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Did I kill my MOSFETS? (ZVS)

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Shaun

Sat Jul 14 2007, 06:51AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Good morning everyone,

So I was just working on my new ZVS driver, and when I got it finished I connected it to an 8.4 volt airsoft battery. Now I know this is a bad supply, but until I rewind my MOT I figured I would at least be able to tell if it worked, and because airsoft batteries are made for high-amp discharges. Nothing happened. I then realized that in my haste I had accidentally switched the flyback center-tap with one of the end taps

.

With the connection fixed, I tested everything I could, and all the diodes, zeners, capacitor, inductor, resistors, were fine. But I do not know how to test the MOSFETS (irfp250) with my multimeter. Can anyone tell me if this error would have killed them?

With everything known good except the MOSFETS, I tried the airsoft battery again, and again nothing. So, figuring maybe 8.4 volts is not enough, I got 3 9 volt batteries and series-ed them. Again nothing. Any ideas anyone?

I actually wouldn't mind too much If they were dead, because the driver itself looks pretty badly done and I would like an excuse to rebuild it.

I'll post pix tomorrow (actually later today), cause its 3 am right now and I gotta sleep... (excuses, excuses...)

Myke

Sat Jul 14 2007, 07:23AM

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

What MOSFETs are they? The driver won't usually work at 8.4V. You really should search Google. Look at the first result for "testing MOSFETs". I've tried this way of testing and it works.

Ken M.

Sat Jul 14 2007, 03:45PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

I just test them like a diode using a DMM, you should get about .7 in ONLY one direction.

...

Sat Jul 14 2007, 06:52PM

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

To Test A MOSFET/IGBT Using A Multimeter:
1. set meter to diode test- Note: Some larger silicon may need more than that 3v provided by most meters, a 12v supply with 1k resistor in series with it and a voltmeter across the resistor/ground is prefered.
2. Connect + lead to source (pin towards the right), - lead to gate (left pin). Should read full scale (3v)
3. Connect + to source, - lead to drain (middle pin). Should read about 2v (from internal diode)
4. Connect + to drain, - to source. Should read full scale
5. Connect + to gate, - to source. Should read full scale
6. Connect + to drain, - to source. Should read 0v for a MOSFET, or about 2v for an IGBT.

Electroholic

Sat Jul 14 2007, 06:53PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

that only test the internal diode.
you can use the "diode" function on most dmm.
use the red probe to charge the gate, then test DS, most dmm should have enough voltage to turn the fet on to some degree, say 100s of ohm range. Then touch all pins with yoru finger to discharge teh gate. and resistance should go up again.

edit:
heh, I guess I'm a bit of a slow typer, I was referring to whitearc

Firnagzen

Sun Jul 15 2007, 03:39AM

Registered Member #567 Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147

As a side note:
The ZVS won't work with only 8.4v, moreover, by '9v' battery, do you mean those rectangular ~5cm high batterys? Those can't supply anywhere NEAR enough current to run a ZVS.

Myke

Sun Jul 15 2007, 04:35AM

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

He said he seriesed them not just one. I've accidentally revered the batt and the ZVS driver was fine. Double check that the wiring of the ZVS is correct.

...

Sun Jul 15 2007, 05:01AM

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

@electroholic, my method checks both the diode/transistor.

But don't worry, there is a very slim chance that your fets are damaged.

I would agree that 9v'sprobably won't be able to run the circuit regardless of how many are in series. However, you should be able to make it work by using 2 series 9v's to run the gates (ie, connect the 2 470r resistor to the + of the 9v, and the sources of the fets to the - of the 9v) and then connect the high power 8v battery's + to the inductor only (not the 470r resistor). Really, if would be nice to put a scope across the gates of the fets, and see if it is oscillating...

Shaun

Sun Jul 15 2007, 05:05PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Okay, I have done some of the things you all suggested and have gotten mixed results, with the final being that I can't really tell if my FETS are dead, but I know something is wrong. I have heard that these (like other HV components) do not die quietly, which is good because they both look fine.

As far as the DMM tests, I think most of the tests suggested are for a loose MOSFETS, whereas mine are currently soldered into the circuit and cannot be easily removed (next time I'll use sockets...learn from mistakes). The tests are unable to distinguish readings from other parts of the circuit.

However, I got the exact same readings for both of the FETs, to +/- about .002 volts, so I think they might be okay.

Also, I tried powering it with 2 8.4V airsoft batteries in series. Still nothing, but my inductor got hot very quickly.
In the picture the batteries are about 6 inches long, each cell is a "mini-C" size.

I'm at a loss as to what to do next, besides start over. Here's a few pics of the setup.

That's 20 gauge wire to the flyback

I tried to make the solder thick for high power. The inductor is 135 turns of 24 gauge wire on a medicine bottle.

The last pic is a toroid I removed from a computer PSU. It was the biggest one in there, and had transformer-type windings on it. I want to make a new inductor from it, but I don't know whether or not it is powdered iron. It definitely sticks to a large NIB magnet much stronger than my ferrites do. Will it work?

...

Sun Jul 15 2007, 06:05PM

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

That core is the correct type. Also, what are the to-220 devices? With irfp250's you can use the internal diode...

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