# Thermal Stability MCQ’s

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

1. 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

2. 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

3. 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

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

5. 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

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

7. 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

8. 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

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

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