Thermal runaway and thermal stability

PC = VCB IC
A d-c power PC is developed across the collector junction.
VCE = VCB + VBE

This d.c power will be dissipated in the form of heat.
The temperature of the junction will become higher than the surrounding temp. Due to increase in the temperature of collector junction, ICO will increase, VBE will decrease.

The regenerative heating cycle thus produced is called thermal runaway.
In a self-biased CE amplifier a d.c power PC = VCEIC is developed at the collector junction of the transistor. With no signal present, this d.c power is completely dissipated in the form of heat.
For dissipating power (heat) to the surrounding the junction temp because higher than the surrounding temperature. Increase in junction temp is accompanied by an increase in collector current IC. If increase in collector current IC produces further increase in power dissipation, junction temp will rise further.
The regenerative cycle thus produced is called thermal runaway. If the phenomena of thermal runaway is allowed to continue, the device may burn out or may be destroyed completely.

Thermal resistance:- Under steady state or equilibrium condition, the temp rise at the collector junction is proportional to power dissipated at the collator junction. 

Or        Tj – TA = θPD---------(1)
Where  Tj --> junction temp in ˚C

            TA --> Ambient temp. (Temp of the surrounding)in ˚C

            PD --> Power dissipated in the collector junction.

            θ --> proportionality constant called thermal resistance.

∂Tj – θ = θ ∂PD

 

Condition for avoiding thermal runaway:


 

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