Body
One of the other threads was discussing the destruction of MOSFETs,
which motivated me to write these few posts, where I normally just
lurk here.
First an aside in case you really do want Magick Smoke:
'Magick Smoke' Resistors are available off-the-shelf, and you will
have a hard time buying them with out jumping through hoops (I've
tried).
Anyone that has been around electronic devices for any length of time
know that when the devices fail, they tend to go up in smoke, leading
to the idea that electronic parts are run by Magic Smoke. Alas our
sterilized, sanitized, paranoid society is taking the *fun* out of
such things as hands on learning.
Vishay has upgraded their old line of Electro-Pyrotechnic Initiator
Chip Resistor (EPIC) (and Design Guide and App Notes) to the new
Massive Electro-Pyrotechnic Initiator Chip Resistor (MEPIC).
"Electro-Pyrotechnic Initiator Chip Resistor (EPIC)"
http://www.vishay.com/company/press/releases/2007/071219resistor/
Design Guides:
http://www.vishay.com/product?docid=53041&tab=documents
"Massive Electro-Pyrotechnic Initiator Chip Resistor (MEPIC)":
http://www.vishay.com/product?docid=53058
MEPIC resistors, also known as bridge resistors, are resistive
elements that convert electrical energy into heat energy in a precise
electro-thermal profile for the purpose of initiating a series of
pyrotechnic events in a controlled energetic reaction. [They go *BOOM*
on command, which is different than Rapid Spontaneous
Self-Disassembly.]
The new Vishay Sfernice resistor is optimized for electronic igniter
applications in automotive safety systems for the deployment of
airbags and other safety devices; digital blasting in mining
applications; and in fireworks applications for better synchronization
of fireworks, music, and special effects.
With firing energy down to 1.5 mJ and a typical ohmic range of 2 Ohms
(+/- 10 %), the device provides designers with very predictable,
reproducible, and reliable behavior.
Offered in the standard 0805 case size for the wraparound and flip
chip versions, with other sizes available upon request, the resistor
features easy set-up of firing levels, and is compatible with various
pyrotechnic compositions.
Offering ESD withstanding to 25 kV without extra protection, the MEPIC
resistor's performance meets no fire/all fire conditions and the
requirements of USCAR, AKLV16, and major car manufacturer standards.
The device is RoHS-compliant and conforms to Vishay "Green" standards.
[Is it not great that Fuzes are 'Green'?]
Almost lost in the mists of time is that in the past manufactures made
military specific parts, before someone thought that Commercial Off
The Shelf technology (COTS) was a good idea (it wasn't). National
Semiconductor's, now part of TI, Application Note #761:Electronic
Fuzing covers the basic terminology of Fuzing:
http://www.ti.com/lit/an/snoa217/snoa217.pdf
Fuzing mechanisms are devices used to "safe", "arm" and detonate
explosive military munitions (such as missiles, mines, demolition
charges, explosive shells ranging in size from 20 mm to 16 inches,
unguided bombs and various submunitions). Early electronic fuzes
developed for 5-inch naval air defense...
Sadly even tho I'm it good standing with my Vishay Rep. Firm, samples
of (M)EPIC parts are restricted to people that can show good cause for
getting them, so Homeland Security can relax. To bad, think of the fun
the people at Hack A Day or Make Magazine could have with some
these...as well as those great educational experiences that are being
lost...
----------------------------------------
Now to my real point:
Q: Why does my FET, IGBT, SCR keep going up in smoke?
A: Spot Heating
A common problem I see in message boards is that someone's circuit
failed, and went up in smoke. People in the more esoteric realm's
blame this on things like "Subtle Energy" overload and other such
minutia. Here is the far more realistic explanation:
The very old "GE SCR Manual" goes into all of the Gorey details of
what is happening inside the part, when the "Magick Smoke comes out",
as it is unlikely you have the Manual at hand, in a nutshell:
What lets the Magick Smoke out of IGBTS, FETS and SCRs in most cases
is turn them on too slowly, causing 'Spot Heating' of the die.
Think of a FET as hundreds of thousands, possibly millions, of very
small resistors all in parallel, where each one can be turned on and
off individually. The 'resistors' closest to the gate turn on first,
and as the gate potential spreads across the die the rest turn on. The
ones farthest from the gate turn on last.
With a slow gate turn on, a few of the small resistors nearest the
gate are trying to carry all of the load, which they can't do, so they
burn up, but the device does not fail quite yet. The next time the
device is turned on, which may be only milliseconds away depending on
your switching frequency, or days away depending on the application,
some more of the resistors further in burn up. When the point is
reached that there is simply not enough of the 'resistors' left to
carry the load is when the Magick Smoke escapes, and the part dies a
catastrophic death.
This is why the parts generally run "for a while" before failing. If
it fails as soon as you fire it up the first time, you either had a
catastrophic short in the load, possibly shorted caps that take a bit
of time to 'wake up' before they hold a charge, generally fixed with
'Soft Start', or the gate drive REALLY sucked big time.
There needs to a be a few *amps* of current pumped in the gate of the
larger parts, for short periods of time, to get the gate potential to
spread across the entire die as fast as possible.
You also want to get the thing turned off as fast as possible.
If you are not familiar with the concept of Magick Smoke, this is
where all electronic parts run on Magick Smoke, because once the smoke
comes out of the part, it no longer runs...
--
http://blog.softwaresafety.net/
http://www.designer-iii.com/
http://www.wearablesmartsensors.com/