What fails a FET?

Maybe the gate voltage is being exceeded due to some noise somewhere? Maybe terminate the gate/source with a 10-50K resistor would help?

-Bill

A shorted gate means the gate oxide has broken down, which implies excessive Vgs. That, in turn, could be from the circuit, or ESD damage.

Best, James Arthur

Hello James,

I'd second that. To prevent it Andrew could look up the max Vgs rating (the 2N7000 is usually 20V), then place a resistor between driving source and FET gate, preferably close to the gate. 10k or so, depending on how fast the signal needs to be. Then run one diode to the 14V supply and another to ground which limits the excursion to a range of -0.7V to

14.7V. A BAV99 is nice for that if space is tight.

--
Regards, Joerg

http://www.analogconsultants.com

Yes, I've blown a few mosfets with excessive gate voltage. Adding a 10K resistor from gate to source usually fixes the ESD problems.

BTW, the 16 white LED driver is working well at 86% efficiency. It's almost ready to mount inside the flashlight. I have it running here projecting a beam of light on the wall. Tomorrow will tell if it fits inside the housing.

-Bill

Thanks for the update--let the blazing begin.

James

Antistatic soldering stations? ESD protection and overvoltage protection? Clamping using reverse biased zeners will lower performance but it will protect so will decoupling and the like. Many things ... can kill a FET ... they like BJTs can melt. A touch of a finger on the gate to internal ruptures can kill them... High temperatures can open up a lead bond in a circuit leading the circuit to fail then someone may try testing a FET... but that is something really to avoid your very last shot because you can SO easily kill it. As said above the thin oxide layer at the gate electrode ensures high input impedance but also makes it vulnerable to ESD. You need a low voltage ohm meter when testing it plus it has to be set to the highest resistance reading.

lemonjuice

Thanks all for your input. I'm still a little new to effectively using transistors and the things that can happen with them in the "real" world. I didn't take any ESD care whatsoever when handling the transistors and when soldering. It is poor practice that experience has never forced me to change. Guess I've been lucky so far. I didn't realize transistors were so sensitive to ESD. This could certainly be what happened, and would explain what happened to the transistor on the board that failed before any minutes/hours were even put on it (besides testing - some of which was done with a DMM). I have plenty of room to try a 10k resistor from gate to source (which would be gate to ground, in my case) if the working board doesn't hold up. I'll be testing more this week so I'll try to update everyone.

I'm curiou, though, is it possible to damage a transistor with ESD while handling/soldering, then install it, have it work perfect for a few hours, then fail suddenly because of the original ESD damage prior to/during install? Or would a failure like this be unlikely to be ESD and more likely to be a circuit problem?

I really don't think I am over-voltaging the gate. With 4.8V max going in now, a 20V max Vgs, and no noise to speak of, I find it unlikely that it was over-voltaged, but I could be wrong.

Thanks again for all your help!

One more thing I thought of, now that we are on the ESD topic (sort of). With the current board (as opposed to previous) I am now using a foam packing type insert to press the board against its plastic case, instead of letting it free float. The entire plastic box that houses the device can undergo some serious vibration. Is it possible that this foam pad is generating ESD or something? I don't know anything about ESD, I'm just thinking outloud. It is contacting the bottom of the board, where there are no compontents, but all through hole connections and vias (the transistor in question is through hole) are being contacted by the foam piece.

Any chance this is related? Or very unlikely?

Andrew wrote: [....]

If two different material rub over each other, the electrons from one will tend to transfer to the other and build up a charge. You don't get a high voltage out of it until you move the charges apart. I would worry about anything that allows one plastic to move against another. It wouldn't be just the foam but also the plastic housing against mounting parts.

You want to make sure that you have a much shorter spark path to something non-harmful and/or make sure that none of the parts are good insulators.

Very often you end up solving the static problem when you solve the EMI problem. The conductive material you put in as shielding also works to take static charges away. You still need to make sure that the plasic foam between the metal and the PCB can't build up a charge.

I might have been a bit unclear. It is a plastic BUD type box with the circuit board inside. I don't know the composition of the foam piece, but it is foam and is stuffed between the lid of the plastic box and the bottom of the circuit board.

I really doubt this is the problem, though, because I have "fried" one of these transistors without having the board in the box.

