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Problems in ZVS: IGBT/FETS Overvolting

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Mike

Thu Feb 16 2006, 03:47AM

Registered Member #58 Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317

Hi, well I built the 'zvs' circuit (not sure what to call it hehe.) Anyways, the problem is after running it for about 20 seconds, the fets are usually dead, but my IGBT's last for about a minute until they are extremely burning hot, and they die, and the G and S pins show conductivity, which means over volting if I remember correctly. The only thing I see sending the power to the IGBT's are the 470ohm resistors, I currently have 4 470 ohms, 2x two in series, that are in parallel, going to each G leg. kind of confusing, but they are 1/2watts, so it totals 4 watts @470ohm per IGBT/FET. The IGBT's I'm using are 18N120BND 56Amps 1200v Fairchild IGBT's 390watts.

Thanks for the help.
Mike

Part Scavenger

Thu Feb 16 2006, 04:35AM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

Do you have them both on the same heatsink?

Mike

Thu Feb 16 2006, 04:40AM

Registered Member #58 Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317

Gah I forgot to mention all I hvae for heatsinks are those crappy Radioshack ones that are the size of the component, but I still dont think the IGBT's should be getting hot this fast

...

Thu Feb 16 2006, 06:37AM

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

In the zvs circuit the fets should not get hot very quickly. My best guess as to why this is happening is that your series inductor is too small. I ship my zvs kits with a full 80 turns on the core to get enough inductance.

Do you have zener diodes on them? having a 12v zener diode directly across the gate/source is crucial for the fet to survive.
If you do have them then check them to make sure they are still alive; if you do not use a separate supply for the zeners they dissipate a lot of heat when you are running at high voltages (>40v kills my 1w zeners), but you don't know because they go open and you never even think of touching the circuit while there are huge ass sparks to be played with...

Steve Conner

Thu Feb 16 2006, 11:37AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

What DC voltage and current are you feeding it with? What frequency is it oscillating at? IME, almost any cause of death can make an IGBT go short between gate and emitter, it's not unique to overvolting.

uzzors2k

Thu Feb 16 2006, 05:00PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

My fets don't even get close to warm. Did you use the ZVS driver made by Vladimiro Mazzilli or a different one?
Post a picture of the setup so we can see if anything is wrong.

Mike

Thu Feb 16 2006, 10:54PM

Registered Member #58 Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317

Hi, Thanks for the replys, sorry I should have also said my psu for the ZVS. It is two radioshack transformers (I know I know :P) In series. One is 26v@2a, one is 12v3a. I parallel'd the primaries, and series the secondaries to a rectifier, after the rectifier the voltage measures **link** (I am not sure if the Amp figure is right, my multimeter has a max of 10 Amps, and when i measured it said 14A, so who knows.) Also yes I have 12v 1watt zener diodes on the board, however my inductor may be the problem, I did not have any cores that I knew were ferite, just a core that I took from a powersupply that was covered in green resin, I scraped off some and the core looked dark black like ferite, but I am not sure, however it is wound pretty crapily, should i be using 20gauge wire or 28gauge magnet wire on it? And yes I used the circuit by Vladmir on the old board and Wiki.

Thanks, Mike

Edit: So I checked my zeners after again 1 of the IGBT's died and got really hot, one of the zeners was dead i think, so i replaced it and im getting better arcs, however the igbt's are still getting pretty hot.
I think the circuit may need to be rethought if the life of the main component relies soley on a whimpy 12v 1w zener :p

Thanks, Mike

Edit2: After playin with the arc again it seemed fine but the IGBT's were still hot, so i unplugged it, then i plug it in to play again, and no work, so i check and one of the IGBT's is showing conductance again~!!!!

...

Fri Feb 17 2006, 12:32AM

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

you need to use an iron powder core for the inductor, a good place to get these is an ATX power supply. Take the big yellow toroid and wrap like 50-100 turns of 20awg wire on it.

The one you have does sound like it might be powdered iron, how many turns do you have? I use 80 turns of 20 awg magnet wire and they take running at 50v/10a with no problems.

The way to solve the problem of the circuit dying because of the zeners can be solved a few ways. The easiest would be to use a separate gate power supply, so the fets cant see more than 12v on them... Or you could put a led+resistor in parallel with it so that if it goes open it you will see the led light up..

Do you have a 10k (or almost any resistor) across the fet's gate and source when testing? Without it the fet's gate will charge up on its own making it look shorted source/drain...

Also, what diodes are you using? If they are not fast enough they could be your problem...

Good luck!

Mike

Fri Feb 17 2006, 01:15AM

Registered Member #58 Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317

Well I am using Ultrafast 400v fairchild diodes, im using 10k resistors yes, and I was using a big yellow toroid the iron powder type, but I heard from others I am not suppose to use those o.0

Oh well, maybe just a huge heatsink will fix it?

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