Ch 18 Regulators

7/31/2019 Ch 18 Regulators

1/64

Chapter 18Voltage Regulators


2/64

Objectives

Describe the basic concept of voltage regulation

Discuss the principles of series, shunt, and switching

voltage regulation

Discuss IC voltage regulators and some of theirapplications


3/64

Introduction

The purpose of voltage regulation is to maintain aprecise voltage output from a power supply despite loadand input voltage variations. There are basically twocategories: linear and switching. As with many systems,discrete device circuits are being replaced withintegrated circuits. We will discuss a few of the IC typeregulators.


4/64

Voltage RegulationLine regulation is the maintenance of a specific output

voltage despite changes in input voltage. How well aregulator performs line regulation can be determined bythe formula below.

Line Regulation = ( VOUT/VIN )100%


5/64

Voltage RegulationLoad regulation is the maintenance of a precise output

voltage despite changes in load resistance. How well aregulator performs load regulation can be determined by theformula below.

Load Regulation = (VNL - VFL) /VFL100%


6/64

Figure 183 Thevenin equivalent circuit for a power supply with a load resistor.

Thomas L. Floyd

Electronic Devices, 7e

Copyright 2005 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.


7/64

Basic Series RegulatorsWith series regulation the control elementis in series with the input and output.


8/64

Basic Series RegulatorsThe zener diode sets the reference voltage for the noninverting input of theop-amp. Any changes in the output are fed back to the inverting input of

the op-amp. The difference voltage output of the op-amp biases thetransistor to correct the output voltage for the overall circuit. The output ofthis regulator can be determined by the formula below.

VOUT (1 + R2/R3)VREF


9/64

Figure 186 Illustration of series regulator action that keeps VOUT constant when VIN or RL changes.

Thomas L. Floyd





10/64

Figure 187

Thomas L. Floyd





11/64

Basic Series RegulatorsOverload protection for a series regulator protects the control element inthe case of a short or an unusually heavy load (low R

L). Q

2is biased by the

voltage drop across R4. When load current exceeds the predetermined levelQ2 diverts current from the base of Q1 causing Q1 to conduct less. The loadcurrent maximum can be determined by the formula below.

IL(max)= .7V/R4


12/64

Figure 189

Thomas L. Floyd





13/64

Basic Series RegulatorsFold-back current limiting allows operation up to peak load current.With a shorted output the current is dropped to a lower value (folded

back). VR5 + VBE must be overcome before Q2 conducts to limitcurrent.


14/64

Figure 1811 Fold-back current limiting (output voltage versus load current).

Thomas L. Floyd





15/64

Basic Shunt RegulatorsIn shunt regulation the controlelement is in parallel with the load.


16/64

Figure 1813 Basic op-amp shunt regulator with load resistor.

Thomas L. Floyd





17/64

Figure 1814 Sequence of responses when VOUT tries to decrease as a result of a decrease inRL orVIN (opposite responses for an

attempted increase).

Thomas L. Floyd





18/64

Figure 1815

Thomas L. Floyd





19/64

Basic Switching Regulators

The switching regulator is more efficient than thelinear series or shunt type. This type regulator is ideal forhigh current applications since less power is dissipated.

Voltage regulation in a switching regulator is achieved bythe on and off action limiting the amount of current flowbased on the varying line and load conditions. Withswitching regulators 90% efficiencies can be achieved.


20/64

Basic Switching RegulatorsWith the step-down (output is less than the input) configurationthe control element Q

1

is pulsed on and off at variable rate basedon the load current. The pulsations are filtered out by the LC filter.


21/64

Figure 1816 Basic step-down switching regulator.

Thomas L. Floyd





22/64

Figure 1817 Switching regulator waveforms. The VC waveform is shown for no inductive filtering to illustrate the charge and discharge

action(ripple). L and C smooth VC to a nearly constant level, as indicated by the dashed line forVOUT.

Thomas L. Floyd





23/64

Figure 1818 Basic regulating action of a step-down switching regulator.

Thomas L. Floyd





24/64

Basic Switching RegulatorsThe step-up configuration works much the same as the step-

down. The difference is in the placement of the inductor andthe fact that Q1 is shunt configured. During the time when Q1 isoff the VL adds to VC stepping the voltage up by some amount.


25/64

Figure 1820 Basic action of a step-up regulator when Q1 is on.

Thomas L. Floyd





26/64

Figure 1821 Basic switching action of a step-up regulator when Q1 turns off.

