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SLR secondary overheats!!! Need an expert advice!

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GeordieBoy

Sun Nov 30 2008, 07:51PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Also make sure you wind the HV secondary in one single layer to reduce stray capacitance across the winding. You don't need many pF of capacitance across the HV winding to set up large circulating currents even when the ends of the winding are open-circuit. The more extreme the turns-ratio is the more sensitive the transformer becomes to inter-winding capacitance on the secondary side. This is because the effective capacitance seen at the primary side is equal to the secondary side self-capacitance multiplied by the turns-ratio squared!

-Richie,

PS. The reason why windings in flyback transformers can overheat in the vicinity of the air gap is because the fringing flux is greatest here. As mentioned this increases the local current density in the winding and can overheat it by exceeding the amps per circular mils the wire is rated for.

baleworker

Mon Dec 01 2008, 05:18PM

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

GeordieBoy wrote ...

Also make sure you wind the HV secondary in one single layer to reduce stray capacitance across the winding. You don't need many pF of capacitance across the HV winding to set up large circulating currents even when the ends of the winding are open-circuit.

The thing I am trying right now is a diode split secondary design, it has multiple sections connected thru diodes. Each section is only 2mm wide but has many layers, voltage per section is about 2kV. We will see if this design makes any difference.

I also increased a thickness of dielectric between primary and secondary to 12.5mm
Next thing to try would be a different dielectric. Unfortunately teflon costs arm and leg and also potting epoxies dont bond to it :(

I am kinda new to dielectric losses, what if I use a PVC bobbin lets say 1.5 mm thick and then a teflon spacer 10 mm thick? Will PVC still take the heat? Is stacking different dielectrics similar to connecting resistors in series?

Firefox

Mon Dec 01 2008, 08:29PM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

If you add more dielectric, you will probably make the problems Geordie stated worse, because the dielectric will increase the interwinding capacitance by it's dielectric constant. Also, a diode split transformer won't help. SLR requires that you use full wave rectification, as Steve Ward found out the hard way.

Steve Ward wrote ...
This

failure was a result of (foolishly) operating with a half-wave rectifier on the output of the transformer. I believe this may have caused saturation of the transformer. Always use full-wave rectification on your transformers!

baleworker

Mon Dec 01 2008, 09:09PM

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

Firefox wrote ...

If you add more dielectric, you will probably make the problems Geordie stated worse, because the dielectric will increase the interwinding capacitance by it's dielectric constant. Also, a diode split transformer won't help. SLR requires that you use full wave rectification

I use a fullwave rectified diode split secondary.
More dielectric thickness should actually decrease a capacitance since C=e0*e*A/d, so by adding more dielectric I will increase the distance between primary-secondary and reduce interwinding capacitance. Also swithching to lower dielectric constnat material, i.e. e=2.1 for Teflon vs e=3.7 CPVC should decrease the capacitance even further. Or may be I should just leave an e=1 Air gap between primary and secondary
Please, correct me if I am wrong

uzzors2k

Mon Dec 01 2008, 10:09PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Interwinding capacitance, is as far as I know made up of the microscopic capacitances between parallel wires in a winding. So the winding former should have very little effect on the parasitic capacitance which causes parallel resonance.

baleworker

Mon Dec 01 2008, 11:50PM

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

Uzzors wrote ...

Interwinding capacitance, is as far as I know made up of the microscopic capacitances between parallel wires in a winding. So the winding former should have very little effect on the parasitic capacitance which causes parallel resonance.

What you described is an intra-winding capacitance, I am concerned about inter-winding one(between primary and secondary) since if I move the secondary to the other leg, all my problems disappear

baleworker

Fri Dec 05 2008, 04:10AM

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

tested a new bobbin and after 10 minutes its barely warm to touch. Not anythere close to what I had before. I dont know if its a diode split design or more distance between primary and secondary helped. It was also wound on a bigger core , with primary spread other the whole length vs beeing tight wound. And secondary was made of multiple sections and only every other section had wire in it.
All these thing reduced an interwinding capacitance.

Now I am curious what helped it the most. Will try going back to smaller core, and tightly wound primary. I really hope that diode split design would be a solution for this problem.

baleworker

Mon Dec 08 2008, 07:52PM

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

Back to the smaller core and no overheating :) :) :)

I still dont understand why but I guess diode split technic really helped with reducing capacitive loses in the dielectric.

The open circuit current dropped approximately 10 times vs non diodesplit!!!

So I have about 25W of open circuit power loss right now and probably most of it is in the core, since with an open circuit Vdt is so large, it pushes core close to its limits.

I already tried PVC vs CPVC on non diode split transformers and CPVC losses are 4 times lower! So my guess is the loss was indeed a dielectric loss.

Anybody knows why diode split design eliminates a capacitive coupling between primary and secondary?

Dr. Dark Current

Mon Dec 08 2008, 08:57PM

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

baleworker wrote ...

Anybody knows why diode split design eliminates a capacitive coupling between primary and secondary?

Becuase there is only a small AC component on the individual windings; the windings are effectively un-coupled for AC with the diodes.

The capacitive coupling is the same, it's just that there is mostly DC on the secondary so the AC coupling is nearly gone.

baleworker

Mon Dec 08 2008, 09:38PM

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

Dr. Kilovolt, you seem to be an expert in diode split secondaries, in your opinion, what would be an acceptable voltage per section?

I am running the latest coil at 15kV output and have about 6mm insulation to the core/primary. 6mm should be sufficient up to 100kv+ DC, but what about AC? Secondary is vacuum impregnated, primary and core are not.

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