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Berni

Wed Dec 03 2008, 06:11AM

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

Well i plan on putting on two secondarys and put the primary inside of them.As i sead the transformer in the picture is not built all that seriously its just something quickly sloped together to test the concept. I will make two new secundarys.(And probably have a catastrophic failure at some point, having the thing arc over and burst in to flames)

But first i need to get the protection circuit going.When it hits resonance the thing apears close to a short and if the IGBTs are switching at the amp peeks and 300V input its a kaboom

Dr. Dark Current

Wed Dec 03 2008, 11:23AM

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

Steve Ward wrote ...

Oh any i wonder how many W dose a typical flyback core handle?

*cringe*

Without getting deep into transformer design, you cant just stamp a power rating on a core. I will say that 500W is very optimistic given the tiny amount of copper on there. Perhaps 500W is realistic if you had even a 50% window fill of copper windings. Then again, for pulling arcs, where the load is low duty cycle, it may do 500W, and you just wont run it long enough to see it melt, or spontaneously combust :P.

Aren't the ratings in VA? This is important especially when drawing arcs, when the reactive power is very large.

Like I said before, I ran one flyback in Jacobs ladder duty for 6 hours, pulling an average peak of 500W. This means the xfmr saw 1-2kVA (dependent on arc length). This was with two standard flyback secondaries on one core, I would be surprised if the copper fill was above 5%.

I believe that the "flyback cores" from old large transformers can push several kVA when the windings are designed well, with good copper fill.

Berni

Wed Dec 03 2008, 12:55PM

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

Well im not relay aiming for anything like a kW but sounds good if its true. My bridge probably cant handle a kW anyway with its small fanless heatsink, also switching on zero cross is probably a must.

Also i plan on keeping the secondary single layer, since i don't want to tackle the big insulation problems of multilayer designs.Im aiming for about 10kV output.

jpsmith123

Wed Dec 03 2008, 01:10PM

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

I don't see how you can get 10 kv out of a single layer coil of 100 turns or so.

Berni

Wed Dec 03 2008, 02:37PM

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

I will be making a bigger secondary if you haven't read the previous posts, also i will have 2 secondarys in series

jpsmith123

Thu Dec 04 2008, 03:01AM

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

Well you stated that you intend to "improve" the secondary, in part with "more turns". To me this wording implies squeezing a few more turns on there kind-of-a-thing, whereas it seems to me that you'll need at least 3 or 4 times more turns than what you have now, and possibly larger insulating margins, which means you'll need to use much smaller wire...IOW a complete redesign of the secondary.

Also, with regard to putting two secondaries in series, are you going to "ground" the common connection for a 10kv-0-10kv output?

(I don't mean to nitpick, the reason I'm interested is because I'm planning something very similar myself).

Berni

Thu Dec 04 2008, 04:44AM

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

Thinner wire is not nesesery, because you can wind it thicker, also i don't want to use thiner wire because of the quite high currents involved at these powers. And yes the "center tap" of the secondarys will be ground, that way you can have double the output without having any further insulation problems that are involved with a higher voltage.

As for insulation its good to have lots of clearance between the core and secondary, also paint the secondarys with some kind of varnish o help insulate it better, helps even more if you varnish your bobin and wind the wire on while its still wet, since that will make varnish all around and no air bubles in it, since they can cause a corona arc forms in them and starts eating the insulation until it arcs over.(If your secondary is glowing violet thats a bad thing)

As for the primary in this two secondary design its best to put it under the primary, this reduces leakage inductance to almost nothing, but some driver designs need some inductance to work so sometimes another inductor in series is needed.

Dr. Dark Current

Thu Dec 04 2008, 06:13AM

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

Berni wrote ...

As for the primary in this two secondary design its best to put it under the primary, this reduces leakage inductance to almost nothing, but some driver designs need some inductance to work so sometimes another inductor in series is needed.

ALL drivers need inductance if you want to draw arcs. Remember that arcing is considered as a short-circuit. If you shorted a transformer with little leakage, it would draw huge currents and overheat, or blow your bridge.
So if you're using simple hard-switching design, it is the best to integrate as much of the leakage inductance as possible into the transformer.

Berni

Thu Dec 04 2008, 06:25AM

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

well under the secundary is a quite conveniant place for it. no problem if i add a external inductor.Its also esayer to adjust the inductance then. But i seen even my test winding seams nearly like a short to the driver when under resonance.

Berni

Sat Dec 06 2008, 08:17PM

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

The protection circuit is built, (Could have made a real PCB but too lazy so prefboarded it)

Anyway you can here also see my GDT driver that the protection board is connected to and also a 555 timer that generates the frequency.The 9V battery is for testing the protection.

It needed a bit of tinkering but now it works perfectly, I also used a full wave rectifier on the current transformer so it can sense current both ways. The button on the board is the reset switch for it and the two LEDs tell when its working and when in error mode.

As for the current transformer its a ferrite toroid with some relatively thin wire. Also its dunked in varnish to protect it and make it more solid.Then attach some wires and glue it down to keep them from tearing off.

Now i need to fix my IGBT bridge since i blew it and test it live.

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