electronic-devices-9th-edition-by-floyd pp17a

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Electronic Devices, 9th editionThomas L. Floyd

Electronic DevicesNinth Edition

Floyd

Chapter 17



Power Supply Regulation

An ideal power supply provides a constant dc voltage despite changes to the input voltage or load conditions.

SummarySummary

00 Current

Voltage

Ideal power supply

The output voltage of a real power supply changes under load as shown in the second plot. The output is also sensitive to input voltage changes.

00 Current

Voltage

Real power supply

VNL VNL

VFL



Line Regulation

Line regulation is a measure of how well a power supply is able to maintain the dc output voltage for a change in the ac input line voltage. The formula for line regulation is

SummarySummary

OUT

IN

Line Regulation = 100%VV

Line regulation can also be expressed in terms of percent change in VOUT per volt change on the VIN (%/V).

OUT OUT

IN

/ 100%Line Regulation =

V VV



Load Regulation

Load regulation is a measure of how well a power supply is able to maintain the dc output voltage between no load and full load with the input voltage constant. It can be expressed as a percentage change in load voltage:

SummarySummary

NL FL

FL

Load Regulation = 100%V V

V

Load regulation can also be expressed in terms of percent change in the output per mA change in load current (%/mA).

Sometimes a maximum error voltage is given in the specification as illustrated in the next slide for a commercial power supply.



Load Regulation

SummarySummary

Commercial power supplies, such as you have in lab, have excellent line and load regulation specifications.

The BK Precision 1651A is an example of a triple output supply (two 0-24 V outputs and a fixed 5 V output). Voltage regulation specifications for this power supply are: Line regulation: ≤0.01% +3 mV (Main supply)

≤5 mV (Fixed 5 V supply) Load regulation: ≤0.01%+3 mV (Main supply)

≤5 mV (Fixed 5 V supply)



Load Regulation

Sometimes the equivalent Thevenin resistance of a supply is specified in place of a load regulation specification.

SummarySummary

RTH = ROUT

RLVTH = VNL

VOUT

Power Supply

In this case, VOUT can be found by applying the voltage divider rule:

LOUT NL

OUT L

RV VR R

In terms of resistances, load regulation can be expressed as:

OUT

FL

Load regulation 100%RR



Load Regulation

SummarySummary

A power supply has an output resistance of 25 m and a full load current of 0.50 A to a 10.0 load.

(a) What is the load regulation? (b) What is the no load output voltage?

OUT

FL

0.025 Load regulation 100% 100%10.0

RR

= 0.25%(a)

(b) By Ohm’s law, VOUT = 5.0 V.

OUTNL

L

OUT L

5.0 V10.0

0.025 + 10.0

VVR

R R

= 5.013 V



Series Regulators

Series Regulator block diagram:

SummarySummary

Basic series regulator circuit:

VIN VOUT

Errordetector

Samplecircuit

Controlelement

Referencevoltage

VIN VOUT

R1

Control element

Q1

D1

VREF+

– Error detector R2

R3

Samplecircuit

The control element maintains a constant output voltage by varying the collector-emitter voltage across the transistor.



Series Regulators

SummarySummary

The output voltage for the series regulator circuit is: 2OUT REF

3

1 RV VR

(a) What is the output voltage for the series regulator?(b) If the load current is 200 mA, what is the power

dissipated by Q1?

2OUT REF

3

1

100 k1+ 3.9 V47 k

RV VR

(a)

(b)

VIN VOUT

R1Q1

D1

VREF+

– R2

R3

47 k

100 k3.9 V

18 V

4.7 k

= 12.2 V

P = VI = (18 V – 12.2 V)(0.2 A)

= 1.16 W



Series Regulators

SummarySummary

Current limiting prevents excessive load current. Q2 will conduct when the current through R4 develops 0.7 V across Q2’s VBE. This reduces base current to Q1, limiting the load current.

VIN VOUT

R1

Q1

R3

R2

Q2

Current limiter

R4

+

–

The current limit is:

L(max)4

0.7 VIR

For example, a 1.4 resistor, limits current to about 0.5 A.



Series Regulators

SummarySummary

5R5 OUT

5 6

RV V

R R

An overload causes VR5 to drop because VOUT drops. This means that less current is needed to maintain conduction in Q2 and the load current drops.

+VIN VOUT

R1

Q1

+

–Q2

R4

D1

R3

R2

R6

R5

Fold-back current limiting drops the load current well below the peak during overload conditions. Q2 conducts when VR5 +VBE = VR4 and begins current limiting. VR5 is found by applying the voltage-divider rule:



Shunt Regulators

Shunt Regulator block diagram:

SummarySummary

Basic shunt regulator circuit:

VIN

R1VOUT

Controlelement(shunt)

Samplecircuit

Errordetector

Referencevoltage

VIN

VOUT

R2

Q1

–

+D1

R4

R3RL

VREF

Error detectorControlelement

Samplecircuit

R1

The control element maintains a constant output voltage by varying the collector current in the transistor.



