Test: Thermal Runaway & Thermal Stability - UPSC MCQ

We know, T_J-T_A=HP_D
T_J=T_A+HP_D=27+8*3=51°C.

P_D=(T_J-T_A)/ H_J-C +H_C-S +H_S-A
=200-40/0.5+0.6+1.5=61.5W.

P_D =(T_J-T_A)/ H
=200-75/20=6.25W.
Now, I_C = 6.25/4=1.56A.

H_J-C is thermal resistance between junction and case and H_C-A is thermal resistance between case and ambient. The circuit designer has no control over H_J-C. So, a proper approach to dissipate heat from case to ambient is through heat sink.

P_C is the power dissipated at the collector junction. T_J is junction temperature which varies. The difference between these temperatures is directly proportional to the power dissipation. Here, Q is called as thermal resistance which is proportionality constant.

As power transistors handle large currents, they always heat up during operation. Generally, power transistors are mounted in large metal case to provide a large area from which the heat generated by the device radiates.

 Thermal runaway is a self destruction process in which an increase in temperature creates such a condition which in turn increases the temperature again. This uncontrolled rise in temperature causes the component to get damaged.

 Thermal runaway is a self destruction process in which an increase in temperature creates such a condition which in turn increases the temperature again. This uncontrolled rise in temperature causes the component to get damaged.

Before the feedback is applied, when the temperature is increased, the reverse saturation increases. The collector current also increases. When the feedback is applied, the drop across the emitter resistor increases with decreasing collector current and the thermal runway too.

Before the feedback is applied, when the temperature is increased, the reverse saturation increases. The collector current also increases. When the feedback is applied, the base current increases with decreasing collector current and the thermal runway too.

The T1 transistor is having more power dissipation as it is drawing 0.55A. When power dissipation increases, the temperature increases and this leads to the ultimate further increase in the current drawn by T1. The current drawn by T2 will be reduced as the sum of currents drawn by T1 and T2 should be constant.

 As the collector current is increased, the emitter releases more number of electrons. This causes more collisions of electrons at collector. This happens in a cycle and produces such a condition in which temperature is further more increased.

 The T_J is called as junction temperature which varies and T_A is called as the ambient temperature which is fixed. The difference between these temperatures is directly proportional to the power dissipation. Here, θ is called as thermal resistance which is proportionality constant.

The power dissipation is directly proportional to thermal resistance. We have, T_J – T_A = θP_d in which we can observe θ ∝ 1/P_d. So, a device with low power dissipation has high thermal resistance.

The collector current of a fixed bias transistor is I_C= β(V_CC-V_BE)/R_B. When the temperature is increased, the reverse saturation increases. The collector current also increases. This in turn increases the current again which leads to damage of transistor.

We know, T_J-T_A=HP_D
T_J=T_A+HP_D=27+8*3=51°C.

P_D =(T_J-T_A)/ H
=200-75/20=6.25W.
Now, I_C = 6.25/4=1.56A.

The collector current of a fixed bias transistor is I_C= β(V_CC-V_BE)/R_B. When the temperature is increased, the reverse saturation increases. The collector current also increases. This in turn increases the current again which leads to damage of transistor.

A field-effect transistor does not exhibit thermal runaway; the thermal runway is mostly a feature of devices with bipolar charge carriers

Thermal runaway takes place when V_CE > 1/2 V_CC 

So, to eliminate thermal runaway we should have

Concept:

1) The thermal runway is not possible in FET because as the temperature of the FET increases, the mobility decreases, i.e. if the Temperature (T) ↑, the carries Mobility (μn or μ­p) ↓, and Ips↓

2) Since the current is decreasing with an increase in temperature, the power dissipation at the output terminal of a FET decreases or we can say that it’s minimum.

So, there will be no Question of thermal Runway at the output of the FET.

Important Point

• The thermal runaway takes place in a BJT.
• Thermal Runway in BJT is a process of self-damage of BJT because of overheating at the collector junction due to an increase in Ic with Ico
• If T↑, then Ico (Reverse separation current) ↑, which results in an increase in the collector current, i.e. Ic ↑.
• Power dissipation at the collector junction increases in the form of heat which again raises the temperature and the cycle continues.
• If the above cycle becomes repetitive then the collector junction gets overheated and thereby thermal runway takes place.