Test: Thermal Runaway & Thermal Stability - Electrical Engineering (EE) MCQ

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

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.

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
=200-25/20=8.75W.
Now, V_CEI_C = 8.75/4=2.19A.

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.

 The collector current of a fixed bias transistor is IC= β(VCC-VBE)/RB. 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.

 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.

 The self destruction of a transistor due to increase temperature is called thermal run away. It is avoided by the negative feedback produced by the emitter resistor in a self bias. The IC which is responsible for the damage is reduced by decreased output signal.

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.

The self destruction of a transistor due to increase temperature is called thermal run away. It is avoided by the negative feedback produced by the base resistor in a collector to base bias. The I_C which is responsible for the damage is reduced by decreased output signal.

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.