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Mysterious MOSFET/IGBT deaths

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Dr. Dark Current

Sun Sept 21 2008, 10:41AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Post any of your mysterious transistor deaths here. By that I mean "cold" deaths, when the device was operated well within its safe operating area. Maybe we can find an explantation for these deaths.

While designing the demo Jacobs ladder, I was operating a flyback transformer with halfwave doubler on the output. I was using 22A MOSFETs in halfbridge powered from 330VDC, driven by a GDT (22ohm turn-on gate resistor, turn off shorted by antiparallel diode). It was putting out approx. 300W of power, when one of the transistors died cold, I guess with ~10-15A peak current.
The gate signal was clean with minimal deadtime, and clamped by 18V zeners.
I have no idea why it popped.

Do you think that a RC damper on the bridge output gives any advantages?

Will

Sun Sept 21 2008, 11:59AM

Registered Member #57 Joined: Thu Feb 09 2006, 05:28AM
Location: San Diego, CA
Posts: 27

Were the FETs you used always handled with proper ESD handling precautions?

I've had some cold deaths before and came to the conclusion that they were ESD damaged.

Dr. Dark Current

Sun Sept 21 2008, 12:19PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Will wrote ...

Were the FETs you used always handled with proper ESD handling precautions?

I've had some cold deaths before and came to the conclusion that they were ESD damaged.

They weren't handled with any special precautions, but considering that the device itself has a few pF at most of self capacitance and the gate is a few nF, it should survive several kV ESD discharge no problem.
I think the ESD becomes a problem for sensitive components (when talking about FETs, small low power/RF ones)

aonomus

Sun Sept 21 2008, 02:11PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

I recall that testing IRF640 MOSFETs by hand using the diode test function on a multimeter showed 2/10 of my new mosfets had an abnormally high (relatively speaking, 0.1-0.2 compared to 0.0 average) forward voltage. I put those two parts in a separate portion of my MOSFET storage in anticipation of failure, fortunately they were $0.60 a piece so I'm not out much.

Steve Ward

Mon Sept 22 2008, 12:10AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

It was probably a voltage related failure when running 330V bus.

Dr. Dark Current

Mon Sept 22 2008, 09:37AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Steve Ward wrote ...

It was probably a voltage related failure when running 330V bus.

They were 500V FETs so I don't see a problem here.

ragnar

Mon Sept 22 2008, 10:33AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Dr. Kilovolt wrote ...

I was using 22A MOSFETs in halfbridge powered from 330VDC, driven by a GDT (22ohm turn-on gate resistor, turn off shorted by antiparallel diode). It was putting out approx. 300W of power, when one of the transistors died cold, I guess with ~10-15A peak current.

500V / 330V, 22A / 10A, I doubt this is within the SOA. Do you have an oscilloscope?

rp181

Mon Sept 22 2008, 12:39PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529

wow, coincdence, i was going to post a thread. My IGBT is being used for a boost converter, 14v ~3A in. The IGBT is fast, 600v 60A. The IGBT doesnt burn, but it gets really hot fast. Ive used this IGBT for a converter before, and it didnt even get warm. Last time it did this, i was using a normal (not fast diode) diode, when i replaced it it works fine. I replaced it with a ultrafast piece, any other reasons this could happen?

Dr. Dark Current

Mon Sept 22 2008, 02:28PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Matt Bingham wrote ...

500V / 330V, 22A / 10A, I doubt this is within the SOA. Do you have an oscilloscope?

Well the device will never see 330V at 10A

It does see 330V with near-zero current or 10A @ forward voltage drop (few V at most).
Edit: Well, it DOES see 330V@10A at the turn-off transition but this is just for a few tens of ns, the device is rated at 88A peak (iirc).

Yes I do have an O-scope, but it always goes funky around switched circuits (displays enormous spikes at the transitions) and often gets "overloaded" so I can't see anything at all.

GeordieBoy

Mon Sept 22 2008, 06:06PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

> displays enormous spikes at the transitions

Really? Most oscilloscopes will round edges and suppress narrow spikes by limiting dv/dt if their bandwidth is insufficient to faithfully reproduce switching waveforms.

If you are seeing enormous current or voltage spikes _AND_ have "unexplained" device failures the first thing to do is to get a decent storage scope and set of probes so you can see what is really going on.

-Richie,

PS. Always ensure that the scope probe ground clip is attached to a ground point in the system under test that is as close as possible to the voltage being measured. This minimises "ground bounce". At very high frequencies (or when probing noisy flyback converters!) it is best to un-clip the flying ground lead from the probe and solder a ground pin to the earthed sleeve around the tip of the probe to minimise the area in the measurement loop. I have found this method very useful for testing off-line flyback PSUs when using properly compensated probes with a decent DSO.

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