Middleton W.M. (ed.) Reference Data for Engineers: Radio, Electronics, Computer and Communications

14-12

REFERENCE

DATA

FOR ENGINEERS

Transient-Voltage Suppressors

Transient-voltage suppressors have a terminal char-

acteristic such that, above a certain voltage level, the

voltage drop across the device increases slightly for

large changes

in

current magnitude. Some of these

devices exhibit voltage clamping characteristics for

both voltage polarities, whereas others exhibit clamp-

ing characteristics for only one voltage polarity. Two

types are currently available. One is similar

to

a silicon

zener diode, selected for a high-pulse-power, low-

average-power duty cycle. The second type is com-

posed of a polycrystalline nonlinear resistive material.

Both of these categories of devices come in a range of

voltage and power ratings. They are used as transient-

voltage clippers on incoming ac or dc utility lines and

as transient-voltage limiters for semiconductors and

other voltage-sensitive equipment. Fig.

16

shows a

comparison of several types and sizes of devices as a

function of standardized pulse current waveforms and

supply voltages.

AC-DC CONVERTER CIRCUITS

This section describes ac-to-dc converter circuits,

both unregulated and electronically regulated, using

rectifiers, phase-controlled thyristors, linear transis-

tors, and switching-mode transistors. The basic design

equations together with some of the technical advan-

tages, disadvantages, and common applications are

given and discussed.

Rectifier Circuits

Table

1

shows seven of the most commonly used

power-rectifier circuits and general design information

for each type. Their advantages, disadvantages, and

common applications follow.

Single-phase Half-Wave Rectifier-Since only

half of the input wave is used, the efficiency is low and

the regulation is relatively poor. Capacitors

are

com-

monly used in half-wave circuits to increase the output

voltage and decrease the voltage ripple. The output

voltage and degree of filtering are determined by the

value of capacitance used in relation

to

the

load cur-

rent. Transformer design is complicated, and the unidi-

rectional secondary current causes core saturation and

poor regulation. Most half-wave circuits operate either

directly from ac lines or at a high voltage with a rela-

tively low current.

Single-phase Full-Wave Center-Tap Rectifier-

The efficiency is good, but the transformer ac voltage

is approximately

2.2

times the dc output voltage. The

circuit requires

a

larger transformer than an equivalent

bridge rectifier, with the added complication of a cen-

ter tap. Each

arm

of the center-tap circuit must block

the full terminal voltage of the transformer. Because of

this, center-tap connections are economical only in

voltage ranges where not more than one rectifier per

arm

is required. If series units must be used

to

obtain

the required output voltage, a bridge circuit is prefera-

ble. This circuit is used with low-voltage logic-level

INSTANTANEOUS CURRENT IN AMPERES

Fig.

16.

Voltage-current

characteristics

of

four transient-suppressor devices.

(From

Transient

Voltage

Suppression,

3rd ed.

Auburn,

NY.

General

Electric Co., Semiconductor Products

Dept.,

1982.)

14-1

3

power supplies where the efficiency is improved

because there is only one diode in series with the out-

put.

Single-phase Full-Wave Bridge Rectifier-If sin-

gle-phase full-wave output is required, a bridge circuit

is commonly used. Efficiency is good and transformer

design is easy. Filtering is simplified because the ripple

frequency is twice the input frequency.

Three-phase Wye (or Star) Half-Wave Recti-

fier-This circuit is commonly used if dc output-volt-

age requirements are relatively low and current

requirements

are

moderately large. The dc output volt-

age is approximately equal to the phase voltage. How-

ever, each of the three

arms

must block the line-to-line

voltage, which is approximately 2.5 times the phase

voltage. For this reason, it is desirable to use a three-

phase half-wave connection only where one series unit

per arm will provide the required dc output. The trans-

former design and utilization are somewhat compli-

cated because there is a tendency to saturate the core

with unidirectional current in each winding.

Three-phase Full-Wave Bridge Rectifier-This

circuit is commonly used if high dc power is required

and if efficiency must be considered. The ripple com-

ponent

in

the load is 4.2% at a frequency six times the

input frequency, and

so

additional filtering is not

required

in

most applications. The dc output voltage is

approximately 25% higher than the phase voltage, and

each

arm

must block only the phase voltage. Trans-

former utilization is good. This is the most common

three-phase rectifier connection.

Three-phase Diametric Half-Wave Rectifier-

The characteristics of this circuit approximate those of

the three-phase double-wye circuit without an inter-

phase transformer. Popular applications include

requirements for very high dc load currents in low-to-

medium voltage ranges (approximately

6

to 125 volts

dc).

