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Full Bridge for flybacks and more

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foreign1

Sat Jan 31 2009, 08:40AM

Registered Member #940 Joined: Sat Aug 04 2007, 03:43PM
Location:
Posts: 7

Hello all! After years of fascination with all things high voltage, I finally took it upon myself to build a full bridge and pulse width modulator for driving flybacks and, eventually, Tesla coils. Pleased with the results, I thought I'd post some pictures/details on the forum.

The full bridge uses the 4 STW20NK50Z MOSFETs, 2 gate drive transformers (model SD250-4L with ratio 1:1.5:1.5) from CoilCraft, and a UC3825 chip with 2 of each of the BJTs 2SA1307 and STD13007. The 4 BJTs were just what I had on hand and are used in push-pull configuration to drive the gate drive transformers. A scope snapshot was taken prior to driving one of my flybacks and is included in the following set. The full bridge portion is powered by a variac with a rating of 1.4kW, but I won't be trying to push anything that far... yet ;)

The transformer included in the following set of pictures is homemade, using the U core from a deceased flyback. Although I am not equiped with a proper high voltage meter, I estimate it outputs around 2000VAC before I decide it gets too warm. I was considering using it in a desktop spark gap tesla coil if I manage to get enough voltage out of it. Perhaps I should try a voltage multiplier? Also, has anyone found Teflon plumbing tape useful in helping to boost breakdown voltages? Currently I'm using it in conjunction with spray-on lacquer.

Though I'm very satisfied with the flyback transformer driver, I'd like to finally get to work on a Tesla coil. Some rolls of 30AWG and 36AWG magnet wire are coming to me soon so that I can get to winding some coils, but in the meantime I've completed a secondary using spare wire with a quarter wavelength frequency of about 1.157MHz (resonant is about 2.131MHz). Until I can wind something within my flyback driver frequency's limits (gate drive seems to stay square-ish up to around 200kHz or just a li'l less), I'm debating whether I should just try to use an NST or a homemade flyback in a SGTC, or even implement a Class E driver using a Pierce oscillator and a power-FET or something (a la Richie Burnett's).

I'd love any suggestions y'all might have for experimenting or projects! Thanks for reading, and enjoy the low-res. pics ^_^

...

Sat Jan 31 2009, 06:27PM

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

Looks good, just make sure that your driver is hardened enough to handle tesla coil duty (it really is a lot tougher than drawing arcs on a smps supply)

As to winding a new transformer, you might check out

in the 2nd post I described the winding process for the transformer used in

and

KLH

Mon Feb 02 2009, 02:37PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137

Amidon sells both teflon tubing and polyimide Scotch 92 to insulate wire. The tape has a 7kV breakdown rating (I think). The teflon tubing can be used to insulate the ends of the windings on the transformer.

The tubing is cheap, but the tape is not.

Tom540

Mon Feb 02 2009, 05:19PM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
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Posts: 617

Looks good. I have a few of the same exact flyback. At least the same shape. Not sure about the size. The ones I have came out of a 5KW power supply and were used as the output transformer. So they are capable of quite a of of power. I have yet to use them for anything permanent.

foreign1

Tue Feb 03 2009, 02:07AM

Registered Member #940 Joined: Sat Aug 04 2007, 03:43PM
Location:
Posts: 7

Thanks so much for your follow ups so far, folks. By the by, I just threw together a baby SGTC the other day! :D I used a 10mL syringe tube I had in my bin of parts, as well as some wire that appears to be about 36 AWG. The gauge is an estimate by eye, since I pulled it from a solenoid of some kind and had to use a ruler/loupe to count turns/mm :P

It seems to work relatively well using a li'l HeNe laser supply (1.4kV at about 3.5mA I'm told). I haven't got it tuned to its resonant frequency (I just guessed at pri. turns and tank cap.), but it does throw li'l baby sparks and make luminous tubes glow without wired electrical contact, so at least it's a neat li'l desktop proof-of-concept device. Makes my U-shaped xenon tube glow green. The spark gap necessary to get it to run nicely is tiiiiiny. Using my much more powerful full bridge flyback seems like a *bad* idea, however... I like the notion of using minimal watts for such a teensy itty bitty coil. That way, I can still easily bring it to campus to show off ^_^.

It's alarming how conductive regular dielectrics become at such high frequencies!! Impedance really does shrink quite quickly... I don't like the corona I see from primary to secondary one bit... Also: any recommendations for SSTC stuff? I looked at extremeeelectronics for the Nano Tess project and it looks like the series logic gate oscillator is the way to go... keepin' it simple and low power. Any other suggestions you folks have? I'd love to step up the power to maybe 10W or so, see what I can pull from this li'l sucker, not to mention better tuning. I also have a 27mm dia. by 190mm length secondary with the 1.15MHz quarter wavelength frequency... perhaps a 4046 and half-bridge? I'm wondering if maybe a half-bridge is a better choice for such high frequencies (perhaps even a single power FET but at reduced input power...)

Duality

Tue Feb 03 2009, 04:18PM

Registered Member #1951 Joined: Sun Feb 01 2009, 01:59PM
Location:
Posts: 105

wow i have the same farriet core.
i was planning to build a new flyback with that core
i am getting offtopic.

but you have done a nice job with a fullbrige on you flyback :)

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