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ZVS driver diode failure.

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Pripyat

Sun Aug 08 2010, 09:01PM

Registered Member #3086 Joined: Sun Aug 08 2010, 08:35PM
Location:
Posts: 9

I have been doing a lot of research on this site regarding flyback transformer drivers. I built the Mazzilli ZVS flyback driver earlier today and it seemed to work fine for all of a minute or so. With a 28V, 20A supply and eight windings on the primary, I was getting ~1" of arc.

I was getting ready to check the temperature of my mosfets when one pair of diodes popped and went up in smoke. Three items were obviously damaged. Both diodes and one of the zeners were shot. The mosfets were only warm. The only component I was really unsure of was the capacitor but these are the components in question:

CAPACITOR SILVER MICA 680PF, 500V, RADIAL
FAST SWITCH DIODE, 1A, 400V, AXIAL
ZENER DIODE, 5W, 12V

I can post up pictures of my circuit and the damage if it would help. I am just looking for guidance as to why my circuit failed. Thanks in advance.

[edit] I should also add that I replaced all three failed components and the power supply is having an overload fault every time I try to power the circuit up.

[edit 2] I replaced all the components except for the resistors and the mosfets. Still getting an overload fault. This is my circuit shortly after the failure:

Physical Circuit

I also checked the resistance between a couple nodes. As per the image, I found the following resistances with all the diodes and the zeners replaced:

Node Diagram
Node 2-3: 866 ohm
Node 2-4: .5 ohm

Matt Edwards

Mon Aug 09 2010, 07:09PM

Registered Member #2838 Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333

I would try a faster switching/higher current diode. I will post the one that I used when I get home. It worked well! Also, can you check current during arc? I think that you may possibly be drawing more than one amp. If you get a chance place a photo of your flyback and windings as well. You should be able to get much greater than one inch with the mazilli circuit.

...

Mon Aug 09 2010, 07:46PM

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

You do not need a high current diode, it only has to be able to charge/discharge the gate of your mosfet.

I suspect that your problem is that the mosfet died, and took everything else with it. I have never had a failure where the uf4007 diode was damaged without the failure of something else causing it to fail.

HazzWold 1993

Mon Aug 09 2010, 08:03PM

Registered Member #2563 Joined: Mon Dec 21 2009, 10:17AM
Location: Australia
Posts: 280

If the capacitor your using is this one off farnell

,+500V,+RADIAL
try this one, Ive used it to great success on my ZVS

Also to note your original cap is 680pf, you'd need the 680nf

Dr. Dark Current

Mon Aug 09 2010, 08:25PM

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

I think the first cause was the zener diode going short circuit and the other MOSFET couldn't turn off so it got fried. Put some sort of heat sinks on the zener diode leads close to their cases.

Pripyat

Mon Aug 09 2010, 09:12PM

Registered Member #3086 Joined: Sun Aug 08 2010, 08:35PM
Location:
Posts: 9

Thanks a lot for all the replies. It seems obvious now that I got my units confused and wound up using a 680pF instead of a 680nF.

Would this explain why my arcs were only ~ 1"? And explain why my circuit failed? I was wondering why the arcs so unimpressive. That could explain it.. This is sickening. At least I got to see some arcs before it failed. :D

Pripyat

Tue Aug 10 2010, 12:44AM

Registered Member #3086 Joined: Sun Aug 08 2010, 08:35PM
Location:
Posts: 9

I just spent an hour trying to find the components I need somewhere other than Farnell. Seems that Newark is out of IRFP250s. And it would cost me 20usd to get that single capacitor shipped that was linked at Farnell.

I would really appreciate some help on locating a capacitor for this application. I looked all over at different US distributors such as mouser, jameco, and digikey and none of them have the film capacitor that was linked by Teravolt Labs earlier. Which would be the best type of capacitor for this application? Or rather, what can I use that will be easy to find in a 500v package? Thanks in advance.

[edit] If I could delete this post, I would. I realize I broke the rules by posting twice. My apologies.

teravolt

Tue Aug 10 2010, 02:56AM

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

hi Pripyat, try thease

What you are looking for is a capacitor that has a low ESR and a low ESL wich is your basic snubber capacitor. Just remember to place them as close to your flyback primary as pausible. The circuit uses a parallel tank circuit for the primary wich is push-puled by the two FETs. I have used many types of fets or igbts you just need to keep an eye on the VGon and make shure thay ar under your Zenar voltage. Find a IRFP450APBF or a common IRFP450 to-247 wich I found at newark and mouser for about 3$ us

I also forgot I used this diode for my fast diode just duble up the anodes

Dr. Dark Current

Tue Aug 10 2010, 01:06PM

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

The X2 MKP caps work well in this circuit, actually you can find 330nF 275VAC X2 MKP caps in PC power supplies, two of these in parallel should work ok. Or you can buy these caps, they are cheap.

Pripyat

Tue Aug 10 2010, 02:57PM

Registered Member #3086 Joined: Sun Aug 08 2010, 08:35PM
Location:
Posts: 9

I really appreciate the input. I'm an EE major and have a lot of interest in going further with this hobby. Sad that I'm one semester away from having my 4 year degree in EE and made a silly units error. There was a time when I would really let it discourage me but this is one lesson among many I'll learn in my future with electronics: Don't assume you remembered correctly.

I ordered some more mosfets and the capacitor teravolt linked off mouser. I'm really excited for these parts to come in! I'll post up pics of my transformer later today. It's a flyback from an old CRT monitor.

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