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Test: Thermal Runaway & Thermal Stability


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20 Questions MCQ Test Analog Electronics | Test: Thermal Runaway & Thermal Stability

Test: Thermal Runaway & Thermal Stability for Electrical Engineering (EE) 2022 is part of Analog Electronics preparation. The Test: Thermal Runaway & Thermal Stability questions and answers have been prepared according to the Electrical Engineering (EE) exam syllabus.The Test: Thermal Runaway & Thermal Stability MCQs are made for Electrical Engineering (EE) 2022 Exam. Find important definitions, questions, notes, meanings, examples, exercises, MCQs and online tests for Test: Thermal Runaway & Thermal Stability below.
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Test: Thermal Runaway & Thermal Stability - 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).

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 1

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

Test: Thermal Runaway & Thermal Stability - Question 2

Which of the following are true?

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 2

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.

Test: Thermal Runaway & Thermal Stability - 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?

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 3

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

Test: Thermal Runaway & Thermal Stability - 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.

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 4

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

Test: Thermal Runaway & Thermal Stability - 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?

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 5

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

Test: Thermal Runaway & Thermal Stability - Question 6

Which of the following is true?

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 6

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.

Test: Thermal Runaway & Thermal Stability - Question 7

 The condition to be satisfied to prevent thermal runaway?

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 7

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.

Test: Thermal Runaway & Thermal Stability - Question 8

Thermal stability can be obtained by_________

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 8

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.

Test: Thermal Runaway & Thermal Stability - Question 9

Thermal stability is dependent on thermal runaway which is_________

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 9

 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.

Test: Thermal Runaway & Thermal Stability - Question 10

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

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 10

 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.

Test: Thermal Runaway & Thermal Stability - Question 11

Thermal runaway is_________

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 11

 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.

Test: Thermal Runaway & Thermal Stability - Question 12

The thermal runway is avoided in a self bias because_________

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 12

 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.

Test: Thermal Runaway & Thermal Stability - Question 13

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

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 13

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.

Test: Thermal Runaway & Thermal Stability - Question 14

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

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 14

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.

Test: Thermal Runaway & Thermal Stability - Question 15

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

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 15

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.

Test: Thermal Runaway & Thermal Stability - 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_________

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 16

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.

Test: Thermal Runaway & Thermal Stability - Question 17

 When the collector current is increased in a transistor_________

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 17

 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.

Test: Thermal Runaway & Thermal Stability - Question 18

 Which of the following are true?

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 18

 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.

Test: Thermal Runaway & Thermal Stability - Question 19

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

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 19

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.

Test: Thermal Runaway & Thermal Stability - Question 20

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

Detailed Solution for Test: Thermal Runaway & Thermal Stability - Question 20

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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