제24장 학습문제

제24장 Regulated Power Supplies
Self Test
1. Voltage regulators normally use
  1. Negative feedback
  2. Positive feedback
  3. No feedback
  4. Phase limiting

2. During regulation, the power dissipation of the passtransistor equals the collector-emitter voltages times the

  1. Base current
  2. Load current
  3. Zener current
  4. Foldback current

3. Without current limiting, a shorted load will probably

  1. Produce zero load current
  2. Destroy diodes and transistors
  3. Have a load voltage equal to the zener voltage
  4. Have too little load current

4. A current-sensing resistor is usually

  1. Zero
  2. Small
  3. Large
  4. Open

5. Simple current limiting produces too much heat in the

  1. Zener diode
  2. Load resistor
  3. Pass transistor
  4. Ambient air

6. With foldback current limiting the load voltage approaches zero and the load current approaches

  1. A small value
  2. Infinity
  3. The zener current
  4. A destructive level

7. A capacitor may be needed in a discrete voltage regulator to prevent

  1. Negative feedback
  2. Excessive load current
  3. Oscillations
  4. Current sensing

8. If the output of a voltage regulator varies from 15 to 14.7V between the minimum and maximum load current, the load regulation is

  1. 0
  2. 1 percent
  3. 2 percent
  4. 5 percent

9. If the output of a voltage regulator varies from 20 to 19.8V when the line voltage varies over its specified range, the source regulation is

  1. 0
  2. 1 percent
  3. 2 percent
  4. 5 percent

10. The output impedance of a voltage regulator is

  1. Very small
  2. Very large
  3. Equal to the load voltge divided by the load current
  4. Equal to the input voltge divided by the output current

11. Compared to the ripple into a voltage regulator, the ripple out of a voltage regulator is

  1. Equal in value
  2. Much larger
  3. Much smaller
  4. Impossible to determine

12. A voltage regulator has a ripple rejection of -60dB. If the input ripple is 1V, the output ripple is

  1. -60 mV
  2. 1 mV
  3. 10 mV
  4. 1000 V

13. Thermal shutdown occurs in an IC regulator if

  1. Power dissipation is too low
  2. Internal temperature is too high
  3. Current through the device is too low
  4. Any of the above occur

14. If a linear three-terminal IC regulator is more than a few inches from the filter capacitor, you may get oscillations inside the IC unless you use

  1. Current limiting
  2. A bypass capacitor on the input pin
  3. A coupling capacitor on the output pin
  4. A regulated input voltage

15. The 78XX series of voltage regulators produces an output voltage that is

  1. Positive
  2. Negative
  3. Either positive or negative
  4. Unregulated

16. The LM7812 produces a regulated output voltage of

  1. 3 V
  2. 4 V
  3. 12 V
  4. 78 V

17. A current booster is a transistor in

  1. Series with the IC regulator
  2. Parallel with the IC regulator
  3. Either series or parallel
  4. Shunt with the load

18. To turn on a current booster, we can drive its base-emitter terminals with the voltage across

  1. A load resistor
  2. A zener impedance
  3. Another transistor
  4. A current-sensing resistor

19. A phase splitter produces two output voltages that are

  1. Equal in phase
  2. Unequal in amplitude
  3. Opposite in phase
  4. Very small

20. A series regulator is an example of a

  1. Linear regulator
  2. Switching regulator
  3. Shunt regulator
  4. DC-to-dc converter

21. To get more output voltage from a buck switching regulator, you have to

  1. Decrease the duty cycle
  2. Decrease the input voltage
  3. Increase the duty cycle
  4. Increase the switching frequency

22. An increase of line voltage into a power supply usually produces

  1. A decrease in load resistance
  2. An increase in load voltage
  3. A decrease in efficiency
  4. Less power dissipation in the rectifier diodes

23. A power supply with low output impedance has low

  1. Load regulation
  2. Current limiting
  3. Line regulation
  4. Efficiency

24. A zener-diode regulator is a

  1. Shunt regulator
  2. Series regulator
  3. Switching regulator
  4. Zener follower

25. The input current to a shunt regulator is

  1. Variable
  2. Constant
  3. Equal to load current
  4. Used to store energy in a magnetic field

26. An advantage of shunt regulation is

  1. Built-in short-circuit protection
  2. Low power dissipation in the pass transistor
  3. High efficiency
  4. Little wasted power

27. The efficiency of a voltage regulator is high when

  1. Input power is low
  2. Output power is high
  3. Little power is wasted
  4. Input power is high

28. A shunt regulator is inefficient because

  1. It wastes power
  2. It uses a series resistor and a shunt transistor
  3. The ratio of output to input power is low
  4. All of the above

