From: "Lukas Louw"
References: <email@example.com> <firstname.lastname@example.org>
Subject: Re: Thermal runaway: please settle this dispute!
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NNTP-Posting-Date: Fri, 03 Jan 2003 12:54:24 GMT
Organization: AT&T Worldnet
Date: Fri, 03 Jan 2003 12:54:24 GMT
> Hi, vbe does fall at 2mv/degC but that has nothing to do with thermal
> runaway, which is caused by the increase in collector-base leakage
> current, which subsequently gets multiplied by the devices gain
> (hfe).Modern silicon devices dont really suffer much from this
> problem, its an issue of yesteryears germainium transistors.
Yes, but consider a typical Class AB power amplifier. With fixed base bias
voltages, the reduced Vbe at elevated temps causes more base current, thus
causing more Ic, thus more heat, hence thermal runaway. The Vbe multiplier
that Phil had mentioned, mounted on the same heatsink as the power devices,
also makes use of its own reduced Vbe at higher temps, to REDUCE the output
device base bias voltage, thus counteracting thermal runaway.
So in most applications, I'd say that Vbe falling with temp is the #1 cause
of thermal runaway....... Silicon BJTs suffer from this big time. Consider
that from 15 C to say, 60 C, their Vbe will drop by 90 mV, a rather large
percentage of nominal 600/700 mV..........