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Royer circuit - blown MOSFET?

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Colin 99

Sat Oct 07 2006, 07:17PM

Registered Member #192 Joined: Fri Feb 17 2006, 03:08AM
Location: Canada
Posts: 44

Well I built a Royer flyback driver circuit this week and all seemed fine unit I connected 3 - 12volt batteries to it and a IRFP460 failed (sparks and smoke : - )) ). After re-reading the specs I saw that they are only good for 18.4 amps, so I either had a bad MOSFET or I was pulling over 20 amps from those batteries ! The MOSFET blew while I was pulling a long arc off of the flyback.

Can I use IGBT's on this circuit? I have some 30N60 IGBTs that are rated for 30amps.

Is there other variations of this circuit or has someone tried different values of parts?

thanks,

Shaun

royer circuit

Dr. Dark Current

Sat Oct 07 2006, 07:53PM

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

<removed> (oops, I must first read the post carefully...)

Yes you can use IGBTs.

Things to try: higher voltage Zeners (15-18v), different primary capacitor
Didn't you have a loose connection somewhere (e.g. to the zeners)? Primary winding shorted from overheating can also cause Mosfet failure.

Marko

Sat Oct 07 2006, 08:35PM

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

Something looks wrong if you are pulling that amount of current at 36V.

That would be 720watts for cripes sake, and no flyback could endure that for more than miliseconds.

At that voltage you should be drawing under 10amps with shorted flyback output.

IGBT's won't help you if your circuit is bad, you could be saturating the inductor, have something bad etc.

That 20amp thing doesn't sound good to me!

Colin 99

Sat Oct 07 2006, 09:19PM

Registered Member #192 Joined: Fri Feb 17 2006, 03:08AM
Location: Canada
Posts: 44

Well I could try higher voltage zeners.

The Inductor is not saturating, it's an air wound inductor of 128 microhenries, calculated and measured.

The capacitor is a Cornell Dublier 0.68uF 600 volt pulse cap, so it should be good.

I was using a 0-35 volt, 0 - 6amp variable power supply when I first started playing with this circuit and once I pulled the arc over ~2 1/2 inches the power supply would current limit and I'd loose the arc. With no arc it was drawing over 1 amp.

I also have a meter / oscilloscope near the circuit and it would pick up the emf and showed a waveform somewhere between a sine wave and a triangle wave, nice and stable. The frequency was 27 Khz open circuit(no arc) and ~50 Khz with an arc. I did not take any direct measurements.

Also with 2 batteries (24 volts) it worked fine, longer arcs without current limit.

Colin 99

Sun Oct 08 2006, 02:52AM

Registered Member #192 Joined: Fri Feb 17 2006, 03:08AM
Location: Canada
Posts: 44

Well I replaced the MOSFETs with IGBTs and I replaced the 13 volt zeners with 15 one, the circuit works again. The source pin on one original MOSFET was burned right off and one zener had bit the dust.

I found that 36 volts input it too much for my flyback!! It flashes over from some of the other windings. 24 volts seems ok. The circuit draws upto 10 amps, I measured it.

Question: What is the right way to determine the ground connection of a flyback transformer?

...

Sun Oct 08 2006, 04:41AM

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

You just take the hv out and wave it arround the pins at the bottom. The one that it arks to like mad is the one...

Electroholic

Sun Oct 08 2006, 07:22AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

try somethign with a little less on resistance.
for irfp250 it's <85mohm and for irfp460 it's <270mohm
igbts would work great, too

Colin 99

Thu Oct 19 2006, 02:44AM

Registered Member #192 Joined: Fri Feb 17 2006, 03:08AM
Location: Canada
Posts: 44

I made a jaccobs ladder using 1/2 inch brass strips(they're nice and sturdy) , I mounted everything on plex and ceramic insulators so it all looks good but I can't get the ladder to restart. I tried to use a thin wire at the base connected to one of the ladders but now it just arcs at that point and stays there.

How can I get this JL to work properly with the royer circuit ?

--- I guess I could try two thin rods insted of metal strips ----- any other suggestions.

thanks,

shaun

...

Thu Oct 19 2006, 05:16AM

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

If you are using a AC (old disk type) flyback you could build a simple voltage doubler out of a small (~1nf@30kv) cap and a diode out of the tv (or string of high speed diodes to give like 50kv).

You could also try adding a 3rd electrode in the middle (I forget what it is called) that is connected to the second rod by a high value resistor. The first electrode arks to it (because it is close enough) because of the high value resistor, but as soon as the ark forms the resistance of the ark is much less that that of the resistor, so the electrode looks like it is floating at the potential of the first electrode, but is now physically much closer to the second one, so it can ark over, and then it all starts.

Short of that all you can do it move the electrodes closer and make nice sharp bends to lower the breakdown voltage...

ragnar

Thu Oct 19 2006, 05:58AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Here's what you're looking for; I think you're referring to a "Gabriel electrode".. the jacob's ladder application of this is documented here:
http://www.emanator.demon.co.uk/bigclive/jacobs.htm

"You will notice that there is a third electrode at the base of the vee, and this is a Gabriel electrode used to make the gap at the bottom of the vee less critical. In a normal Jacobs ladder there is no third electrode and it can sometimes be difficult to balance the shape of the vee for the best results."

"If the base is too close then the arc won't rise all the way to the top of the ladder because it's easier just to jump the gap at the bottom again. If the base is too far apart then there is the serious risk of the arc not striking at all meaning that the unit will look as if it's switched off when in fact the full voltage is across it."

"With the limitation of a 10kV transformer I found that the gap was too critical and I designed the Gabriel electrode to solve the problem. It allows the gap to be much wider than normal, but allows the ionisation of the air via two 1 Megohm resistors that make the middle electrode into a semi desireable path for the arc. When the unit is starting an arc it initially jumps over between the middle electrode and the opposing electrode which ionises the air and then pulls the current limited voltage down on the middle electrode. The arc then jumps across the remainder of the gap. It works very well."

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