Shunt Regulators

SummarySummary

Although it is less efficient than the series regulator, the shunt regulator has inherent short-circuit protection. The maximum current when the output is shorted is VIN/R1.

Shunt regulators use a parallel transistor for the control element. If the output voltage changes, the op-amp senses the change and corrects the bias on Q1 to follow. For example, a decrease in output voltage causes a decrease in VB and an increase in VC. VIN

VOUT

R2

Q1

–

+D1

R4

R3RL

VREF

Error detectorControlelement

Samplecircuit

R1



Switching Regulators

SummarySummary

All switching regulators control the output voltage by rapidly switching the input voltage on and off with a duty cycle that depends on the load. Because they use high frequency switching, they tend to be electrically noisy.

VC

ton toff ton toff ton toff tonton toff ton toff ton toff ton

VC

VC

ton toff ton toff ton toff tonon/off control

VOUT

An increase in the duty cycle increases the output voltage.A decrease in the duty cycle decreases the output voltage.




SummarySummary

A step-down switching regulator controls the output voltage by controlling the duty cycle to a series transistor. The duty cycle changes depending on the load requirement.

VIN

VOUT

R1

Q1

R3

R2

RLD1

–

+

D2

Variablepulse-width

oscillator

VREF

L

C

Because the transistor is either ON or OFF on all switching regulators, the power dissipated in the transistor is very small and the regulator is very efficient. The pulses are smoothed by an LC filter.

onC charges

+ VIN

VOUT

R1

Q1

R3

R2

RLD1

–

+

D2

Variablepulse-width

oscillator

VREF

L

C+

L reverses polarityoff



VINVOUT

R1Q1

R3

R2

RL

D1 C

D2

Variablepulse-widthoscillator

L

++

+–

C discharges

+

on

offL field builds


SummarySummary

In a step-up switching regulator, the control element operates as a rapidly pulsing switch to ground. The switch on and off times are controlled by the output voltage.Step-up action is due to the fact the inductor changes polarity during switching and adds to VIN. Thus, the output voltage is larger than the input voltage.

VINVOUT

R1Q1

R3

R2

RL

D1 C

D2


L

++

+–

on

off

C charges

+

L field collapsesVIN

VOUT

R1Q1

R3

R2

RL

D1 C

D2


L

++

+–



+VIN

–VOUT

R1

Q1

R3

R2 RL

D1

D2


L

+

–

C+

on

off

L field builds

C discharges


In a voltage-inverter switching regulator, the output is the opposite polarity of the input. It can be used in conjunction with a positive regulator from the same input source.Inversion occurs because the inductor reverses polarity when the diode conducts, charging the capacitor with the opposite polarity of the input.

SummarySummary

+VIN

–VOUT

R1

Q1

R3

R2 RL

D1

D2


L

+

–

C

on

L field collapses

+ C charges

off+VIN

–VOUT

R1

Q1

R3

R2 RL

D1

D2


L

+

–

C



IC Voltage Regulators

Integrated circuit voltage regulators are available as series regulators or as switching regulators. The popular three-terminal regulators are often used on separate pc boards within a system because they are inexpensive and avoid problems associated with large power distribution systems (such as noise pickup).

SummarySummary

12

3

1 2 3

4

Type number Output voltage78057806780878097812781578187824

+5.0 V+6.0 V+8.0 V+9.0 V

+12.0 V+15.0 V+18.0 V+24.0 V

78XX Regulators

The 78XX series is a fixed positive output regulator available in various packages and with standard voltage outputs.

TO-220 case

D-PAK




The only external components required with the 78XX series are input and output capacitors and some form of heat sink. These IC’s include thermal shutdown protection and internal current limiting.

SummarySummary

Positiveinput

Gnd

Positiveoutput78XX

(1)

(2)

(3)The 78XX series are primarily used for fixed output voltages, but with additional components, they can be set up for variable voltages or currents.

12

3

1 2 3

4

Heat-sink surface




The 79XX series is the negative output counterpart to the 78XX series, however the pin assignments are different on this series. Other specifications are basically the same.

SummarySummary

Negativeinput

Gnd

Negativeoutput79XX

(1)

(2) (3) 79057905.2790679087912791579187924

–5.0 V–5.2 V–6.0 V–8.0 V

–12.0 V–15.0 V–18.0 V–24.0 V

Type number Output voltage




The LM317 is a adjustable positive output IC regulator. There is a fixed reference voltage of +1.25 V between the output and adjustment terminals. There is no ground pin.

