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Flyback Driver Topology question

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LAN_403

Inducktion

Mon Sept 26 2011, 01:32AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

From what I understand, a typical 555 timer basic flyback driver uses a mosfet's avalanching ability to effectively create a high voltage spike on the primary of the flyback, and thus stepping it up on the secondary.

So, would this not mean, using a higher voltage rated mosfet/IGBT that's rated for avalanching, would result in a larger voltage spike? Or just the opposite?

And, would it not be possible to use a half/full bridge circuit, with large delays between on time and off time to recreate a flyback topology scenario for driving the transformer?

Thanks!

dude_500

Mon Sept 26 2011, 01:50AM

Registered Member #2288 Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179

A one-transistor flyback driver is just connecting ground to the primary letting current build up in the core, then this built up energy discharges ideally through the arc, and whatever doesn't discharges through the MOSFET's reverse recovery diode. A half or full bridge operates in a somewhat similar manner (although mathematically there are some key differences between the two setups). A half/full bridge will allow you to put a lot more power into the transformer at a lower thermal dissipation in the transistor(s).

Not sure exactly what you mean by a higher avalanche rated part, you should not be avalanching your transistors in a properly designed transformer driver, that would indicate you have insufficiently rated parts (unless perhaps by avalanche this just refers to using the reverse recovery diode, in which case indeed you do want a part with a sufficient reverse recovery body diode, especially if not using a half/full bridge since the excess magnetic field must collapse somewhere and you don't want to kill your FET)

Inducktion

Mon Sept 26 2011, 02:04AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Makes sense. But, I guess what i'm trying to get at is, what is the best way to get the most VOLTAGE out of a flyback, not necessarily current. I've already built a ZVS driver, and the flame arcs are great, but I want to see some corona!

Kiwihvguy

Mon Sept 26 2011, 02:29AM

Registered Member #3395 Joined: Thu Nov 04 2010, 08:42AM
Location: Christchurch, New Zealand
Posts: 193

I don't know much when it comes to flyback topology, but Dr. Kilovolt's 'Super' flyback driver is an excellent choice for you, lots of voltage and corona.

Check out his video on YT, it has the schematic: "Super flyback driver final version".

GrantX

Mon Sept 26 2011, 02:34AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Inducktion wrote ...

Makes sense. But, I guess what i'm trying to get at is, what is the best way to get the most VOLTAGE out of a flyback, not necessarily current. I've already built a ZVS driver, and the flame arcs are great, but I want to see some corona!

This is exactly what I want too! I haven't built a ZVS ever, but I've played with enough NST's and a few MOT's, so I'm now kinda bored with flamy arcs, unless of course I ever find a pole pig.

I've looked at a few half bridge flybacks on youtube, but they seem to be quite fat arcs.

dude_500

Mon Sept 26 2011, 02:50AM

Registered Member #2288 Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179

You could get tons of voltage out at no-load by using an LC series-resonant driver (and conveniently also reduces losses as ZCS is achieved at the most output voltage). This lets the primary current ring up since inductive impedance is reduced. Be careful, you can get so much voltage out of a flyback that it will just destroy itself.

Inducktion

Mon Sept 26 2011, 04:06PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

dude_500 wrote ...

I think I'll give designing a series resonant flyback driver a shot! It shouldn't be too hard, hopefully. Just a matter of tuning, unless I design a PLL or schmitt trigger based version!

haxor5354

Mon Sept 26 2011, 10:46PM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

I think i've saw an instructables tutorial to drive a FBT using a CFL ballast. arcs are pretty long with lots of corona

EDIT: ok found it

Jamie Cansdale

Tue Sept 27 2011, 12:45AM

Registered Member #3904 Joined: Sat May 21 2011, 10:32AM
Location:
Posts: 28

Are you driving your flyback at its resonant frequency, because this makes a massive difference to the maximum output voltage?

Here are a couple of examples of the flyback output voltage when a sin wave is applied. In the first screenshot it's being driven at 100kHz and in the second at 330kHz (its resonant frequency). I'm applying a tiny voltage to the primary and using x10 probe on the high voltage side. When driven at 100kHz it's showing a maximum of 500V, but at its resonant frequency its showing 1200V!

Patrick

Tue Sept 27 2011, 01:13AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Jamie Cansdale wrote ...

Are you driving your flyback at its resonant frequency, because this makes a massive difference to the maximum output voltage?

Here are a couple of examples of the flyback output voltage when a sin wave is applied. In the first screenshot it's being driven at 100kHz and in the second at 330kHz (its resonant frequency). I'm applying a tiny voltage to the primary and using x10 probe on the high voltage side. When driven at 100kHz it's showing a maximum of 500V, but at its resonant frequency its showing 1200V!

Jamie, what you speak of in the above quote is true, but i wonder what happens at resonance to that high voltage, if you draw a tiny current? does the V drop significantly? with corona i would expect it too.

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