29. A switching regulator is considered

  1. Quiet
  2. Noisy
  3. Inefficient
  4. Linear

30. The zener follower is an example of a

  1. Boost regulator
  2. Shunt regulator
  3. Buck regulator
  4. Series regulator

31. A series regulator is more efficient than a shunt regulator because

  1. It has a series resistor
  2. It can boost the voltage
  3. The pass transistor replaces the series resistoR
  4. It switches the pass transistor on and off

32. The efficiency of a linear regulator is high when the

  1. Headroom voltage is low
  2. Pass transistor has a high power dissipation
  3. Zener voltage is low
  4. Output voltage is low

33. If the load is shorted, the pass transistor has the least power dissipation when the regulator has

  1. Foldback limiting
  2. Low efficiency
  3. Buck topology
  4. A high zener voltage

34. The dropout voltage of standard monolithic linear regulators is closest to

  1. 0.3 V
  2. 0.7 V
  3. 2 V
  4. 3.1 V

35. In a buck regulator, the output voltage is filtered with a

  1. Choke-input filter
  2. Capacitor-input filter
  3. Diode
  4. Voltage divider

36. The regulator with the highest efficiency is the

  1. Shunt regulator
  2. Series regulator
  3. Switching regulator
  4. DC-to-dc converter

37. In a boost regulator, the output voltage is filtered with a

  1. Choke-input filter
  2. Capacitor-input filter
  3. Diode
  4. Voltage divider

38. The buck-boost regulator is also

  1. A step-down regulator
  2. A step-up regulator
  3. An inverting regulator
  4. All of the above


Self Test Answers
문제번호answer 문제번호answer 문제번호answer 문제번호answer 문제번호answer
1a 2b 3b 4b 5c
6a 7c 8c 9b 10a
11c 12b 13b 14b 15a
16c 17b 18d 19c 20a
21c 22b 23a 24a 25b
26a 27c 28d 29b 30d
31c 32a 33a 34c 35a
36c 37b 38d      


Problems
1. A power supply has VNL = 15V and VFL = 14.5V. What is the load regulation?

2. A power supply has VHL = 20V and VLL = 19V. What is the line regulation?

3. If line voltage changes from 108 to 135V and load voltage changes from 12 to 12.3V, what is the line regulation?

4. A power supply has an output resistance of 2Ω. If the minimum load resistance is 50Ω, what is the load regulation?


5. In Fig. 24-4, Vin = 25V, RS = 22Ω, VZ = 18V, VBE = 0.75V, and RL = 100Ω. What are the values of output voltage, the input current, the load current, and the collector current?

Fig. 24-4

6. The shunt regualtor of Fig. 24-5 has these circuit values: Vin = 25V, RS = 15Ω, VZ = 5.6V, VBE = 0.77V, and RL = 80Ω. If R1 = 330Ω and R2 = 680Ω, What are the approximate values of output voltage, the input current, the load current, and the collector current?

Fig. 24-5

7. The shunt regualtor of Fig. 24-6 has these circuit values: Vin = 25V, RS = 8.2Ω, VZ = 5.6V, and RL = 50Ω. If R1 = 2.7kΩ and R2 = 6.2kΩ, What are the approximate values of output voltage, the input current, the load current, and the collector current?

Fig. 24-6


8. In Fig. 24-8, Vin = 20V, VZ = 4.7V, R1 = 2.2kΩ, R2 = 4.7kΩ, R3 = 1.5kΩ, R4 = 2.7kΩ, and RL = 50Ω. What are the output volltage? What is the power dissipation in the pass transistor?

Fig. 24-8

9. What is the approximate efficiency on Prob. 8?

10. In Fig. 24-15, the zener voltage is changed to 6.2V. What is the approximate output voltage?

Fig. 24-15

11. In Fig. 24-16, Vin can vary from 20 to 30V. What is the maximum zener current?

Fig. 24-16

12. If the 1-kΩ potentiometer of Fig. 24-16 is changed to 1.5kΩ, what are the minimum and maximum regulated output voltages?

13. If the regulated output voltage is 8V in fig. 24-16, what is the load resistance where current limiting starts? What is the approximate shorted-load current?


14. What is the load current in Fig. 24-33? The headroom voltage? The power dissipation of the LM7815?

Fig. 24-33

15. What is the output ripple in Fig. 24-33?

16. If R1 = 2.7kΩ and R2 = 20kΩ in Fig. 24-20, what is the output voltage?

Fig. 24-20

17. The LM7815 is used with an input voltage that can vary from 18 to 25V. What is the maximum efficiency? The minimum efficiency?


18. A dc-to-dc converter has an input voltage of 5V and an output voltage of 12V. If the input current is 1A and the output current is 0.25A, what is the efficiency of the dc-to-dc converter?

19. A dc-to-dc converter has an input voltage of 12V and an output voltage of 5V. If the input current is 2A and the efficiency is 80 percent, what is the output current?


