2. During regulation, the power dissipation of the passtransistor equals the collector-emitter voltages times the
3. Without current limiting, a shorted load will probably
4. A current-sensing resistor is usually
5. Simple current limiting produces too much heat in the
6. With foldback current limiting the load voltage approaches zero and the load current approaches
7. A capacitor may be needed in a discrete voltage regulator to prevent
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
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
10. The output impedance of a voltage regulator is
11. Compared to the ripple into a voltage regulator, the ripple out of a voltage regulator is
12. A voltage regulator has a ripple rejection of -60dB. If the input ripple is 1V, the output ripple is
13. Thermal shutdown occurs in an IC regulator if
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
15. The 78XX series of voltage regulators produces an output voltage that is
16. The LM7812 produces a regulated output voltage of
17. A current booster is a transistor in
18. To turn on a current booster, we can drive its base-emitter terminals with the voltage across
19. A phase splitter produces two output voltages that are
20. A series regulator is an example of a
21. To get more output voltage from a buck switching regulator, you have to
22. An increase of line voltage into a power supply usually produces
23. A power supply with low output impedance has low
24. A zener-diode regulator is a
25. The input current to a shunt regulator is
26. An advantage of shunt regulation is
27. The efficiency of a voltage regulator is high when
28. A shunt regulator is inefficient because
29. A switching regulator is considered
30. The zener follower is an example of a
31. A series regulator is more efficient than a shunt regulator because
32. The efficiency of a linear regulator is high when the
33. If the load is shorted, the pass transistor has the least power dissipation when the regulator has
34. The dropout voltage of standard monolithic linear regulators is closest to
35. In a buck regulator, the output voltage is filtered with a
36. The regulator with the highest efficiency is the
37. In a boost regulator, the output voltage is filtered with a
38. The buck-boost regulator is also
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?
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?
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?
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?
11. In Fig. 24-16, Vin can vary from 20 to 30V. What is the maximum zener current?
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?
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?
17. The LM7815 is used with an input voltage that can vary from 18 to 25V. What is the maximum efficiency? The minimum efficiency?
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?
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?
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?
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?
31. In Fig. 24-10, we want current limiting to start at approximately 250mA. What value should we use for R4?
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.
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?
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.
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.
|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?|