Chap 7 Voltage Regulators

Welcome

Voltage Regulators

2. Performance measures of voltage regulators

1. Introduction

3. Voltage Regulator circuits

(i) Zener Diode Regulator

(ii) Emitter Follower Regulator

(iii) Feedback Voltage Regulator

(iv) Feedback Voltage Regulator using Op-amp

4. Short Circuit Protection

5. Foldback Current Limiting

6. Thermal Shutdown

Introduction

Fig 7.1 (a) Block diagram of ac to dc converter

Fig 7.1 (b) Block diagram of dc to dc converter

2. Performance measures of voltage regulators

The factors for poor regulation are:

(i) The line voltage changes and the ripple content in the dc voltage

(ii) The voltage drop across the internal resistanceof the regulator due to load current changes

(iii) The temperature dependence of the device parameters which caused change in the output voltage

TS IR VSV

as defined becan regulator a of tageoutput volin change The

TLoiVo

2. Performance measures of voltage regulators

TS IR VSV

as defined becan regulator a of tageoutput volin change The

TLoiVo

0,0 TIi

ov

L

V

VS

0,0 TVL

oo

i

I

VR

0,0 Li IV

oT

T

VS

where Sv is the voltage stability factor, Ro is the output resistance and ST is the temperature coefficient

Fig 7.2 Voltage variations in ideal and practical regulators with load current

2. Performance measures of voltage regulators

3. Voltage Regulator Circuits

(i) Zener Diode Regulator

Zener Diode Regulator

The zener diode voltage VZ remains almost constant in the breakdown region so long as

(max)(min) zzz III

Z

Zz(max) Zz(max)

V

PI diode,zener theofP ratingpower by the decided is I where

z

zz

ΔI

ΔVr ,resistancezener finite a has diodezener practicalA

Vz. source, oltageconstant v a as acts diodezener idealAn

The value of Rs is selected such that it fulfill the following requirements:

1) When the input voltage is minimum and load current is maximum, Iz is sufficient to keep the zener diode under breakdown region.

2) When the input voltage is maximum and the load current is maximum, the zener diode current must not exceed the maximum value(Iz(max)).

Zener Diode Regulator

The optimum value of Rs can be found using the following equations:

(max)

(min)

(min)

(min)

(max)

(max)

L

s

zin

z

L

s

zin

z

IR

VVI

IR

VVI

(max)(min)

(min)

(max)

(min)(max)

(max)

(min)

andLz

zin

s

Lz

zin

s

II

VVR

II

VVR

Hence Rs should be chosen such that Rs(min) < Rs < Rs(max)

(min)(max) sss RRR

(i) Zener Diode Regulator

Sv and Ro can be obtained by replacing zener diode by its equivalent zener resistance rz. The values are:

0,0 TIi

ov

L

V

VS

0,0 TVL

oo

i

I

VR

zs

zv

rR

rS zso rRR ||

Disadvantages:

1. The maximum load current is limited to Iz(max) – Iz(min)

2. A large amount of power is wasted in the zener resistance and series resistance.

3. The regulation factor Sv and output resistance Ro are not very low.

Voltage Regulators

2. Performance measures of voltage regulators

1. Introduction

3. Voltage Regulator circuits

(i) Zener Diode Regulator

(ii) Emitter Follower Regulator

(iii) Feedback Voltage Regulator

(iv) Feedback Voltage Regulator using Op-amp

4. Short Circuit Protection

5. Foldback Current Limiting

6. Thermal Shutdown

(ii) Emitter Follower Regulator

Vo = Vz - VBE

OCECBBEo VVIIVV

1

)(||rR and z

o

rrRR

RRr

rS xs

sz

zv

where rx – base spreading resistance and r small signal resistance between base and emitter terminals of the transistor

(iii) Feedback Voltage Regulator

OO

ZO

nVV

R

RRVV

2

2B

2

12

RR1

RV and

2

222

eq

)1(R Therefore

n

rrrR zxB

ib2 and ic2 denote the incremental changes in base and collector currents

eq

O

cR

Vi 2

1

113

O

3

1

)||(R and

rrRR

RR

RS

xeq

eq

eq

v

b22c2

222

2

ii

)1(

and

rrrR

Vni

zxB

O

b

(iv) Feedback voltage regulator using op-amp

Voltage Regulators

2. Performance measures of voltage regulators

1. Introduction

3. Voltage Regulator circuits

(i) Zener Diode Regulator

(ii) Emitter Follower Regulator

(iii) Feedback Voltage Regulator

(iv) Feedback Voltage Regulator using Op-amp

4. Short Circuit Protection

5. Foldback Current Limiting

6. Thermal Shutdown

(iv) Feedback voltage regulator using op-amp contd..

(iv) Feedback voltage regulator using op-amp contd..

4. Short Circuit Protection

SC

BEDR

R

VVI

SC

2

SCR

Di

SC IR

VVI

3

2

5. Foldback Current Limiting

SC

(max)R

6.0 VI L

6. Thermal Shutdown

Voltage Regulator ICs

General Purpose Voltage Regulator IC

Positive low voltage (2 V to 7 V) regulator using IC 723

21

2

RR

RVV refout

21

21

213

||

RR

RR

RRR

High Voltage (above 7 V) regulator using IC 723

2

11R

RAV

volts115.72

1

R

RVout

Current limiting and current foldback in 723 circuits

Current limiting Current foldback

Thermal Shutdown

Voltage Regulator ICs

General Purpose Voltage Regulator IC

Positive low voltage (2V to 7V) regulator using IC 723

21

2

RR

RVV refout

High voltage (above 7V) regulator using IC 723

2

11R

RAV

volts115.72

1

R

RVout

Current limiting and current foldback in 723 circuits

scsc

sense

RR

V 6.0 I limitingCurrent limit

Current limiting and current foldback in 723 circuits

V 6.04 regR VV

43

414

RR

RVVR

SCreg IRVV1

)(43

44 SCregR IRV

RR

RV

current knee theis This , )6.0(4

43

sc

reg

reg

SC R

VV

RR

RRI

)6.0(4

43

sc

scRR

RRI

At shirt circuit, Vreg=0, I=Isc. Therefore,

Current Boosting

Negative Voltage Regulator

Fixed Voltage Regulators

Tracking Voltage Regulator

Adjustable Voltage Regulator

Switched Capacitor Voltage Converters

Voltage Inverter

Voltage Inverter

Voltage Inverter

Voltage Inverter

Switching Regulators

Buck Regulator

Bulk Regulator

Buck Regulator

Boost Regulator

Boost Regulator

Buck-Boost ( Inverting )Regulator

Switching IC Regulators

Inverting Buck-Boost Regulator using LM 2575

LM 2577 boost regulator