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Mazilli flyback driver as a step down?

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Firnagzen

Sat Mar 21 2009, 05:24AM

Registered Member #567 Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147

I was thinking it would be nice to have a high power power supply, but I don't feel like shelling out a lot or getting a huge clunky transformer, and while mulling over it, I wondered: would it be possible to use a Mazilli flyback driver as a high frequency transformer driver? Rectify the mains, hook up the driver using 1200V IGBTs (I suppose I'd need something clever to stop the zeners on the gates from burning out.) and use a high frequency transformer.

But then, I'm relatively new to this. Would it work?

Ps. What kind of a core size am I going to need for 1.5kW?

aonomus

Sat Mar 21 2009, 05:51AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

What is the intended purpose and target voltage range/current?

The easiest thing that comes to mind would be using a MOT with a re-wound secondary for low voltage, high current. This only works if you have a AC source, if you need a DC-DC stepdown then that is a different story...

Firnagzen

Sat Mar 21 2009, 05:58AM

Registered Member #567 Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147

Yes, it's from the wall to about 40 volts. Problem is, I don't have a MOT, and I can't get one easily, either. (I live in Singapore. Junkyards are notably absent.)

Dr. Dark Current

Sat Mar 21 2009, 10:15AM

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

I say don't bother, if you try, it'll most likely end up as an explosive expereince.

Littlew

Sat Mar 21 2009, 01:03PM

Registered Member #1448 Joined: Sat Apr 19 2008, 01:16PM
Location: Russia/Moscow
Posts: 21

i used zvs driver to step down from 48v to 12v
but zvs driver works bad on 220v(or 110v) even with igbts.
You can make half bridge or full bridge. Or try to rewound solid state MOT

Firnagzen

Sun Mar 22 2009, 05:29AM

Registered Member #567 Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147

Mmm. Why explosive? And thanks, for the replies, all.

Dr. Dark Current

Sun Mar 22 2009, 08:17AM

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

Firnagzen wrote ...

Mmm. Why explosive? And thanks, for the replies, all.

Well, if you want to talk about that...

The gate drive in this circuit is imperfect, spurious oscillations of high frequency can happen within this gate drive scheme. I've found that increasing voltage increases the possibility of these oscillations, possibly because the gate voltages fall quicker. Also, smaller gate resistors seem to increase the possibility of these oscillations.
When powering it from rectified mains, you'll need to use a small step-down transformer to power the gates. Now, selecting the gate resistor value is very important because too big resistors decrease efficiency and possibly break the feedback loop, stopping the oscillation and resulting in failure. On the other hand, as I said earlier, too small resistors result in unwanted high-frequency oscillations, which damage the transistors.
I really would not be surprised if "too small" and "too large" for the gate resistors overlap by a great margin when running the circuit from mains, meaning that proper selection of the resistors is impossible and the circuit will never run reliably.

Another thing is that during the switching transition, the current may be interrupted for a moment and the series choke could spike above 1200 volts destroying your transistors. Also, improper selection and design of the choke may result in the collector (drain) peak voltages to reach higher values than they would normally do, again damaging your transistors.

Finally, from principle this circuit stores a great amount of energy in the core and resonant cap, meaning that the core and primary winding will run MUCH hotter than with a half-bridge drive.
This circuit seems to be benefical only when driving a highly reactive load, such as drawing arcs, but for a voltage power supply, a bridge is a wiser choice.

Edit: This is especially for running it from rectified 230V mains, for 120V it might be "slightly more doable" but still.

Firnagzen

Sun Mar 22 2009, 09:02AM

Registered Member #567 Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147

I see. What, then, would be the easiest* way, then, other than a big transformer, to get a nice high current power supply? A full blown SMPS? I'm not too concerned about regulation.

*I do realize that it won't be easy by any stretch, but eh.

Thought: I wonder if the high frequency oscillations are preventable.

Dr. Dark Current

Sun Mar 22 2009, 09:44AM

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

Firnagzen wrote ...

I see. What, then, would be the easiest* way, then, other than a big transformer, to get a nice high current power supply? A full blown SMPS? I'm not too concerned about regulation.

*I do realize that it won't be easy by any stretch, but eh.

Well, if you don't need high output voltage regulation, then it can be as simple as a half-bridge (like in SSTC with no PWM), transformer, rectifier and filter capacitor.

Firnagzen wrote ...

Thought: I wonder if the high frequency oscillations are preventable.

I've had some success with active feedback (w/ gate drivers), which ran quite smoothly at ~70V DC, but it blew an IGBT anyway and I gave up that circuit. It's good if you want to get quick big arcs easy out of a TV transformer, but for not much else.

Firnagzen

Wed Mar 25 2009, 04:50AM

Registered Member #567 Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147

Hm. Do those CCFL circuits work with FETs, or IGBTs?

I realize that first of all, I'm going to have to get the voltage down to about 10V to prevent the gates from burning out. But is there anything else?

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