Test: Thermal Runaway & Thermal Stability


20 Questions MCQ Test Electronic Devices | Test: Thermal Runaway & Thermal Stability


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QUESTION: 1

 For a given transistor, the thermal resistance is 8°C/W and for the ambient temperature TAis 27°C. If the transistor dissipates 3W of power, calculate the junction temperature (TJ).

Solution:

We know, TJ-TA=HPD
TJ=TA+HPD=27+8*3=51°C.

QUESTION: 2

Which of the following are true?

Solution:

The TJ¬ is called as junction temperature which varies and TA 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.

QUESTION: 3

 A silicon power transistor is operated with a heat sink HS-A=1.5°C/W. The transistor rated at 150W (25°C) has HJ-C=0.5°C/W and the mounting insulation has HC-S=0.6°C/W. What maximum power can be dissipated if the ambient temperature is 40°C and (TJ)MAX=200°C?

Solution:

PD=(TJ-TA)/ HJ-C +HC-S +HS-A
=200-40/0.5+0.6+1.5=61.5W.

QUESTION: 4

 The total thermal resistance of a power transistor and heat sink is 20°C/W. The ambient temperature is 25°C and (TJ)MAX=200°C. If VCE=4V, find the maximum collector current that the transistor can carry without destruction.

Solution:

 PD =(TJ-TA)/ H
=200-25/20=8.75W.
Now, VCEIC = 8.75/4=2.19A.

QUESTION: 5

The total thermal resistance of a power transistor and heat sink is 20°C/W. The ambient temperature is 25°C and (TJ)MAX=200°C. If VCE=4V, find the maximum collector current that the transistor can carry without destruction. What will be the allowed value of collector current if ambient temperature rises to 75°C?

Solution:

PD =(TJ-TA)/ H
=200-75/20=6.25W.
Now, I= 6.25/4=1.56A.

QUESTION: 6

Which of the following is true?

Solution:

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

QUESTION: 7

 The condition to be satisfied to prevent thermal runaway?

Solution:

PC is the power dissipated at the collector junction. TJ 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.

QUESTION: 8

Thermal stability can be obtained by_________

Solution:

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.

QUESTION: 9

Thermal stability is dependent on thermal runaway which is_________

Solution:

 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.

QUESTION: 10

Which of the following biasing techniques are affected by thermal runaway?

Solution:

 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.

QUESTION: 11

Thermal runaway is_________

Solution:

 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.

QUESTION: 12

The thermal runway is avoided in a self bias because_________

Solution:

 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.

QUESTION: 13

 When the temperature is increased, what happens to the collector current after a feedback is given?

Solution:

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.

QUESTION: 14

The thermal runway is avoided in a collector to base bias because_________

Solution:

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 IC which is responsible for the damage is reduced by decreased output signal.

QUESTION: 15

When the temperature is increased, what happens to the collector current after a feedback is given?

Solution:

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.

QUESTION: 16

Discrete transistors T1 and T2 having maximum collector current rating of 0.75A are connected in parallel as shown in the figure. This combination is treated as a single transistor to carry a single current of 1A, when biased with a self bias circuit. When the circuit is switched ON, T1 had draws 0.55A and T2 draws 0.45A. If the supply is kept ON continuously, it is very likely that_________

Solution:

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.

QUESTION: 17

 When the collector current is increased in a transistor_________

Solution:

 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.

QUESTION: 18

 Which of the following are true?

Solution:

 The TJ is called as junction temperature which varies and TA 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.

QUESTION: 19

When the power dissipation increases in a transistor, the thermal resistance_________

Solution:

The power dissipation is directly proportional to thermal resistance. We have, TJ – TA = θPd in which we can observe θ ∝ 1/Pd. So, a device with low power dissipation has high thermal resistance.

QUESTION: 20

Which of the following biasing techniques are prone to thermal runaway?

Solution:

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.

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