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Off-Line flyback - Quick Q's

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uzzors2k

Fri Nov 24 2006, 01:55PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I whipped up an off-line flyback driver from spares I have left over from the smps, but I haven't plugged it in yet. What bothers me is current limiting, as you can see by the circuit I have none. Will this be a problem? Or will the core size, primary resistance, reactance, something limit current automatically? I just don't want a catastrophic failure from my first arc...

So will it limit itself, or will I need a series light bulb in the circuit forever? (I'll use one the first run of course.

)

...

Fri Nov 24 2006, 02:18PM

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

The forum's software is broken with gif images, you need to manually add the link to the full picture like

1164376553 95 FT0 Offline Flyback Driver

As to current limiting, it will self limit itself somewhere, but I cannot say if that value is a reasonable one...

I would use a variac and see how high you can go, then remove/add turns to the stepdown transformer until you get the desired output.

Marko

Fri Nov 24 2006, 02:39PM

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

You could use a SG3525 and drive the GDT directly, or maybe wire TL494's outputs into a push-pull and use a 1:2:2 GDT.

Depending on the frequency you want to run, you can safely use a much smaller nubmer of turns on GDT.

For some 40KHz you'l probably be good with twice as less, and rather make care that your GDT is well insulated and that wires are fat. Use cat5 or shielded cable and wrap it around the core, then use all white wires (or braid) as primary and paralell others for secondary.

You shouldn't go much over 40kHz with mosfet's body diodes, since their slow recovey.

Not much else to say I guess, just set your duty cycle low at beggining, ensure gate drive works and increase it slowly.

Good luck with it...

Dr. Dark Current

Fri Nov 24 2006, 03:25PM

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

you should put some inductor in series with the mains, if you dont want a big bang every time you switch it on

(maybe the diodes in the bridge can also fail because of this current pulse)
edit> discharge resistors on the filter caps are also a good idea.

Steve Ward

Fri Nov 24 2006, 10:24PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

With 80 primary turns it should be OK i think, but you may require more (like 100 or more). I ran a flyback driver from mains (120VAC) and i recall using around 50 primary turns for good results. Just uhhhh.... put a fuse on the mains, ok? 2A should be *plenty*. Even 1A will be enough for a flyback.

Dr. Dark Current

Sat Nov 25 2006, 08:40AM

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

Steve Ward wrote ...

With 80 primary turns it should be OK i think, but you may require more (like 100 or more). I ran a flyback driver from mains (120VAC) and i recall using around 50 primary turns for good results. Just uhhhh.... put a fuse on the mains, ok? 2A should be *plenty*. Even 1A will be enough for a flyback.

I am not sure if the fuse won't blow when the caps are being charged, use a slow-blow type.
As for the number of turns, I used 60 turns for halfbridge on 230V, but that was with "brute force" driving. You could maybe use waay more turns (200? 300?) when you tune your frequency to the flyback's resonant one, because on my flyback when operating "in tune" it gives ~5-10x the output voltage of that when not in tune.

Marko

Sat Nov 25 2006, 10:22AM

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

I don't think you need such monster amount of primary turns.

IIRC, ATX transformers (with about same cross sections as smaller flybacks) use 40...60 turns and they work fine.

As steve ward says, 50 turns would be appropriate, and you should even reduce that just until you come to edge of saturation.

You can regulate power by changing TL's deadtime and putrting a lightbulb in series to save your fets from saturation current.

Limiting power with deadtime is much more efficient than doing it with monstrous amounts of primary turns.

Set a higher frequency, 40..50kHz and lower it if fet's get hot.

In order to limit inrush current: you can put a limiting NTC thermistor from ATX supply in series with AC in, and then hopefully use a smaller fuse.

vasil

Sat Nov 25 2006, 11:58AM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

Do you have some arc pics from this mains driver?

uzzors2k

Mon Nov 27 2006, 10:24AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I just blew my last half-bridge.

The one I was going to use had been destroyed earlier, so I used my smps to drive the flyback. Original primary, no current limiting. All I got was puny arcs though, smaller than my ZVS! Then I foolishly turned it on while it could arc and I blew my last two IRFP450s...

Afterwards I realized I was running the flyback at 5% duty cycle

, so when I put some new mosfets in place I'll try full power!

I've got 20 200v 14 amp mosfets, will these work? If not can I series 2 of them for 400 volts?

Marko

Mon Nov 27 2006, 12:07PM

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

One thing you aren't clear with: what FREQUENCY are you running at?

Frequency is what is important for your output power and number of turns you use.

With too low frequeny you can use hundreds of turns and still saturate the transformer at few watts trough it.

If you are going >40kHz body diodes staart to make trouble and you need some external FRED's n that case.

Diodes are actually a very good idea no matter the frequency.

When it is done right core should be just somewhat away from saturation at 50 duty cycle, if it blows on 5% somethig is very wrong there!

You say you use flyback's internal primary?

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