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4hv.org :: Forums :: High Voltage
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Very HOT Mosfet in ZVS

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buttim
Thu Apr 28 2011, 06:22PM Print
buttim Registered Member #3853 Joined: Thu Apr 28 2011, 06:16PM
Location:
Posts: 10
Hi all. I successfully built the ZVS driver and amcurrently enjoying a good Jacob's ladder.
Just one problem: one of the 2 FETs becomes terribly hot after a while. If I keep the thing on for more than a couple of minutes it burns. Since the ZVS circuit is simmetrical this seems very strange. I tried with 2 different flybacks with the very same result.

Any help would be MUCH appreciated!
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James
Thu Apr 28 2011, 07:33PM
James Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506
Is the primary winding you added to the flyback nice and symmetrical? Are these AC flybacks without an internal diode?

I've built a couple of ZVS drivers and the mosfets hardly get warm to the touch, even with no heatsink and ~30V in.
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buttim
Thu Apr 28 2011, 09:02PM
buttim Registered Member #3853 Joined: Thu Apr 28 2011, 06:16PM
Location:
Posts: 10
Yes, pretty symmetrical windings (not really perfect but good enough I hope). I thought the same thing, that's why I changed flyback. The strange thig is the other FET is hardly getting warm! This is driving me crazy. In the meanwhile I burned another IRFP250. I took them from 2 TV sets, so they might be DC. Weren't DC flyback supposed to be good for a ZVS drive? One of them is marked Grundig M29201
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magnet18
Thu Apr 28 2011, 09:09PM
magnet18 Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
Location: 1307912312 3766 FT117575 Indiana State
Posts: 624
They're DC unless the TV's were really, really old. Do they have air-gaps? Are the fet's insulated from each other?
not sure if either of those would cause what you're seeing, but it couldn't hurt.
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James
Thu Apr 28 2011, 09:12PM
James Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506
AC flybacks are much better for push-pull drive like the ZVS circuit, but I've run DC ones too without any heating. Are you sure you wired it correctly? Are your diodes fast enough? Are all your components matched? Do you have a scope to look at the waveforms? Something is not right here.
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buttim
Thu Apr 28 2011, 09:20PM
buttim Registered Member #3853 Joined: Thu Apr 28 2011, 06:16PM
Location:
Posts: 10
I'm pretty scared at putting the probes from my oscilloscope at less than 2 meters from the ZVS!
I checked the diodes, they seem OK. May any SLIGHT difference be the cause? I'll try to swap some of them to see if the other FET starts to heat up. I have no better idea!
How can I check for the air gaps? I cant see any
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James
Thu Apr 28 2011, 09:22PM
James Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506
So wind a different transformer on a ferrite core so you don't have HV in the picture. I've used the same circuit to produce much lower voltages with an appropriate transformer. Heck you don't even really need a secondary at all to test it.
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magnet18
Thu Apr 28 2011, 09:29PM
magnet18 Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
Location: 1307912312 3766 FT117575 Indiana State
Posts: 624
look along the ferrite core, it will be a little gap, bout the thickness of a piece of paper, lack of one could cause the transformer to do funky things, like walk itself into saturation, but listen to James, try it with a low voltage coil and scope it.
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buttim
Thu Apr 28 2011, 09:29PM
buttim Registered Member #3853 Joined: Thu Apr 28 2011, 06:16PM
Location:
Posts: 10
Am I really THAT stupid? Why can't i think of such simple things? I'll definitely do it, thanks for your help. And I'll let you know what I find
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buttim
Thu Apr 28 2011, 10:25PM
buttim Registered Member #3853 Joined: Thu Apr 28 2011, 06:16PM
Location:
Posts: 10
Did what you said. Here is what I saw attaching to the gates of the 2 FETs
1304029530 3853 FT114336 Zvs1

1304029530 3853 FT114336 Zvs2
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