# Thermal runway

Discussion in 'Homework Help' started by zamansabbir, Jun 1, 2008.

1. ### zamansabbir Thread Starter Member

May 27, 2008
15
0
Hi
i got question in my previous year question paper that Define thermal runway in power amplifier section but could not able to get myself satisfied with the answer that got from my book.

from Sabbir

2. ### Dave Retired Moderator

Nov 17, 2003
6,960
170
What is the definition in your book?

Dave

3. ### circuitashes Active Member

May 13, 2008
37
0
Which specific part of the definition in your book are you not satisfied with? You need to give us that information so we can give you the needed help.

4. ### zamansabbir Thread Starter Member

May 27, 2008
15
0
i just want the definition a proper one to understand the thermal runway .
i just could not able to understand what my book said.

5. ### circuitashes Active Member

May 13, 2008
37
0
If its a definition you want, you can check this link: http://en.wikipedia.org/wiki/Thermal_runaway
However, i do not think what you need is a definition. Rather, what you need is a good understanding of the underlying principles described in the definition. You should do some reading on the ff:
1. The effect a change in temperature has on the resistance of a semiconductor, and by extension on a P-N junction diode.
2. The 'diode-equation' and how a variation in temp. affects the other parameters in that equation.
3. The I-V characteristics of a P-N junction, and how a change in temp. affects the characteristics (specifically, how the slope is affected).

This is one clue you can start with: For a semiconductor, an increase in temp. results in a decrease in resistance.
For the diode eqn. check this link: http://www.allaboutcircuits.com/vol_3/chpt_3/1.html
To see how thermal runaway can be dealt with, check this link: http://www.allaboutcircuits.com/vol_3/chpt_4/10.html

6. ### mofokeng New Member

May 14, 2008
3
0
thermal runaway is an increase in collector current due to a small increase in base current of a bjt transistor which leads to a Rc heat up.To overcome thermal runaway we connect an emitter resistor in parallel to Rc.