Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
References: <email@example.com> <firstname.lastname@example.org> <4KfR9.92768$hK4.email@example.com>
Subject: Re: Thermal runaway: please settle this dispute!
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Date: Sat, 4 Jan 2003 09:35:42 -0000
NNTP-Posting-Date: Sat, 04 Jan 2003 09:35:52 GMT
Lukas Louw wrote:
>> 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
It is nothing to do with base current. The transistor is voltage
> thus causing more Ic, thus more heat, hence thermal runaway.
It is the variation in Is(T) with temp that causes the increase in
current at the *same* value of Vbe.
> 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..........
As I noted in my other post, this is correct, although there may be a
bit of confusion here on the term "Vbe". If the voltage applied to the
base emitter is fixed, it *is* the Vbe. That is, Vbe can't change!
What is really meant here is that at some forced, *fixed* current of
base emitter voltage, i.e. say Vbei, if temperature is increased and
Vbei is adjusted such that the current remains constant, than Vbei will
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