# Electronic Devices and Circuits Questions and Answers – Thermal Stability

This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Thermal Stability”.

1. For a given transistor, the thermal resistance is 8°C/W and for the ambient temperature TA is 27°C. If the transistor dissipates 3W of power, calculate the junction temperature (TJ).
a) 51°C
b) 27°C
c) 67°C
d) 77°C

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

2. Which of the following are true?
a) TJ-TA=θpd
b) TJ-TA=θ/pd
c) TJ-TA=θ+pd
d) TJ-TA=θ-pd

Explanation: 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.

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?
a) 70.6W
b) 61.5W
c) 37.8W
d) 56.9W

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

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.
a) 3.67A
b) 7.56A
c) 2.19A
d) 4.16A

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

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?
a) 3.67A
b) 7.56A
c) 2.19A
d) 1.56A

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

6. Which of the following is true?
a) HC-A = HJ-C – HJ-A
b) HC-A = HJ-C + HJ-A
c) HJ-A = HJ-C – HC-A
d) HJ-A = HJ-C + HC-A

Explanation: 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.

7. The condition to be satisfied to prevent thermal runaway?
a) ∂PC/∂TJ > 1/Q
b) ∂PC/∂TJ < 1/Q
c) ∂PC/∂TJ > 1/Q
d) ∂PC/∂TJ < 1/Q

Explanation: 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.

8. Thermal stability can be obtained by_________
a) shifting operating point
b) increasing power supply
c) heat sink
d) decreasing current at collector

Explanation: 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.

9. Thermal stability is dependent on thermal runaway which is_________
a) an uncontrolled positive feedback
b) a controlled positive feedback
c) an uncontrolled negative feedback
d) a controlled negative feedback

Explanation: 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.

10. Which of the following biasing techniques are affected by thermal runaway?
a) self bias
b) collector to base bias
c) fixed bias
d) the biasing technique is identified by temperature effect

Explanation: 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.

Sanfoundry Global Education & Learning Series – Electronic Devices and Circuits.

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