Holley 4 Brrl gm ecm

[Garfield Willis]

On Mon, 22 May 2000 03:01:13 -0400, Chris Conlon <synchris at ricochet.net>
wrote:

>Now I freely admit that when it comes to anything high power I'm
>a mosfet guy, but the times I've fried bipolars (not *that* many)
>they all ended up as either dead shorts (which quickly led to) (or)
>opens. But, if the driver is a Darlington, it's possible depending
>on how it's set up to fry the driven/"big" transistor and leave
>the driver/"small" transistor alive. And further depending on how
>things are set up (and what pin opens), you could maybe possibly
>be able to still drive a LED, running off the driver only.

Very true. Your idea about how a Darlington might fail is another good
possibility. If the collector on the main driver opened, and the
base-emitter remained intact, you're left with a small transistor where
there used to be a big one, just as you said. BTW, they *are* almost
certainly constructed as darlingtons internally.

I think I've mentioned this here before, but when I said earlier that
bipolar devices "current hog", I was referring to an interesting
distinction/phenom betwixt bipolar and mosfet that's useful to know. In
general, the bipolars have a negative tempco while the mosfets have a
positive one. That's why it's a no-no to parallel a bunch of bipolar
drivers together, without VERY careful matching and circuit design,
whereas it's standard practice in power stuff now to see bunches of big
mosfets brute-force wired in parallel to increase drive capability. In
fact, this is one of the main reasons why mosfets have virtually taken
over the high-power driver arena. Hence the ruleOthumb, bipolars current
hog, mosfets current share.

Here's how this works: if you have two resistors in parallel, and they
have negative tempcos, the one that gets a bit hotter will hog the
current (it's R will drop more than the other, it will get more of the
current, it will then get hotter still, etc.), and thermally runaway. If
OTOH you have two resistors in par with pos tempcos, the one that heats
a tad more will *share* the current; it's R will *rise* a tad more, so
the current thru it will drop some and the other devices will take up
the load. Works that way with bipolars vrs. mosfets.

Now in a Darlington, as much as they want to make the final driver
devices one single transistor for this reason, beyond a certain size,
you get *localized* current hogging (hot spots) in that one largo
transistor, so they rely on the uniformity of multiple transistors
within the same die, and they DO parallel up transistors. Either way,
it's quite possible for some of those to blow open, leaving you with not
much of an overall big driver anymore. I've seen pix of failed large
bipolar devices covered with bomb craters; the device still passed
current, but the gain was pretty low. :)  Same thing would happen if as
Chris suggested, the final driver totally failed open, and the predriver
was still intact and able to conduct thru the base-emitter of the final.
You end up with a whimp of a transistor.

Gar


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