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Mazzilli Flyback Driver Failure

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DYI

Mon Oct 12 2009, 12:28AM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
Location:
Posts: 62

On and off for a while now, I've been trying to build a Mazzilli driver. I don't claim semiconductors among my areas of expertise, and as a result I've been building the driver using exactly the parts and configuration shown in the circuit diagram. So far, this effort has failed completely.

The inductor I'm using is a 100uH model, rated at 10A, 'FETs are IRFP250. Capacitors are film/foil type with polypropylene dielectric, 0.68uF, rated at 660V. Primary is 5+5 turns, using 14AWG copper wire (not magnet wire, but the normal stuff, used for household wiring).
For the testing I've been using a 12V lead-acid car battery. On the first test, I somehow got the polarity reversed and, predictably, fried everything in sight.
On the second test, I replaced all the semiconductors, got the polarity right, and hooked up the battery. I got a short (~5mm), weak arc to the pin nearest the HV lead, followed by sudden failure. After this run of maybe two seconds, the MOSFETs were too hot to handle.
Think I must have not bolted the heat sink on well enough during soldering, or some other such mishap that destroyed one of the fragile parts, I replaced the semiconductors and tried again. This time, I used large 6" x 6" aluminum heat sinks, one for each 'FET, during soldering, and left them bolted on afterward. This time, when I tested the circuit and tried to find which was the second HV pin, I got roughly equal length arcs from all pins to the insulated lead for about 30 seconds of testing, followed by failure again. This time, both of the large heat sinks were too hot to handle.

I'm stumped. Everything was done completely to spec, no modifications at all, using brand new parts. I could create an oil bath to dump the thing in to sink heat off the heat sinks, but in my eyes that's just covering up what is obviously a severe problem. I've come up with a few possible problems, and thought I'd run them by you to see if any of them sound plausible;
1. Insufficient coupling between the new primary and the ferrite core. Other people have done fine using 14AWG PVC insulated wiring, so it seems unlikely to me that this is the problem.
2. Operating at the very bottom of the recommended voltage range is causing the 'FETs to overheat. I really don't know enough about them to tell if this is likely to be the problem or not.
3. I got a bad batch of diodes / zener diodes / 'FETs. I don't know what kind of manufacturing defect would allow operation followed by overheating failure, but I imagine such a defect exists.

I'm aware that flyback driver problems are common topics, but I've yet to find a topic addressing a problem such as I have. Any ideas on the source of the failure, and how to correct it, would be greatly appreciated.

rp181

Mon Oct 12 2009, 01:22AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

The heating may be caused by parasitic oscillations, try replacing the 410ohm resistors with a high value, around 910 ohms. Try two of the 410 ohm resistors in series.
Are the mosfet backs isolated from one another? You mentioned separate heatsinks, but making sure. I am not sure if this does anything, but I like to keep it all isolated.

Do you have a O-scope available? If so, look at the mosfet gates, it should be a shopped sine wave (I think :p)

teravolt

Mon Oct 12 2009, 04:45AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

hi dyi,your car battery should work fine but it can supply as much current as you wish that's why your heat syncs are so hot. I also had truble with this circuit when it was functioning I had to add or subtract caps to tune it. Can you show some pictures

Myke

Mon Oct 12 2009, 04:59AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Make sure you diodes are good and that they are all put in the right way. As suggested before, check with an oscilloscope whether there is oscillations on the gate. try to aim for a freq around 35-50KHz (determined by the tank cap and the primary inductance). Could you take a picture of it so we can see your setup? Not much tuning is needed for this circuit nor is too much available current going to do anything.

DYI

Tue Oct 13 2009, 01:41AM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
Location:
Posts: 62

Thanks for the replies.
The FET backs are isolated, and the diodes should have been fine. I will, however, replace them at the same time I replace the 'FETs. I'll try increasing the resistance like rp181 suggested as well.
Unfortunately, I don't yet have access to an oscilloscope. I'm aware that this makes troubleshooting circuits similar to doing brain surgery with a sharp stick, but there's not much I can do about it right now.

Below is a picture of the assembled circuit. It does look like a bit of a rat's nest at the moment...

[Edit: Big picture]

Ugly as it looks, the connections are all solid, and nothing is touching that shouldn't be.

rp181

Tue Oct 13 2009, 01:57AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

That is ugly... :)
Try and aim for something like this:

The only question I have from the pic is the inductor, is it aircored? I do not think it can be that high inductance with that. Do you have the means to measure its inductance? If not, Tell me the specs of the coil (if it is aircored), and I will try to wind one and measure the inductance with a meter.

BTW, there is a 400px wide limit for images here.

...

Tue Oct 13 2009, 02:58AM

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

The inductor looks like it is wound on a ferrite rod, which while some members have reported sucessed using such inductors, I would recomend replacing it with one wound on a yellow/white torroidial core (such as the one in just about any ATX computer power supply). Just use ~50-100 turns on it, and see if it affects operation.

teravolt

Tue Oct 13 2009, 03:52AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

I think you will have better sucsess when it is more orginized. I can see some thing right off and that is that your heat syncs may be making contact through the bar that holds the flyback together

Herr Zapp

Tue Oct 13 2009, 05:31AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

DYI -

While it may appear to be quick and cheap, the "rat's nest" construction style makes it difficult to trace the circuit, very difficult to inspect for shorts and cold solder joints, and makes it easy to overheat small components like diodes during soldering.

Do yourself a favor, invest a few dollars in some perfboard and push-in terminals, and rebuild your circuit. Especially if you don't have any test equipment which will allow you to "see" what the circuit is doing, building a neat, clean layout will improve your chances of success.

Regards,
Herr Zapp

Dr. Dark Current

Tue Oct 13 2009, 07:14AM

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

Well,
1) are the diodes connected correctly, from gate 1 to drain 2 and vice versa?
2) are your primary turns insulated (you say copper wire)?
3) are both your primary "halves" wound in the same direction? (I have seen people who after bringing the center tap, actually wind the other primary half in the other direction

)
4) if everything seems ok, look at the schematic and check your connections once again.

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