I also experimented today with purposely trying to fry a transistor in the fashion that the ones are failing on my board, and believe it or not, I had some trouble. I put 37V (max from single power supply that we have) from gate to source and left it for 15 minutes or so. It was alright. I tried pulsing it and that didn't work either. I put the

37V from source to gate instead and still it was alright both pulsed and continuous. I linked two supplies together and put up to 60V to both gate to source and source to gate, pulsed and continuous (for a few maybe 30 seconds) and it was still alright. I put 72V (max of both supplies in series) from gate to source and this did not take long at all to "fry" the transistor. The resistance from gate to source was very similar to the results of the transistors that failed on my board.

With this type of voltage to fry these things, unless there is some other serious problem, I must have damaged the 3 transistors that failed in boards with ESD somehow. I seriously doubt (certainly have not seen it) that there is enough noise on the 4.8V signal to achieve

60V+.

Wow. I'm stunned. I can reliably get one of these transistors to fail with ESD. There's nothing like seeing is believing! For anyone who is skeptical that ESD easily fails transistors, here is the experiment I just fried 5 transistors in a row easily doing.

I grounded the source terminal to earth ground (used the one on the power supply). I stood on my padded chair, rubbed my shoes on the (nylon?) padded chair a couple times (don't need many at all) and touched the gate. One time out of 5 I felt/heard/thought I saw the shock, the other 4, I thought nothing happened. Tested resistance between gate and source, and BOOM, it was fried... very low resistance.

I then considered that the soldering iron tip is earth ground and figured that it wouldn't surprise me for a second that that his how I damaged some transistors (that eventually prematurely failed), and fried other transistors on the spot. To confirm this theory, I held the gate pin with one hand, the soldering iron (on the rubber boot/handle) with the other, stood on the padded chair, then rubbed my feet a bit on the padded chair while intermittently touching the soldering iron tip to the source. Sure enough, after a couple tries the transistor was fried.

Although I don't solder while standing on my padded chair, I can easily see how ESD is transferred to devices. I may have transferred a smaller charge while standing on the linoleum floor and soldering that simply damaged the FET. This damage, from what I've read, may often times still allow the chip to work alright - for a while - then fail, which is what some of my boards/transistors have done.

ESD is much more serious than I thought. Regarding FETs and other sensitive devices at least. Needless to say I'll be taking MUCH more care in the future.

Sorry there are so many posts by me, but I'm trying to fill everyone in and keep some info in here for future searchers.

I just wanted to note that right after I posted, I went to see if I could fry a transistor by holding the gate pin, rubbing my feet on the linoleum, and touching the soldering iron tip to the source pin. Sure enough, first try, I fried it. I'm really beginning to think this is case closed!

Thanks for your help again everyone.

Thanks for your updates!

I'm new to electronics design, and this info is very helpful for my future design endeavors.

Andrew wrote:

I have one article re ESD damage where the authors claim that ESD damage is a thermal effect and therefore latent damage is almost non-existant. Don't believe it really, but have used the article to support the argument when my customer has asked me the same question.

What are other people finding re latent damage?

M Walter

This is the article that convinced me that there could be latent damage:

.

Seems plausable. An ESD spark can damage the chip in the same way that welding or arcing can. Check out the picture in that writeup, it makes sense to me. The chip works fine until it gets some time on it, excessive heat, or slightly more current than the damaged track will allow, which would have otherwise be acceptable on an undamaged chip.

Interesting article, thanks

M Walter

Further regarding latent damage... I just did another experiment where I tried to damage a transistor in the same fashion(s) and I had one that failed where the gate to source resistance measured like 9 mega ohms or so. Typically, when the transistors were failing, they were reading approx. 120 ohms, and a "good" or "new" one usually reads open on my fluke. This 9 mega ohm gate to source transistor appeared to work "alright" (no further damage done anyway) at 5 volts or so, and even at 10 volts, but once I put 37 volts (skipped to it, should have incremented, doh!) it blew instantly (so that gate to source resistance read 120 ohms, same as the other "blown" ones). As I mentioned before, even though they aren't rated for it continuous, the "new" "good" transistors will take the 37 volts no problem, for at least 15 seconds (most time I've done it for, but I've never blown a "good" one with 37 volts before).

Seems like proof to me that ESD can damage a chip such that it might

*seem* ok, but will fail where an undamaged chip wouldn't.