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Flyback: What do I need to know? Please, help a first time builder!

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LAN_403

baleworker

Sun Jul 20 2008, 05:50PM

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

So i decided I need to built one myself to meet all the requirements.
It will be driven by PWM controlled full bridge. duty cycle changes to keep a preset output current

Primary voltage 320V
Secondary idle voltage 24kV, required for initial breakdown
Load voltage ~8kV DC under 100mA of current, determined by preset current value and gas discharge characteristics = 800W continues

So far I am planning on machining a bobbin out of plastic, and potting secondary with a potting resin compound, separating each layer with paper/mylar

Here are the questions I have:
1: is that possible to have a 24kv idle 8kv load from the same transformer?
2: should I just use a diode/capacitor or a voltage doubler circuit?
3: which end of the secondary(inner or outer) should be a 0V one?
4: Any formulas/design procedures for a HV flyback?
5: Should the secondary winding direction be constant(i.e. from left to right on every layer), or can it alternate?
6 :Any caveats, am I missing anything?

Thanks tons! Any input is highly appreciated!

Dr. Dark Current

Sun Jul 20 2008, 06:16PM

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

baleworker wrote ...

1: is that possible to have a 24kv idle 8kv load from the same transformer?

Yes thats perfectly possible, use an inductor in series with primary winding (air gap can be varied to change output current)

baleworker wrote ...

3: which end of the secondary(inner or outer) should be a 0V one?

Inner should be at 0V and preferrably connected to core

baleworker wrote ...

5: Should the secondary winding direction be constant(i.e. from left to right on every layer), or can it alternate?

It can alternate, this is how most flybacks are wound.

Sulaiman

Sun Jul 20 2008, 08:23PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

800 Watts is quite a lot for a flyback - not to say it can't be done - just more attention to details required.
24 kV peak is difficult to insulate for, unless you vacuum-encapsulate or immerse in oil etc.
Fast recovery 30 kV+ 100 mA dc diodes will not be cheap - or rugged.

8 kV @ 100 mA dc would be easily achievable with two MOT's and diodes/capacitors.
Using a separate eht pulse generator to 'start' the plasma.
A variable inductor or variac could provide output voltage control.
Just my thoughts.

I would wind in one direction only (e.g. right-to-left) then at the end of each layer (left) run the wire (axialy) along the cylinder back to the start (right) - insulate the wire from both the winding below and the one to go on top.
This has two benefits; 1/2 the voltage stress on insulation compared to continuous left-right and right-left winding
AND lower self-capacitance which is really important for such a high inductance winding in an smpsu.
If you can pot under vacuum then OK, if not I'd go for oil.

How will pwm of a full bridge alter the eht voltage?

Dr. SSTC

Mon Jul 21 2008, 03:32AM

Registered Member #1407 Joined: Fri Mar 21 2008, 07:09AM
Location:
Posts: 222

if you want a 800 watt flyback try to find a rear projection tv

Andyman

Mon Jul 21 2008, 03:43AM

Registered Member #1083 Joined: Mon Oct 29 2007, 06:16PM
Location: Upland, California
Posts: 256

The flybacks from rear projection TVs are AMAZING. I have one and I've pumped an unbelievable amount of power through it without it failing.

baleworker

Mon Jul 21 2008, 03:47AM

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

I was thinking of using a separate eht for initial breakdownd, but my other application requires around 20 kV working voltage at 25mA, so I want to use the same transformer and kill 2 birds with one shot.
Besides I am looking at that tiny asian flyback I posted in a different thread, if they could do it why cant I?

I need a constant output current, voltage will be adjusted to keep the current at the set level. It will be done thru feedback loop, not manually, so variacs etc are unfortunately out of the picture.

thanks!

baleworker

Mon Jul 21 2008, 03:50AM

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

I have a few applications and they all require different parameters, so its in my best interest to learn how to make those myself, may be outsource coil winding/vacuum encapsualtion. My requirements are very specific and it seems like nobody really makes off the shelf flybacks and i have no choice but go for custom.

Arcstarter

Mon Jul 21 2008, 04:18AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

"Load voltage ~8kV DC under 100mA of current, determined by preset current value and gas discharge characteristics = 800W continues"

Use two mot's with a doubler. that is the easiest way i know how to get that kinda voltage and current. Ballast it down to keep it from drawing unnecessary current. For the 24kv just use a flyback or ignition coil.

baleworker

Mon Jul 21 2008, 04:17PM

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

MOTs are bulky and heavy, 60Hz solution does not look attractive to me. Besides I need a constant output current!
Any advise on the output cascade?
Should I use a diode/capacitor or a doubler circuit?

Sulaiman

Mon Jul 21 2008, 04:52PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Newer microwave ovens have quite a large ferrite transformer - may be worth a look.

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