20. A buck regulator has VREF = 2.5V, R1 = 1.5kΩ, and R2 = 10kΩ. What is the output voltage?

21. If the duty cycle is 30 percent and the peak voltage of the pulses to the choke-input filter is 20V, what is the regulated output voltage?

22. A boost regulator has VREF = 1.25V, R1 = 1.2kΩ, and R2 = 15kΩ. What is the output voltage?

23. A buck-boost regulator has VREF = 2.1V, R1 = 2.1kΩ, and R2 = 12kΩ. What is the output voltage?


Critical thinking

24. Figure 24-34 shows an LM317 regulator with electronic shutdown. When the shutdown voltage is zero, the transistor is cut off and has no effect on the operation. But when the shutdown voltage is approximately 5V, the transistor saturates. What is the adjustable range of output voltage when the shutdown voltage is zero? What does the output voltage equal when the shutdown voltage is 5V?

Fig. 24-34

25. The transistor of Fig. 24-34 is cut off. To get an output voltage of 18V, what value should the adjustable resistor have?

26. When a bridge rectifier and a capacitor-input filter drive a voltage regulator, the capacitor voltage during discharge is almost a perfect ramp. Why do we get a ramp instead of the usual exponential wave?

27. If the load regulation is 5 percent and the no-load voltage is 12.5V, what is the full-load voltage?

28. If th line regulation is percent and the low-line voltage is 16V, what is the high-line voltage?

29. A power supply has a load regulation of 1 percent and a minimum load resistance of 10 Ω. What is the output resistance of the power supply?

30. The shunt regulator of Fig. 24-6 has an input voltage of 35V, a collector current of 60mA, a load current of 140mA. If the resistance is 100Ω, what is the load resistance?

Fig. 24-6

31. In Fig. 24-10, we want current limiting to start at approximately 250mA. What value should we use for R4?

Fig. 24-10

32. Figure 24-12 has an output voltage of 10V. If VBE = 0.7V for the current-limiting transistor, what are the values of shorted-load current and the maximum load current? Use K=0.7 and R4 = 1Ω.

Fig. 24-12

33. In Fig. 24-35, R5 = 7.5kΩ, R6 = 1kΩ, R7 = 9kΩ, and C3 = 0.001uF. What is the switching frequency of the buck regulator?

34. In Fig. 24-16, the wiper is at the middle of its range. What is the output voltage?

Fig. 24-16


Troubleshooting

Use Fig. 24-35 for the remaining problems. In this set of problems, you are troubleshooting a switching regulator. Before you start, look at the OK row in the troubleshooting table to see the normal waveforms with their correct peak voltages. In this exercise, most of the troubles are IC failures rather than resistor failures. When an IC fails, anything can happen. Pins may be internally open, shorted, and so on. No matter what the trouble is inside the IC, the most common symptom is a stuck output. This refers to the output voltage beging stuck at either positive or negative saturation. If the input signals are OK, an IC with a stuck output has to be replaced. The following problems will give you a chance to work with outputs that are stuck at either +13.5 or -13.5V.

Fig. 24-35

35. Find Trouble 1.

36. Find Trouble 2.

37. Find Trouble 3.

38. Find Trouble 4.

39. Find Trouble 5.

40. Find Trouble 6.

41. Find Trouble 7.

42. Find Trouble 8.

43. Find Trouble 9.

troubleshooting
Trouble VA VB VC VD VE VF
ok NIMJKH
T1 PIUTIL
T2 TLVORO
T3 NQMVIT
T4 PNLTQL
T5 PVLTIL
T6 NQMORT
T7 PIUIQL
T8 pIULQV
T9 NQMORV

Fig. 22-35-1


Job Interview Questions
1.Draw any shunt regulator and tell me how it works.
2.Draw any series regulator and tell me how it works.
3.Explain why the efficiency of a series regulator is better than that of a shunt regulator.
4.What are the three basic types of switching regulators? Which one steps the voltage up? Which one produces a negative output from a positive input? Which one steps the voltage down?
5.In series regulator, what does headroom voltage mean? How is the efficiency related to headroom voltage?
6.What is the difference between the LM7806 and the LM7912?
7.Explain what line and load regulation mean. should they be high or low if you want a quality power supply?
8.How is the Thevenin or output resistance of a power supply related to the load regulation? For a quality power supply, should the output resistance be high or low?
9.What is the differnece between simple current limiting and foldback current limiting?
10.What does thermal shutdown mean?
11.The manufacturer of a three-terminal regulator recommands using a bypass capacitor on the input if the IC is more than 6 in from the unregulated power supply. What is the purpose of this capacitor?
12.What is the typical dropout voltage for the LM78XX series? What does it mean?