Will a true class B have thermal runaway? - Page 2 - diyAudio
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Old 11th January 2004, 12:16 PM   #11
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Quote:
Originally posted by Tube_Dude
If you stop to drive the heavy load the transistors will return
to near zero standing current...and will cool down!
so you don't get thermal run-away if the amp cools down at idle?

[/B][/QUOTE]Thermal run away is when the current in the transistor increase...the transistor become hoter ...as the transistor become hoter the current increase...and the process ends with the self destruction of the device. [/B][/QUOTE]


yeah. that's my understanding as well and it has no mentioning of being conditional on cool idling.

and from it a class C amp can certainly enter into a thermal run-away.
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Old 11th January 2004, 12:24 PM   #12
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Quote:
Originally posted by millwood


so you don't get thermal run-away if the amp cools down at idle?


If the amp cools down at idle after being pushed to suplying a very heavy load and after being very hot...of course it is not in thermal run away.

Thermal run away is a self induced issue.
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Old 11th January 2004, 12:28 PM   #13
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Quote:
Originally posted by Tube_Dude



If the amp cools down at idle after being pushed to suplying a very heavy load and after being very hot...of course it is not in thermal run away.

Thermal run away is a self induced issue.
Let me ask the question differently:

suppose that you have an amp that will always cool down at idle but when applied a constant signal (say 1vrms sine wave) starts to heat up more and more until complete destruction.

Do you consider such an amp to be in a thermal run-away?
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Old 11th January 2004, 12:31 PM   #14
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I can think of no circumstance where a class-b output stage could suffer thermal runaway. It could overheat due to driving the load but not runaway.

And NFB will not elimininate crossover distortion.
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Old 11th January 2004, 12:38 PM   #15
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Quote:
Originally posted by millwood


Let me ask the question differently:

suppose that you have an amp that will always cool down at idle but when applied a constant signal (say 1vrms sine wave) starts to heat up more and more until complete destruction.

Do you consider such an amp to be in a thermal run-away?
No...the amp has exced the dissipation of the devices but from a cause external to the devices themselfs ( the over dissipation caused by the heavy load driving)..and not for the self thermal instability of the device.
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Old 11th January 2004, 12:39 PM   #16
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Quote:
Originally posted by Richard C
I can think of no circumstance where a class-b output stage could suffer thermal runaway. It could overheat due to driving the load but not runaway.

And NFB will not elimininate crossover distortion.

Amen!
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Old 11th January 2004, 12:45 PM   #17
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I think you are talking about two different types of thermal
runaway.

I think I see millwoods point here. If the transistors have
insufficient cooling for the signal we feed them, they will not
cool down entirely during the idle half cycle, so their average
temperature will slowly rise and they will amplify more and
more making them run hotter and hotter. This may or may
not lead to a runaway, I guess. Since the amplification increases
feedback will try to counteract this. Beta droop in the transistors
will eventually start limiting the current. So, I suppose it depends
on how good the heatsinking is. However, I don't think this
phenomenon is covered by the term "thermal runaway" as it
is commonly used, although it certainly is a kind of thermal
runaway.

The usual meaning of thermal runaway, as I understand it, is
that if the transistor gets hotter for some reason, be it because
of the signal or the room temperature increasing or whatever,
the bias current will increase. More precisely, suppose we keep
the bias voltage fixed and heat the transistor somewhat. The
bias current will then increase slightly due to the positive temp.
coefficient of Ic vs. Vbe. This increased bias current will increase
the average temperature of the transistor, leading to yet
higher bias current, leading to yet higher current etc. etc.
This is why there usually is and should be a temperature
compensation circuit that senses the average temperature
and lowers the bias voltage accordingly when the temperature
rises. I cannot see how this type of thermal runaway could
occur in a class B amp, since the bias voltage is 0V.
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Old 11th January 2004, 12:47 PM   #18
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Quote:
Originally posted by Tube_Dude


No...the amp has exced the dissipation of the devices but from a cause external to the devices themselfs ( the over dissipation caused by the heavy load driving)..and not for the self thermal instability of the device.

but it fits your definition of thermal runaway, word for word.

Quote:
Thermal run away is when the current in the transistor increase...the transistor become hoter ...as the transistor become hoter the current increase...and the process ends with the self destruction of the device.
and you will note that in the above definition, it gives no presumptions on amp topology.
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Old 11th January 2004, 01:00 PM   #19
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Quote:
Originally posted by millwood



but it fits your definition of thermal runaway, word for word

No because if you stop the suplying of current to the heavy load the transistors will cool down...and in a case of thermal run away even if you stop the suplying of current to the load the transistors will continue becoming hoter and self destruct(even if you have stoped the supliyng of current to the load!)...

I hope have been clear now...
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Old 11th January 2004, 01:35 PM   #20
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For what it is worth, I agree with Tube_Dude.

A class C amplifier (or even a class B amplifier that is kept at class B for all temperatures, with bias voltage going down if temperature rises to compensate for lower Vbe) will not suffer from thermal runaway. This has nothing to do with loading or overloading the amplifier. Of course overloading may cause breakdown of the output transistors because of high temperature, but that high temperature was deliberately caused by the overloading itself (external cause).
Thermal runaway is a phenomenon where the dissipation and temperature increases due to the fact that the temperature just has been increased (internal cause). As an example, this might happen with bipolar class AB and class A amplifiers without proper temperature compensation. Here the bias current will increase when temperature increases and this higher bias current causes additional dissipation to increase the temperature even more. Thermal runaway is born.
We have to distinguish between load current and resulting dissipation that will not be the cause itself of thermal runaway, and the idle current (or bias current) that will flow allways and can cause thermal runaway when it is not kept within certain boundaries. Of course it can happen that thermal runaway is started by a big load current that pushes up the temperature with the result that the idle current increases (bad compensation). But then the compensation was already bad and still it is the continuously increasing idle current that causes the thermal runaway and not the (stable) load current.
In a class C amplifier, as described at the start of the thread, this will never happen because no temperature increase will move this output stage into a area where an idle current will flow and certainly not a big idle current.

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