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 nut shell:

What lets the Magick Smoke out of IGBTS, FETS and SCRs in most cases is turn them on to 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...

'Magic Smoke' Resistors available off-the-shelf

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.

While places like Analog Devices' Engineering University are great for learning theory and hands on labs with their hardware, sometimes it is far more educational, and down right *fun*, to learn why things went wrong. Only experience is going to teach one, the important debugging skill, of the differences in smells between burning resistors, burning capacitors, and burning circuit board material. Also teaches the importance of wearing protective eyewear (never know when a backwards part might try to impale itself into the ceiling or ones face if it is closer), and having a electrical rated fire extinguisher next to the workbench.

What brings us to Magic Smoke, is that 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).

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 Spontanious 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:

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...