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"Half dead" semiconductors

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Dr. Dark Current

Sat Aug 25 2007, 02:24PM

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

Hi all, is it possible that a semiconductor (fet/igbt/whatever) gets partially damaged and it is still able to operate in some conditions, but with some "quirks" (less current/voltage ability, slower switching etc.)? I'm asking because I blew ONE igbt in a halfbridge, the other still seems ok at low power tests, should I replace both or is it ok to replace just the blown one?

Zum Beispiel

Sat Aug 25 2007, 02:52PM

Registered Member #514 Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295

I had a failure on my SSTC halfbridge where the upper fet died short cicuit. The other fet seemed to be fine when I tested it so I just replaced the upper one. Powered it up to full power and it ran for a few seconds and then the fuse blew. This time the lower fet died, the upper one seemed to be ok. I replaced both and haven't had failures since then...

Just my 2cents.

Sulaiman

Sat Aug 25 2007, 04:07PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I've had exactly the same experience with SSTC bridges.
One mosfet short-circuit, the other seems ok
but just replacing the faulty mosfet causes failure again.
I learnt (after loosing several mosfets) to replace both
even if one seems ok at low voltage.
e.g. ok with 24Vac in to bridge rectifier for mosfet bridge power,
failure with 240Vac in.

Dr. Dark Current

Sat Aug 25 2007, 04:35PM

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

ok I'll replace both, this is getting a bit expensive

but who cares if it works

Dalus

Sat Aug 25 2007, 05:07PM

Registered Member #639 Joined: Wed Apr 11 2007, 09:09PM
Location: The Netherlands, Herkenbosch
Posts: 512

Just measure the D to S resistence if it doesn't match a new fet within 30% it would fail.

ragnar

Sun Aug 26 2007, 12:15AM

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

Often MOSFETs will fail incompletely.

To measure the (apparent) intactness of a MOSFET, grab a multimeter and set it to resistance (10Mohm range)

If you put the positive probe on the source, and the negative probe on the drain, you should get a reading of about 5 - 15Mohm. This is very temperature dependent -- just warming a small MOSFET with your breath/hand will cause this reading to go down a few Mohms.

Now check that there's an approximately 0.5V drop across the same pins.

If a MOSFET has a D-S resistance of several hundred kohms, it's most likely popped, or about to.

Steve Conner

Sun Aug 26 2007, 08:58AM

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

Bear in mind that static charge on the gate pin can turn the MOSFET partly on and make it look bad. To be fair and consistent, you should connect the gate to the source or something.

ragnar

Sun Aug 26 2007, 09:50AM

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

Ah, thanks Steve -- I forgot to mention; before you make these measurements, smush a probes against all three pins like I do, to dissipate any residual charge.

Dr. Dark Current

Sun Aug 26 2007, 10:36AM

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

BlackPlasma wrote ...

Often MOSFETs will fail incompletely.
...

Ok you mention MOSFETs, but had anyone an IGBT fail incompletely?
I'll replace both anyways, but I don't feel like throwing away a fully functional device.

Marko

Sun Aug 26 2007, 11:08AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I don't think something as such can happen in ordinary circumstances, at least not from what I've seen. Mosfet will always test bad if they are dead. It's not enough to test the diode or off resistance, make sure you give them a full operation test.

I never had a mosfet or IGBT that probed functional to fail for reason of being 'damaged'.
If it was, it was obvious jet at testing. Failures of these semiconductors have a great positive feedback, after it goes just a bit short it will instantly dump large amount of power and vaporise completely.

Lots of IGBT's sold on ebay are from 'dead' bridges where one IGBT has failed, yet we buy them and they work fine.

If you are rich and afraid of using survivor IGBT's give them or sell to someone.

I would rather ask myself why the first IGBT in halfbridge failed after all.

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