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Flyback driver on mains?

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cbfull

Tue Dec 05 2006, 02:04PM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

I searched my files at home and found the picture of the globe filled with industrial purity argon (not as pure as it should be for plasma globes).

I have been thinking about the MOSFET gate capacitor question, and the only thing I can come up with relates to the pull-up resistors. In the drawing, the pull-up resistors would be connected between the blocking caps and the gates, so I am getting the feeling that they are still necessary for a self-starting circuit. If I could set it up (no space for projects right now) I would just test it and be done with it, but that will have to wait, possibly a while.

Marko

Mon Dec 11 2006, 02:51PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi guys...

OK, I got some clue how should such a thing look.

I settled around this relatively simple

halogen lamp 'electronic transformer'.

The trick they use is a small, separate current transformer, with small core cross section connected in series with 'main' transformer.

Small core will saturate much before the main transformer without producing a noticeable effect for the circuit, making it much more robust.

I believe the current transformer is usual small 3:3:5 toroid.

Starting circuit is diac based, and little filtering is used so it retriggers oscillator with each cycle, and keeps power factor high.

If I get time I'l try to have some fun with it, and hopefully put some more serious power through.

MOSFET's could probably also be used if current transformer is redone.

Dr. Dark Current

Mon Dec 11 2006, 06:59PM

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

Firkragg wrote ...

Hi guys...

OK, I got some clue how should such a thing look.

I settled around this relatively simple

I tried exactly this circuit a while ago, but it couldn't start oscillating, I'm sure I had the current transformer done wrong, since I had no idea how many turns and what core to use. Any info on this is appreciated.

Marko

Mon Dec 11 2006, 09:24PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

In practically all such circuits I seen small transformer is a 5:3:3, 5:2:2 etc. usually on a small toroid.
Old CFL's also have such a transformer.

Other things are relatively optional in this circuit, base RC-s are just to make transistor's delays less significant.

Steve Conner

Tue Dec 12 2006, 11:23AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

If the load isn't highly resonant enough, that little transformer sets the oscillation frequency of the circuit by saturating, so it's not as simple to design as it looks. If you get it wrong, the frequency could plummet when you pull an arc, and if it falls low enough the flyback itself could saturate and blow your transistors out. My advice would be to just steal the transformer out of a burnt-out low-energy lamp. Or forget it and drive the devices off an ordinary TL494 arrangement. These self-drive circuits only really come into their own for BJTs, which need a base drive current proportional to the collector current for efficient running. I wouldn't bother if I was using MOSFETs or IGBTs.

FWIW, I think that PC power supplies with BJTs actually use a cross between self-drive and the TL494. I'm sure I saw one with a winding on the base drive transformer that carries the halfbridge output current, as well as a winding driven by a TL494 and a couple of small transistors.

PS, I hate those $%&Â£(! electronic halogen light transformers, they are the reason why I can't use my HF radio unless I turn all the kitchen lights out

Marko

Tue Dec 12 2006, 01:01PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

But, heck, if we use TL494, requiring all the gate drive and additional power supply mess, simplicity and idea of such exclusively off-line driver would be ruined.

I would also like ot use the circuit as small switched power supply, simple enough to be cost-effective over 50Hz transformers.

Core-saturation current transformer is, although tricky, probably simplest way to provide square-like base drive to transistors in such circuit.

Since it's halfbridge uber cheap transistors can be used, and considering just an amp of input current is enough to blow most flybacks up at that voltage there's a really wide choice of them.

Lots of stronger CFL's and smaller halogen supplies use BUT11A's with 5:3:3 transformer (that can be dug out) so I would start around there.

For interference thing, it may be a good idea to somehow eliminate the diac and LC network and use real filtering with a big cap, with some other way to start the circuit.

This would make power factor worse but I think it would actually eliminate HF noise and sharp harmonics fed back into mains.

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