ElectronicsElectronics

Principles & ApplicationsPrinciples & ApplicationsSixth EditionSixth Edition

Chapter 15Regulated

Power Supplies(student version)

©2003 Glencoe/McGraw-Hill

Charles A. Schuler

• Open-Loop Voltage Regulation

• Closed-Loop Voltage Regulation

• Current and Voltage Limiting

• Switch-Mode Regulators

• Troubleshooting

INTRODUCTION

Dear Student:

This presentation is arranged in segments. Each segment is preceded by a Concept Preview slide and is followed by a Concept Review slide. When you reach a Concept Review slide, you can return to the beginning of that segment by clicking on the Repeat Segment button. This will allow youto view that segment again, if you want to.

Concept Preview• Conducting zener diodes show a relatively

constant voltage drop.

• Zener shunt regulators are not practical for large load currents.

• A zener can regulate the base voltage of a series pass transistor.

• Adding an error amplifier provides much better voltage regulation (closed loop operation).

• IC voltage regulators contain a pass transistor, a voltage reference, and an error amplifier.

0246

20

40

60

80

100

120

140

Reverse Bias in Volts

Reversecurrentin mAI

V

V

The voltage across a conductingzener is relatively constant.

Load

The load is in parallel with the zener and will see a relatively constant voltage as

long as the zener is conducting.

Using a Zener Diode as a Voltage Regulator

Unregulated Supply

LoadUnregulated Supply

Series Pass Transistor

The amplified zener regulator is used when large load currents are required. The voltage regulation of this circuit is fair since VBE is relatively constant.

VBE

ErrorAmplifier

Closed-loop is required for demanding applications.

VOUT

If VOUT decreases, the amplifier output goes more positive to increase

the drive to the pass transistor.

The pass transistor, reference, and error amplifier are inside IC regulators.

7805

Concept Review• Conducting zener diodes show a relatively

constant voltage drop.

• Zener shunt regulators are not practical for large load currents.

• A zener can regulate the base voltage of a series pass transistor.

• Adding an error amplifier provides much better voltage regulation (closed loop operation).

• IC voltage regulators contain a pass transistor, a voltage reference, and an error amplifier.

Repeat Segment

Concept Preview

• The current capabilities of an IC regulator can be extended by adding a boost transistor.

• A boost transistor can be protected from overcurrent by adding a current limit transistor.

• Conventional current limiting won’t always protect a pass transistor. Foldback current limiting offers better protection.

• Parallel pass transistors require emitter swamping resistors so they will share the load current.

• A crowbar circuit provides overvoltage protection.

7805

PNP Current Boost Transistor

Load

R

When the drop across R reaches 0.7 volts, the

boost transistor turns on.

IC voltage regulators have modest current limits. This circuit extends their capabilities.

7812

Load

This current limit circuit protects the pass transistor.

R2

When the drop across R2 reaches0.7 V, the current limit transistor

turns on and shunts R1.

R1

Current-limitTransistor

VO

UT

IL

Conventional Current Limiting

Short circuit

Constant currentregion

VO

UT

IL

Foldback Current Limiting

Short circuit

Foldback currentregion

R1

Foldback Current Limiting

VZ

R5

R6

RL

+Vin

R2

R7

Q1

Q2

R3

R4

Parallel transistors need emitter swamping resistors to ensure current sharing.

7812

Crowbar Over-voltage Protection Circuit

Fuse

If the zener conducts, the

SCR turns on and blows the fuse.

Linear Power Supply Quiz

Pass transistors are connected in __________ with the load. series

IC regulators can provide more current withthe addition of a __________ transistor. boost

The two types of current limiting are conventional and __________. foldback

Emitter swamping resistors force paralleltransistors to share __________. current

Crowbar circuits are used to protect a load from excess __________. voltage

Concept Review• The current capabilities of an IC regulator can be

extended by adding a boost transistor.

• A boost transistor can be protected from overcurrent by adding a current limit transistor.

• Conventional current limiting won’t always protect a pass transistor. Foldback current limiting offers better protection.

• Parallel pass transistors require emitter swamping resistors so they will share the load current.

• A crowbar circuit provides overvoltage protection.

Repeat Segment

Concept Preview• Switch mode power supplies use pulse width

modulation and achieve much better efficiency than linear power supplies.

• Switchers operate at tens of kilohertz so that smaller inductors and capacitors can be used.

• The three basic switching configurations are step-down, step-up, and inverting. All three store energy in an inductor.

• Converter-type switchers use transformers.

• Flyback-type switchers use transformers and a critical conduction mode of operation.

Power supplies with pass transistors are linear and are not as efficient as switch-mode power supplies.

Recall: Pulse width modulation (PWM) is one wayto use a digital approach to an analog problem.

PWM

Averagevalue

PC = IC x VCE (The heat loss in a pass transistor can be significant.)

Step-down configuration

Switch-mode Power Supply

PWM

VIN

VLOAD

VLOAD < VIN

Step-up configuration

Switch-mode Power Supply

PWM

VIN

VLOAD

VLOAD > VIN

CEMF

Inverting configuration

Switch-mode Power Supply

PWM

VIN

VLOADCEMF

Converter-type Switch-mode Supply

OSC. &PWM VREF

Switch-mode Quiz

Switch-mode supplies have better __________ than linear supplies. efficiency

Switch-mode supplies use pulse width __________. modulation

A higher voltage is provided by the __________ configuration. step-up

The opposite polarity is provided by the __________ configuration. inverting

A push-pull arrangement is common in __________ type power supplies. converter

Linear Supply Troubleshooting

• A shorted pass transistor produces high output voltage.

• An open pass transistor produces no output voltage.

• Voltage error can be due to overloads, the reference voltage, the error amplifier, etc.

• IC regulators can oscillate. Check bypass capacitors.

• IC regulators are subject to RF fields.

Flyback Switching Power Supply

Flyback Supply Waveforms

Switch-Mode Troubleshooting• May not function unless loaded (use a

dummy load).• Use oscilloscope to view PWM

waveforms.• Frequency can be critical.• May use an optoisolator in the feedback

circuit. Disable and substitute for the feedback signal.

• Check the reference voltage.• Check for overloads.

Concept Review• Switch mode power supplies use pulse width

modulation and achieve much better efficiency than linear power supplies.

• Switchers operate at tens of kilohertz so that smaller inductors and capacitors can be used.

• The three basic switching configurations are step-down, step-up, and inverting. All three store energy in an inductor.

• Converter-type switchers use transformers.• Flyback-type switchers use transformers and a

critical conduction mode of operation.

Repeat Segment

• Open-Loop Voltage Regulation

• Closed-Loop Voltage Regulation

• Current and Voltage Limiting

• Switch-Mode Regulators

• Troubleshooting

REVIEW