Deck 39: Regulated Power Supplies

Question 1

A power supply has V NL = 15 V and V FL =14.5 V. What is the load regulation

Answer 1

Write the expression for load regulation. Here, is the load voltage at no load current is the load voltage at full load current
Substitute 15 V for , and determine load regulation. Therefore, the value of load regulation is .

Question 2

Voltage regulators normally use

A) negative feedback.
B) positive feedback.
C) no feedback.
D) phase limiting.

Answer 2

Voltage regulators are used to maintain constant voltage level. They generally use negative feedback to keep voltage level constant for better regulation. It compares the output voltage with a reference voltage and amplifies the difference to maintain the regulation. Regulation accuracy can be increased by increasing the gain.
Thus, the voltage regulators normally use negative feedback. Hence, the correct option is, .
Positive feedback is used to increase the input which is not a required condition for voltage regulator. Feedback is compulsory for voltage regulation in order to check the condition of the output and phase limiting is not related to voltage regulators. Thus, all other options are correct.

Question 3

A power supply has V HL = 20 V and V LL = 19 V. What is the line regulation

Answer 3

Write the expression for line regulation. Here, is the load voltage at high line is the load voltage at low line
Substitute 20 V for , and determine line regulation. Therefore, the value of line regulation is .

Question 4

During regulation, the power dissipation of the pass transistor equals the collector-emitter voltage times the

A) base current.
B) load current.
C) zener current.
D) foldback current.

Question 5

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

Question 6

Without current limiting, a shorted load will probably

A) produce zero load current.
B) destroy diodes and transistors.
C) have a load voltage equal to the zener voltage.
D) have too little load current.

Question 7

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

Question 8

A current-sensing resistor is usually

A) zero.
B) small.
C) large.
D) open.

Question 9

In Fig. 39-5, V in = 20 V, V Z = 4.7 , R 1 = 2.2 k , R 2 = 4.7 kV, R 3 = 1.5 k , R 4 = 2.7 k , and R L = 50 . What is the output voltage What is the power dissipation in the pass transistor

Question 10

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

A) negative feedback.
B) excessive load current.
C) oscillations.
D) current sensing.

Question 11

What is the approximate efficiency in Prob. 24-8

Question 12

If the output of a voltage regulator varies from 15 to14.7 V between the minimum and maximum load current, the load regulation is

A) 0%.
B) 1%.
C) 2%.
D) 5%.

Question 13

In Fig. 39-7, the zener voltage is changed to 6.2 V. What is the approximate output voltage

Question 14

If the output of a voltage regulator varies from 20 to19.8 V when the line voltage varies over its specified range, the source regulation is

A) 0%.
B) 1%.
C) 2%.
D) 5%.

Question 15

What is the load current in Fig. 39-31 The headroom voltage The power dissipation of theLM7815

Question 16

The output impedance of a voltage regulator is

A) very small.
B) very large.
C) equal to the load voltage divided by the load current.
D) equal to the input voltage divided by the output current.

Question 17

What is the output ripple in Fig. 39-31

Question 18

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

A) equal in value.
B) much larger.
C) much smaller.
D) impossible to determine.

Question 19

If R 1 = 2.7 k and R 2 = 20 k in Fig. 39-11, what is the output voltage

Question 20

A voltage regulator has a ripple rejection of 260 dB. If the input ripple is 1 V, the output ripple is

A) 260 mV.
B) 1 mV.
C) 10 mV.
D) 1000 V.

Question 21

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

Question 22

Thermal shutdown occurs in an IC regulator if

A) power dissipation is too low.
B) internal temperature is too high.
C) current through the device is too low.
D) any of the above occur.

Question 23

A buck regulator has V REF = 2.5 V, R 1 = 1.5 k , and R 2 = 10 k . What is the output voltage

Question 24

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

A) current limiting.
B) a bypass capacitor on the input pin.
C) a coupling capacitor on the output pin.
D) a regulated input voltage.

Question 25

If the duty cycle is 30% and the peak value of the pulses to the choke-input filter is 20 V, what is the regulated output voltage

Question 26

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

A) positive.
B) negative.
C) either positive or negative.
D) unregulated.

Question 27

A boost regulator has V REF = 1.25 V, R 1 = 1.2 k , and R 2 = 15 k . What is the output voltage

Question 28

The LM7812 produces a regulated output voltage of

A) 3 V.
B) 4 V.
C) 12 V.
D) 78 V.

