetically. There's nothing forcing the flux in them to be identical, the only constraint is that it has to (vector) sum to the same as the flux in the centre leg.

Forty

Thu Mar 15 2012, 09:26PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

If you need 8 turns for each primary and insist on using that red wire, then perhaps you could put two 4 turn, 2 layer primaries next to each other. That way they'd still be identical (as opposed to winding one over the other, where they would differ quite a bit) and could both fit on the center leg.

klugesmith

Thu Mar 15 2012, 11:28PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Yes, what Forty said. There is value in solving the problem with materials already at hand, and side-by-side is better than one inside the other.
You get the best of both worlds by -interleaving- two windings of 4 turns x 2 layers. That's what Sulaiman and I mean by a "bifilar" winding. Best coupling, and better matching of the short connections between the coils proper and the solder terminals.

teravolt

Fri Mar 16 2012, 03:53AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

Steve Conner wrote ...

Yes! Teravolt, I reject your transformer design and substitute my own.

your stament only suports my argument that you could put a winding on each limb and a primary in the center. if each winding is electicaly seperated you could series aid them or put them in parallel. its no different than putting two flyback cores back to back and the secondaries on the outsides and the same primary wraped around the center. each secondary might get half the flux and its not tradishional but it would work and it may give the opertunity to stuff more windings in that little core.

here is a understanable websight that migiht be usefull

currentkills91 how much voltage do you want to generate? Steve is also corect in that you could wrap them all in the center and it would probly be a more efishent transformer. The greater L ie more secondary turns the more voltage

currentkills91

Mon Mar 19 2012, 09:23PM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265

Wow that question got some attention...I think I'll go with what Forty suggested, it's simple yet ellegent. Haha, The transformer needs to supply at least 250 volts. So if the core can handle the voltage I think I might just go with a 4 turn primary. That would give me 3 volts per turn and a top output voltage of like 600 volts, under load much smaller no doubt. think the core would handle 3 volts per turn? I've never pushed one of those tiny cores with more than 2V/turn so idk?

Thanks to everyone who replied, I learned a few things that will be useful for future projects :)

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