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Simple Flyback Driver Questions

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Arcstarter

Fri Dec 24 2010, 09:10PM

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

Well you can make a ZVS circuit without zeners, i do it all the time. More times than not the zeners are what actually kill my circuit. Reason being, they clamp, but apparently see too much current and fail short, which turns one gate off and the other gate is on which shorts the mosfet and kills it.

Seeing that the gate supply voltage i usually use is 15v, the gate should not see anything more except for transients which i will just take my chances with.

Ash Small

Fri Dec 24 2010, 09:48PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Yep, I'd considered this scenario.

Do you use more than 15V for the collecter/emitter circuit (or whatever the MOSFET equivalents are) or 15V throughout?

There are other ways to reduce the transients.

(I'm getting some interesting responses in this thread)

Arcstarter

Fri Dec 24 2010, 11:23PM

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

The zeners are from gate to source, there is nothing from drain to source (collector to emitter)...

Ash Small

Fri Dec 24 2010, 11:38PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

But do you use more than 15V on source to drain?....and 15V on gate?

Or do you use 15V for everything?

Sorry if I didn't make this clear earlier.....

Plasmana

Fri Dec 24 2010, 11:57PM

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Location: Worthing, England
Posts: 72

Yes, he would use more than 15v between drain (collector) and source (emitter) of the MOSFET with a flyback primary in series with it. Since the MOSFET's gate (base) can handle maximum 15v or 18v, he has an separate 15v supply for the MOSFET's gate.

(BTW, I'm going this route as I don't have any suitable zeners lying around for a ZVS circuit)

You won't need any zenner diodes if you are going to power your ZVS driver less than 15v.

Ash Small

Sat Dec 25 2010, 12:24AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Thanks Plasmana. Another question I was thinking of asking is can you use the (conventional) ZVS circuits to drive, for example, two flybacks, one 5 turn primary around each one, with the centre tap (or whatever it's called)?

I've been thinking about this for a while, it should reduce overheating, I think. (If it works)

Ash Small

Tue Dec 28 2010, 05:22AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

ubuntupokemoninc wrote ...

you wan't to amplify a signal with the 2n3055?

just put you signal inter the base and ground your signal generator to the emitter of the transistor, then put the primary and your power supply in series with the collector and em miter of the transistor, you want more power put a (around 1uf cap) 250 volt in parallel with the primary, and a diode across the cap.

What value diode would you recommend across the cap? (across the primary)

Arcstarter

Wed Dec 29 2010, 02:52AM

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

Plasmana wrote ...

Yes, he would use more than 15v between drain (collector) and source (emitter) of the MOSFET with a flyback primary in series with it. Since the MOSFET's gate (base) can handle maximum 15v or 18v, he has an separate 15v supply for the MOSFET's gate.

(BTW, I'm going this route as I don't have any suitable zeners lying around for a ZVS circuit)

You won't need any zenner diodes if you are going to power your ZVS driver less than 15v.

As Ash might say, you hit that nail on the head! As long as the gate voltage wasn't getting any higher than maybe 25v i wouldn't worry. But if you are going to do this make SURE you use some sort of thick film resistor! NON INDUCTIVE! Otherwise it could easily ring the voltage way up, though im sure you would lose the mosfets to switching losses before you lost them to gate over-voltage.

@ Ash. You can use two transformers with one ZVS driver, but i am not sure how you are talking about connecting them. If you where to parallel the primarys you would want 2 times the turns on each primary for the overall same power, then just parallel them. You could even put the bare leg of two identical flybacks together and wind one primary around the two legs of the core. That has worked great for some people.

You could series the secondaries like this, but i would hiiiighly advise against that. For one, the obviously problem of killing the flybacks. Also, the ground pin on on of the flybacks will be at 30kv relative to ground, which could easily arcover to the primary. But you could parallel them with ease. The circuit would heat no less, though, unless by some odd coincidence.

As for that schematic ubuntu is talking about, the cap changes the switching topology from flyback to quasi-resonant. It basically runs pretty much like class-e. The diode in parallel is to block reverse currents going through the transistor, so it needs to be fast and rated probably 4x Vin and more than the peak current the transistor will be switching. The MUR series comes to mind...

Ash Small

Wed Dec 29 2010, 07:50AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Arcstarter wrote ...

..@ Ash. You can use two transformers with one ZVS driver, but i am not sure how you are talking about connecting them. If you where to parallel the primarys you would want 2 times the turns on each primary for the overall same power, then just parallel them. You could even put the bare leg of two identical flybacks together and wind one primary around the two legs of the core. That has worked great for some people.

....

I'm still puzzling over how exactly the Mazzilli ZVS circuit works.....I can see that the inductor and capacitor give it resonance.....and that the MOSFETs fire alternately (I think)...It's the way the capacitor is wired that puzzles me.

I was just wondering if it would still work if each 'half' of the primary was on a different transformer, and whether that would result in them 'firing' alternately,

I have a couple of large N27 cores (4 'C' cores in all) that I will be using eventually, (I'll have to wind secondaries for them before I can use them) Ideally I want to fire them alternately, to save on smoothing caps/multiplier components.(my 'back up' plan is to just use an astable multivibrator)

I'm planning to push around 3kW through them, so I may end up having to use IGBTs (I'm not sure yet whether MOSFETs will cope with 3kW)....

The limiting factor will be heating in the cores (core losses due to saturation), as I will be winding my own secondaries to suit.

Anyway....back to my preliminary circuit.....I ran some tests last night to see if the sig. gen. was putting out sufficient current to switch the BJTs. (the maths was a bit inconclusive, as I don't know how well I can trust my DMM and the gain of the 2N3055H's can vary,) but it seemed to be switching a few amps ok through a light bulb. I'll knock up a chicken stick later and see if I can draw an arc.

ubuntupokemoninc

Wed Dec 29 2010, 03:57PM

Banned on 1/22/2011 for repeated rule violations after multiple warnings.
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