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is my power supply set up suitable for zvs use?

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Forty

Tue Mar 06 2012, 02:57AM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

Yea ramping it up from zero is probably a bad idea. You'll want to set the variac so that the circuit gets at least 12 volts when you turn it on. 12v should be enough if your gate resistors are less than 500 ohms. Any higher voltage is fine too (given the constraint of your supply)
You can add bigger heatsinks once you get the circuit working properly so that you can run it for longer periods. When it's working properly the mosfets shouldn't heat up as much (at least this is the case when driving a transformer)
As far as the tank voltage goes, is your volt meter set for AC? I imagine you should see a voltage in the same order of magnitude as the input.

Davebmx

Tue Mar 06 2012, 09:17AM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
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Posts: 40

thanks forty tonight i will try the switch on from a much higher voltage. I'm worried however that i have already fried the mosfets! multimeter is set for dc current. should i have it set to ac as its a pulsed current?

Davebmx

Tue Mar 06 2012, 09:43AM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
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Posts: 40

I found this on a site about tesla coils!

"Insufficient gate drive, (incomplete turn on)
MOSFET devices are only capable of switching large amounts of power because they are designed to dissipate minimal power when they are turned on. It is the responsibility of the designer to ensure that the MOSFET devices are turned hard on to minimise dissipation during conduction. If the device is not fully turned on then the device will have a high resistance during conduction and will dissipate considerable power as heat. A gate voltage of between 10 and 15 volts ensures full turn-on with most MOSFET devices."

sounds like thats my problem i hope!

Forty

Tue Mar 06 2012, 05:33PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

I'd set the meter for AC voltage. I think the signal should be something like a square wave with curved edges. I wouldn't measure the current in the tank because it might exceed the rating of your multimeter (because of the capacitors.)
Yea I don't know a lot about mosfets but I do know that you want to turn them on and off properly. If you were ramping up the power from zero then that was definitely causing problems. I hope you didn't fry your fets. If you turned if off before they were smoking then they should be alright.

Davebmx

Wed Mar 07 2012, 10:37AM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
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Posts: 40

I did some testing of the 3 circuits i've built and it appears that one entire half of each circuit has blown out. Its kind of annoying because i think if i had just turned them on full power they might have worked fine! last nights testing blew up my rectifier so i'm using a much bigger one i have attached to a heat sink. its a 35a one do you think the heat sink is big enough and should i use thermal paste? gonna rebuild with my last remaining set of mosfets tonight and give it another blast! also used 10 slow fuses haha

Forty

Wed Mar 07 2012, 09:40PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

thermal paste is a good idea whenever you heatsink something. It fills in the tiny gaps between the two surfaces. Ah well i hope the fuses weren't too expensive then. Good luck!

Davebmx

Thu Mar 15 2012, 11:03AM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
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Posts: 40

update time. ordered a load of fuses haha nad still waiting on my mosfets to arrive. started fresh again this time. i've cleared out all the copper tracks i dont need so there definately isnt any shorts and i have used brand new components. as you can see my diodes are huge and look like mosfets. they are super fast response and have a handling of like 30a i think. can a diode be too big? looks pretty rediculous next to the tiny 1w zeners haha. also i am using blue powder cores for the chokes im assuming this int the cause of my many failures?

Davebmx

Thu Mar 15 2012, 11:04AM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
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Posts: 40

oh yeah soldered my chokes in the wrong place!

Forty

Thu Mar 15 2012, 09:36PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

I don't think the big diodes will be a problem (I've used TO-220 package heavy duty, ultrafast diodes quite a bit myself.) If they were stud mount or pucks then I'd say you might need to tone them down.
Powdered iron cores are what you want for this application. If you don't know for sure what's inside the core then you can scratch through the lamination a bit, and if you can't tell from there by the color, then do a scratch test (ferrite is way harder than powdered iron.)

Davebmx

Thu Mar 15 2012, 09:47PM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
Location:
Posts: 40

thanks forty i deffo know they are powdered as i ordered them originally in a bag for another application. I've tried to make the circuit as straightforward as possible as i have had so many failures and no successes. do you think on the next test i should skip using the variac and just go straight from the mains? then the circuit will get a straight 20-21 volts.

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