ElectronicsElectronics

Principles & ApplicationsPrinciples & ApplicationsFifth EditionFifth Edition

Chapter 15Regulated

Power Supplies

©1999 Glencoe/McGraw-Hill

Charles A. Schuler

• Open-Loop Voltage Regulation• Closed-Loop Voltage Regulation• Current and Voltage Limiting• Switch-Mode Regulators• Troubleshooting

INTRODUCTION

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 turn

on the pass transistor harder.

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

7805

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 comes 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 areconventional and __________. foldback

Emitter swamping resistors force paralleltransistors to share ________. current

Crowbar circuits are used to protect aload from excess __________. voltage

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.

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.

• Open-Loop Voltage Regulation• Closed-Loop Voltage Regulation• Current and Voltage Limiting• Switch-Mode Regulators• Troubleshooting

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