Question 29

A buck-boost regulator has V REF = 2.1 V, R 1 =2.1 k , and R 2 = 12 k . What is the output voltage

Question 30

A series regulator is an example of a

A) linear regulator.
B) switching regulator.
C) shunt regulator.
D) dc-dc converter.

Question 31

A dc-dc converter has an input voltage of 5 V and an output voltage of 12 V. If the input current is1 A and the output current is 0.25 A, what is the efficiency of the dc-dc converter

Question 32

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

A) decrease the duty cycle.
B) decrease the input voltage.
C) increase the duty cycle.
D) increase the switching frequency.

Question 33

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

Question 34

An increase of line voltage into a power supply usually produces

A) a decrease in load resistance.
B) an increase in load voltage.
C) a decrease in efficiency.
D) less power dissipation in the rectifier diodes.

Question 35

If the load regulation is 5% and the no-load voltage is 12.5 V, what is the full-load voltage

Question 36

A power supply with low output impedance has low

A) load regulation.
B) current limiting.
C) line regulation.
D) efficiency.

Question 37

If the line regulation is 3% and the low-line voltage is 16 V, what is the high-line voltage

Question 38

A zener-diode regulator is a

A) shunt regulator.
B) series regulator.
C) switching regulator.
D) zener follower.

Question 39

A power supply has a load regulation of 1% and a minimum load resistance of 10 . What is the output resistance of the power supply

Question 40

The efficiency of a voltage regulator is high when

A) input power is low.
B) output power is high.
C) little power is wasted.
D) input power is high.

Question 41

A switching regulator is considered

A) quiet.
B) noisy.
C) inefficient.
D) linear.

Question 42

The zener follower is an example of a

A) boost regulator.
B) shunt regulator.
C) buck regulator.
D) series regulator.

Question 43

The efficiency of a linear regulator is high when the

A) headroom voltage is low.
B) pass transistor has a high power dissipation.
C) zener voltage is low.
D) output voltage is low.

Question 44

The dropout voltage of standard monolithic linear regulators is closest to

A) 0.3 V.
B) 0.7 V.
C) 2 V.
D) 3.1 V.

Question 45

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

A) choke-input filter.
B) capacitor-input filter.
C) diode.
D) voltage divider.

Question 46

The regulator with the highest efficiency is the

A) shunt regulator.
B) series regulator.
C) switching regulator.
D) dc-dc converter.

Question 47

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

A) choke-input filter.
B) capacitor-input filter.
C) diode.
D) voltage divider.

Question 48

The buck-boost regulator is also

A) a step-down regulator.
B) a step-up regulator.
C) an inverting regulator.
D) all of the above.

Question 49

A key disadvantage of switch-mode power supplies is its

A) lower efficiency.
B) higher efficiency.
C) noise generation.
D) complexity.

Question 50

A low-dropout regulator

A) is a switching regulator.
B) has very low overhead.
C) consumes more power than a standard linear regulator.
D) is not available In IC form.

Question 51

Question 52

Switching power supplies operated up to which pulse frequencies

A) Less than 50 kHz.
B) 100 kHz to 300 kHz.
C) 500 kHz to 1 MHz.
D) Over 3 MHz.

Question 53

A PWM signal has a pulse amplitude of 5 V and a duty cycle of 35%. What is the average output voltage

A) 1.75 V.
B) 2.5 V.
C) 3.5 V.
D) 4.25 V.

Question 54

A charge pump is a type of

A) regulator.
B) dc-dc converter.
C) linear regulator.
D) inverter.

Question 55

The main purpose of the switching transistor in a SMPS is to

A) vary the series load current.
B) convert ac into dc.
C) average the load current.
D) switch dc into a square wave.

Question 56

A "brick" is a type of

A) regulator.
B) dc-dc converter.
C) linear regulator.
D) inverter.

Question 57

An inverter converts

A) dc to ac.
B) ac to dc.
C) dc to dc.
D) ac to ac.

Question 58

The output of an inverter may be a

A) square wave.
B) pulse wave.
C) sine wave.
D) any of the above.

Question 59

In a self-oscillating inverter, the frequency of operation is set by the

A) transistors.
B) RC network.
C) transformer.
D) an external circuit.

Question 60

PWM can be used to create a sine-wave output.