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Methods of Winding Large Ferrite Cores

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jpsmith123

Mon Apr 28 2008, 11:45PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843

I finally located some plastic bobbins for a ferroxcube U100/57/25 core. While waiting for them to arrive, I'm considering how to wind them. My goal is to have two HV secondaries, each one producing 10 kv peak wrt their common midpoint ground.

I'm thinking it may be possible to wind a 10 kv multilayer coil without any kind of potting, oil, etc., if I use teflon coated wire and arrange the winding geometry and details so as to avoid subjecting the wire to any electric field higher than that implied by the 600 volt rating of the wire.

I'm wondering, has anyone tried anything like this?

Electroholic

Tue Apr 29 2008, 06:08AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

look at the CCPS thread under tesla coil or goto steve ward's site. I'm working on my own verson of the CCPS too, and I have the exact same core as you do, 100/57/25 3C90. I used some 2" vacuum ducting and i was able to wind about 180t of 30gauge on them before i ran out of space.I will be posting my result as soon as i get it running again. I used a full bridge of IRFP264 last time and blew them to hell, lol.
good luck!

jpsmith123

Tue Apr 29 2008, 11:49PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843

Hello Electroholic,

Thanks for your reply.

I'm wondering, what kind of driver were you using when you blew up the IRFP264s? Also, if I may ask, at what frequency were you driving the core and at how many volts/turn?

Lastly, are you using two U pieces, or a UI arrangement, and how do you hold the core together mechanically, i.e., glue, tape or clamp?

Electroholic

Wed Apr 30 2008, 12:52AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

It was a copy of steve ward's driver, 494 followed by UCC, with a 1:1:1:1:1 GDT. My heatsink was way too small, and the fets overheated and latched.
Fres for the series circuit was about 180kHz, and it was running at about 7volt/turn, because I was running it thru a variac.
Two U pieces, held together by an wooden clamp.

Mine was pretty much a copy of Steve's ccps

Edit: link fixed now

Berni

Wed Apr 30 2008, 07:25AM

Registered Member #1132 Joined: Mon Nov 19 2007, 06:15PM
Location: Slovenia
Posts: 84

Btw the link is broken, it included the word ccps in front of the link.

Oh and i was thinking of puting together a simalar transformer. But still havent got around doing it. Seams pretty easy to put together and amazeing results.

jpsmith123

Wed Apr 30 2008, 01:43PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Those cores are apparently capable of handling at least 4 to 5 kw of continuous power and maybe more. Steve says that with 200 turns on each secondary, he gets 20 kv AC (Is that peak I wonder?).

If my bobbins ever arrive, I will experiment with multiple layer unpotted coils.

In the mean time I have to decide how to drive the thing, and that decision will to a certain extent affect the transformer design, e.g., how to wind/place the primary vis-a-vis the secondary.

Berni

Wed Apr 30 2008, 02:03PM

Registered Member #1132 Joined: Mon Nov 19 2007, 06:15PM
Location: Slovenia
Posts: 84

I would recomend you to do the same steve did.

Best thing is to run it off mains so you have a nice high current 300V with a very cheap power supply (4 diodes,capacitor and some kind of balast)
Then for switching i would recomend IGBTs. They proven numorus times when it comes to switching higher voltages the IGBT is the way to go.

As for where to put the primary is if its very close to the secundary that means more current under heavy load but also means its harder on the IGBTs.

Dr. Dark Current

Wed Apr 30 2008, 02:27PM

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

Berni wrote ...

As for where to put the primary is if its very close to the secundary that means more current under heavy load but also means its harder on the IGBTs.

This controls the leakage inductance, that's the inductance "seen" at the primary winding as series inducance. For the CCPS I learned you want to put the primary just under the secondary so you get as small Lleak as possible, then use external inductor to control the inductance. This improves open circuit performance of the transformer.

jpsmith123

Wed Apr 30 2008, 03:31PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843

If I'm looking at Steve's circuit properly, it looks unregulated, i.e., I see no feedback-control loop.

I think I understand the concept of varying the drive frequency wrt the series resonance, (including primary-referred load), but this is usually done as part of a feedback control system, isn't it? Does this topology still have advantages even without active frequency control?

Maybe I'm wrong, but as I see it, under extreme load conditions, where you may have hard-switching, having all the "extra" inductance in the circuit may be more damaging to the Mosfets than hard-switching with a tightly coupled transformer with high Lm and low leakage.

Dr. Dark Current

Wed Apr 30 2008, 04:32PM

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

jpsmith, the CCPS uses a SLR technology, which basically means that you switch a series LC circuit at half its resonant frequency. Because resistance does not change the resonant frequency, no feedback is necessary.

I was once trying to implement feedback in this circuit, but you would really need complex logic to create the needed drive pulses, so it is absolutely not worth it.

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