SummarySummary

The output voltage is calculated by: 2OUT REF ADJ 2

1

1 RV V I RR

AdjustmentR1

R2

C1

C2

C3

Positiveinput

Positiveoutput

(1)

(3) (2)LM317

What is VOUT? (Assume IADJ = 50 A.)

= 16.8 V

R1

R2

C1

C2

C3

(1)

(3) (2)LM317

OUT2 kΩ1.25 V 1 50 μA 2 k

150 V

2 k

150

+20 V +16.8 V




IC regulators are limited to a maximum allowable current before shutting down. The circuit shown is uses an external pass transistor to increase the maximum available load current.

SummarySummary

VIN

RL

Rext

Qext

VOUTC1

C2

78XX

Rext sets the point where Qext begins to conduct:

extmax

0.7 VRI

For example, if Imax is 1.0 A, Rext = 0.7

What minimum power rating is required for Rext?P = I2R = (1 A)2(0.7 ) = 0.7 W




The 78S40 is an IC containing all of the elements needed to configure a switching regulator, using a few external parts.

SummarySummary

It is a universal switching regulator subsystem because it can be configured as a step-down, step-up, or inverting regulator by the user. The data sheet shows typical circuits for these configurations.

–

+

10 11 12 13 14 15 16

7 6 5 4 3 2 1

–

+

Q1

Q2

SOscillator

R

Q

Comp.

Flip-flop

VOUT

R1

L

CO

R2

VINCT

RCS

VCC

1.25 Vreference

D1

8

9

–

+

9 10 11 12 13 14 15 16

Noninvertinput

Invertinput Gnd Timing

capVCC

Ipksense

Drivercollector

Switchcollector

8 7 6 5 4 3 2 1Referencevoltage

Invertinput

Noninvertinput

VCCop-amp Output Switch

emitter Anode Cathode

–

+

Q1

Q2

SOscillator

R

Q

Comp.

Flip-flop

1.25 Vreference

D1

Here is the step-down configuration.



Selected Key TermsSelected Key Terms

Regulator

Line regulation

Load regulation

Linear regulator

Switching regulator

A electronic circuit that maintains an essentially constant output voltage with changing input voltage or load current.

The percentage change in output voltage for a given change in input (line) voltage.

The percentage change in output voltage for a given change in load current.

A voltage regulator in which the control element operates in the linear region.

A voltage regulator in which the control element operates as a switch.



QuizQuiz

1. The load regulation of an ideal power supply is

a. 0%

b. 25%

c. 50%

d. 100%



QuizQuiz

2. A correct formula for load regulation is

a.

b.

c.

d.

OUT

IN

Load Regulation = 100%VV

NL FL

FL

Load Regulation = 100%V V

V

FL

NL FL

Load Regulation = 100%VV V

OUT OUT

IN

/ 100%Load Regulation =

V VV



QuizQuiz

3. An alternate way to express load regulation is in terms of the

a. output resistance and the full-load resistance

b. output resistance and the shorted-load resistance

c. input resistance and the full-load resistance

d. input resistance and the shorted-load resistance



QuizQuiz

4. In the circuit shown, R4 = 0.7 . The output current will be limited to

a. 0.5 A

b. 0.7 A

c. 1.0 A

d. 1.4 A

VIN VOUT

R1

Q1

R3

R2

Q2

Current limiter

R4

+

–

0.7



QuizQuiz

5. The block diagram for a series voltage regulator is shown. The yellow box represents a

a. control element

b. sample circuit

c. error detector

d. reference voltage

VIN VOUT

?



QuizQuiz

6. The block diagram for a shunt voltage regulator is shown. The yellow box represents a

a. control element

b. sample circuit

c. error detector

d. reference voltage

VIN

R1

VOUT

?



QuizQuiz

7. The circuit in the blue shaded area is a

a. high speed switching circuit

b. fold-back current limiter

c. reference source

d. shunt regulator

+VIN VOUT

R1

Q1

+

–Q2

R4

D1

R3

R2

R6

R5



QuizQuiz

8. A major advantage of all switching regulators is

a. low noise

b. high output impedance

c. high efficiency

d. all of the above



QuizQuiz

9. The type of regulator circuit shown is a

a. series linear

b. series switching

c. shunt switching

d. none of the above

+VIN

–VOUT

R1

Q1

R3

R2 RL

D1

D2


L

+

–

C+

on

off

L field builds

C discharges

+VIN

–VOUT

R1

Q1

R3

R2 RL

D1

D2


L

+

–

C

on

L field collapses

+ C charges

off+VIN

–VOUT

R1

Q1

R3

R2 RL

D1

D2


L

+

–

C



QuizQuiz

10. The output voltage from a 7912 is a regulated

a. +5 V

b. +12 V

c. 5 V

d. 12 V



QuizQuiz

Answers:

1. a

2. b

3. a

4. c

5. d

6. b

7. b

8. c

9. d

10. d

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