Thomas L. Floyd




B i S i hi R l


27/64

Basic Switching RegulatorsWith the voltage-inverter configuration the output voltage output is ofopposite polarity of the input. This is achieved by VL forward-biasing

reverse-biased diode during the off times producing current andcharging the capacitor for voltage production during the off times. Withswitching regulators 90% efficiencies can be achieved.


28/64

Figure 1822 Basic inverting switching regulator.

Thomas L. Floyd





29/64

Figure 1823 Basic inverting action of an inverting switching regulator.

Thomas L. Floyd





30/64

Figure 1824 Basic regulating action of an inverting switching regulator.

Thomas L. Floyd





31/64

IC Regulators

Regulation circuits in integrated circuit form arewidely used. Their operation is no different butthey are treated as a single device with associatedcomponents. These are generally three terminaldevices that provide a positive or negative output.Some types are have variable voltage outputs.


32/64

IC Regulators

A typical 7800 series voltage regulator is used for positivevoltages. The 7900 series are negative voltage regulators.These voltage regulators when used with heatsinks can safelyproduce current values of 1A and greater. The capacitors act asline filtration.


33/64

Figure 1825 The 78XX series three-terminal fixed positive voltage regulators.

Thomas L. Floyd





34/64

Figure 1826 The 79XX series three-terminal fixed negative voltage regulators.

Thomas L. Floyd





35/64

IC RegulatorsAdjustable IC regulators are available with either positive or

negative output. They can be set to produce a specific voltageby way of an external reference voltage divider network.

Switching regulators are also available in IC form.


36/64

Figure 1828 Operation of the LM317 adjustable voltage regulator.

Thomas L. Floyd





37/64

Figure 1829

Thomas L. Floyd

Electronic Devices, 7eCopyright 2005 by Pearson Education, Inc.

Upper Saddle River, New Jersey 07458



38/64

Figure 1830 The LM337 three-terminal adjustable negative voltage regulator.

Thomas L. Floyd

Electronic Devices, 7eCopyright 2005 by Pearson Education, Inc.




39/64

Figure 1831 The 78S40 switching regulator.

Thomas L. FloydElectronic Devices, 7e




40/64

IC RegulatorsTo increase the current capability of an IC regulatoran external pass transistor can be used.


41/64

Figure 1833 Operation of the regulator with an external pass transistor.





42/64

IC RegulatorsA current limiting circuit similar to the one discussedearlier can be used to protect the external pass transistor.


43/64

Figure 1835 The current-limiting action of the regulator circuit.





44/64

Figure 1836 The three-terminal regulator as a current source.





45/64

Figure 1837 A constant-current source of 1 A.





46/64

Figure 1838 The step-down configuration of the 78S40 switching regulator.





47/64

Figure 1839 The step-up configuration of the 78S40 switching regulator.





48/64

Summary

Voltage regulators keep a constant dc output despiteinput voltage or load changes.

The two basic categories of voltage regulators are linearand switching.

The two types of linear voltage regulators are series andshunt.

The three types of switching are step-up, step-down,and inverting.

Switching regulators are more efficient than linearmaking them ideal for low voltage high currentapplications.


49/64

IC regulators are available with fixed positive or negativeoutput voltages or variable negative or positive outputvoltages.

Both linear and switching type regulators are available

in IC form. Current capacity of a voltage regulator can be increasedwith an external pass transistor.

Summary


50/64

Figure 1840 Block diagram of the dual-polarity power supply.





51/64

Figure 1841 The dual-polarity power supply circuit board.





52/64

Figure 1842 The dual-polarity power supply schematic.


Copyright 2005 by Pearson Education, Inc.




53/64

Figure 1843 Results of tests on four faulty power supply boards.






54/64

Figure 1844





Figure 1845 Multisim file circuits are identified with a CD logo and are in the Problems folder on your CD-ROM. Filenames


55/64

Figure 18 45 Multisim file circuits are identified with a CD logo and are in the Problems folder on your CD ROM. Filenames

correspond to figurenumbers (e.g., F1845).





Basic Shunt Regulators


56/64

Basic Shunt RegulatorsThe shunt regulator is similar in design to the series. With attemptedchanges in output voltage, Q1 is biased to conduct more or less. This

dynamic collector-emitter resistance and R1 act as a voltage divider networkthat maintains a specific voltage across the load. R1 limits current in thecase of a short.

Fi 18 46


57/64

Figure 1846





Fi 18 47


58/64

Figure 1847





Figure 18 48


59/64

Figure 1848





Figure 18 49


60/64

Figure 1849





Figure 18 50


61/64

Figure 1850





Figure 1851


62/64

Figure 1851





Figure 1852


63/64

Figure 18 52





Figure 1853


64/64

Figure 18 53

Documents

Ch 18 Regulators