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SLR secondary overheats!!! Need an expert advice!

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M3mristor

Thu Nov 06 2008, 01:02PM

Registered Member #1738 Joined: Fri Oct 03 2008, 08:48AM
Location:
Posts: 5

for sure i am not an expert on transformer design.
but as you say the problem disappears when the secondary is moved to the other leg of the core, this must have changed the source of the problem.
the chancing parameters are leakage inductance, capacitive coupling between windings (and something else?)

do you use this topology to charge a capacitor?

Steve Conner

Thu Nov 06 2008, 01:59PM

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

I think what other posters are trying to get at here is the effect of self-capacitance in the secondary. You can think of it as a capacitor shunted across the secondary coil, so it could be carrying an AC current that's large compared to the output current.

If this were the case, the secondary might actually run cooler with its terminals shorted, since this would de-Q any resonance.

Moving the primary to the other core limb may well be a red herring. If the circuit no longer gives the output you want after doing this, then you're not comparing like with like. The secondary could be running cooler just because it's processing hardly any power.

PS, do you have an airgap in the core under the secondary? Apparently this is to be avoided because the windings near the gap get very hot.

baleworker

Thu Nov 06 2008, 02:54PM

Registered Member #1596 Joined: Fri Jul 18 2008, 08:43PM
Location: USA
Posts: 65

Steve McConner wrote ...

Moving the primary to the other core limb may well be a red herring. If the circuit no longer gives the output you want after doing this, then you're not comparing like with like. The secondary could be running cooler just because it's processing hardly any power.

I did finally manage to run it at the same output current by reducing number of primary turns and so leakage inductance. Though the core is very close to its limits now.

Steve McConner wrote ...

PS, do you have an airgap in the core under the secondary? Apparently this is to be avoided because the windings near the gap get very hot.

No, I dont have any airgap, I've already learnt its effects the hard way - molten bobbin ;)

Now I start believing that its really an inter-winding capacitance what causes the heating. I just cant think of anything else.
Is anything I can do to reduce it? I really wanna have more room to play, and not push the core to like 400mT or so.
Though Vsec is high only if the secondary is open. I use transfarmer to draw arcs, so after the 1st spark voltage and Vsec drop considerably.

Dr. Dark Current

Thu Nov 06 2008, 04:14PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Steve McConner wrote ...

PS, do you have an airgap in the core under the secondary? Apparently this is to be avoided because the windings near the gap get very hot.

Why?
Most flyback SMPS transformers I've disassembled have a large (>1mm) gap right under the winding.

Marko

Thu Nov 06 2008, 05:02PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Steve McConner wrote ...

PS, do you have an airgap in the core under the secondary? Apparently this is to be avoided because the windings near the gap get very hot.

Why would an airgap cause windings in it's vicinity to get hot? Did this happen on SLR supplies before?

I would guess that could happen only if windings are wide flat foil (like the primary, not the secondary) and the gap is rather large.

Marko

baleworker

Thu Nov 06 2008, 05:42PM

Registered Member #1596 Joined: Fri Jul 18 2008, 08:43PM
Location: USA
Posts: 65

Marko wrote ...

Why would an airgap cause windings in it's vicinity to get hot? Did this happen on SLR supplies before?

I never used airgap on the transformer itself, though, I tried using it as a mean of adjusting an inductance of a serial inductor, yeah i had at least 6mm of an air gap and it vaporized a hi temp insulation off the magnet wire!!!

jonny5

Tue Nov 11 2008, 08:34AM

Registered Member #1807 Joined: Tue Nov 11 2008, 07:36AM
Location: San Luis Obispo, California
Posts: 19

Me thinks maybe proximity effects are coming into play? With a gapped core (for instance), the fields around the gap may be quite strong! This causes a significant change in the current distribution in the windings. I just had a primary winding melt into a bobbin two nights ago (big gap in the E-core). If I understand correctly, your primary is Litz and secondary (wound directly over the primary) is 26G magnet wire? Litz is pretty wicked stuff! But the current distribution in the secondary windings may be severely squished, leading to a surprisingly high AC resistance! I forgot how much secondary current you were running...I wonder if the Intusoft magnetics software (free!) could shed some light on this. I may be completely wrong on this, but at 100KHz, I wonder how much capacitive reactance there can be. It sounds like when you rearranged the windings (pri and sec no longer in close proximity) some of this problem went away. Seems like the self-capacitance wouldn't change much. Come to think of it, you did mention that you reduced the number of turns, which would significantly decrease the secondary self-capacitance. Question: if the AC secondary current due to self-capacitance was large, wouldn't it appear on the primary (and a lot bigger, due to the large turns ratio)? You have to admit, this is a pretty cool problem. Sorry that you suffer while we reap the benefits of your discoveries!

baleworker

Sun Nov 30 2008, 06:32AM

Registered Member #1596 Joined: Fri Jul 18 2008, 08:43PM
Location: USA
Posts: 65

jonny5 wrote ...

With a gapped core (for instance), the fields around the gap may be quite strong! . I just had a primary winding melt into a bobbin two nights ago (big gap in the E-core).

I dont use a gapped core, so this effect shouldnt play any role

jonny5 wrote ...

If I understand correctly, your primary is Litz and secondary (wound directly over the primary) is 26G magnet wire? Litz is pretty wicked stuff! But the current distribution in the secondary windings may be severely squished, leading to a surprisingly high AC resistance! I forgot how much secondary current you were running...

Yes, primary is litz, tried a regular magent wire too, secondary current around 180-200mA. Secondary is right on top of the primary, they are separated by 6mm CPVC.

I've tried putting an aluminum foil between primary and secondary, Heats up 10 times more than before!!! Here is the pic after foil removed, 3 mins run time, bobbins started to melt. Foil was of a C shape, to make sure it doesnt form a shorted turn.

I also tried it with the shorted secondary, no difference. Its a constant current supply, so it doesnt matter if I short the secondary, however shorting drops its voltage to near zero and excludes corona

This problem really gets me!!!

baleworker

Sun Nov 30 2008, 04:13PM

Registered Member #1596 Joined: Fri Jul 18 2008, 08:43PM
Location: USA
Posts: 65

P.S: You can see that primary is concentrated on one half of a bobbin, the heat is also localized in that area. When I used foil, the whole area was covered with it, and bobbin started to melt even where no priamary turns were located.
I do beleive its some sort of capacitive coupling what is causing overheating, but have no idea why and how to fight it ;(((

...

Sun Nov 30 2008, 05:54PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

You might try using something less lossy than PVC, since it could just be dielectric losses. Teflon comes to mind, or even just paper would work well. A nice heavy cardboard tube that was lacquered seems like a good thing to try.

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