Three-phase Double-Wye Half-Wave Rectifier-

A

three-phase double-wye connection is recom-

mended if a very high direct current is required at a

relatively low dc voltage. Each

arm

is required to

block the full phase voltage of the secondary windings.

The dc output current rating is double that of a three-

phase bridge

or

half-wave connection. However, the

output voltage

is

only

75%

of the phase voltage. The

transformer design is complicated by additional con-

nections and extra insulation, and an interphase trans-

former (or balance coil) is required.

Single-phase Voltage

Multipliers

These circuits use the principle of charging capaci-

tors in parallel from the ac input and adding the volt-

ages across them in series to obtain dc voltages higher

than the source voltage. Filtering must be of the capac-

itor-input type.

Conventional and Cascade Voltage Doubler-In

the conventional circuit (Fig.

17),

capacitors

C,

and

C,

are each charged, during alternate half-cycles, to the

peak value of the alternating input voltage. The capaci-

tors are discharged in series into load

RL,

thus produc-

ing an output across the load of approximately twice

the ac peak voltage.

In

the cascade circuit (Fig. 18),

C,

is charged to the

peak value

of

the ac input voltage through rectifier

CR,

during one half-cycle, and during the other half-cycle

it discharges

in

series with the ac source through

CR,

to charge

C,

to twice the ac peak voltage.

The "conventional" circuit has slightly better regu-

lation, and, since the ripple frequency is twice the sup-

ply frequency, the output is easier to filter, the

percentage ripple being approximately the same

in

both circuits.

In

addition, both capacitors are rated at

the peak ac voltage, whereas

C,

in

the cascade circuit

must

be rated at twice

this

value. With both circuits,

the peak inverse voltage across each rectifier

is

twice

the ac peak. The cascade circuit, however, has the

advantage of a common input and output terminal and,

therefore, permits the combination of units to give

higher-order voltage multiplications. The regulation

of

both circuits is poor,

so

that only small load currents

can be drawn.

Bridge Rectifier or Voltage Doubler-If SW in

Fig.

19

is open, the circuit is a bridge rectifier; if the

peak ac applied voltage is 2Ep, the dc output voltage is

2Ep. If SW is closed, the circuit is a voltage doubler;

an

ac input of

Ep

gives a dc output of 2Ep. This circuit

is used if the same dc voltage is desired and the input

voltage can be either 115 or 230 volts ac.

Further Voltage Multiplication-The cascade

voltage doubler shown in Fig. 18 can be combined

several times to obtain higher dc voltages, as shown in

Fig. 20. The voltage ratings of all the capacitors and

rectifiers are twice the ac peak voltage, but the capaci-

tors must have the values shown. The value of

C

will

be the same as that for the cascade voltage doubler

(Fig.

lS),

which is the basic unit for the circuit

in

Fig.

20.

The load current must be small. The increasing

size of capacitors and

the

deterioration

in

regulation

limit the voltages that can be obtained from this type

of

circuit.

Phase-Controlled Thyristor

Circuits

As

described earlier, the thyristor can block forward

voltage until it is triggered into conduction. The delay

angle

of

gating the thyristor into conduction is desig-

nated as alpha

(a).

With

a

=

0,

the thyristor behaves

like a rectifier.

As

the delay angle is increased, the

average dc output voltage decreases, and therefore

14-1

4

REFERENCE

DATA

FOR ENGINEERS

TABLE

1.

RECTIFIER

Type of Circuit-,

Primary+

Secondary4

One Cycle Wave of Rectifier

Output Voltage

(No Overlap)

Single-phase Single-phase Single-phase Three-phase Star

Half-Wave Center Tap Bridge (Wye)

Number of rectifier elements

Rms dc volts output

Peak dc volts output

Peak reverse volts per

rectifier element

Average dc output current

per rectifier element

Rms current per rectifier

element:

Resistive load

Inductive load

rectifier element:

Resistive load

Inductive load

Ratio of peak

to

average

current per element:

Resistive load

Inductive load

%

Ripple (rms of ripple/

average output voltage)

Ripple Frequency

Peak current per

Transformer secondary

rms

volts per leg

Transformer secondary

rms

volts line-to-line

Secondary line current

Transformer secondary

Transformer primary

Average of primary and

Primary line current

volt-amperes

rms

amperes per leg

volt-amperes

secondary volt-amperes

Line power factor

1

1.57

3.14

1.41

1

.oo

1

.oo

1.57

-

3.14

-

3.14

-

121%

1

Resistive Load

2.22

1.57

3.49

1.21

2.69

3.09

1.21

-

2 4 3

1.11 1.11 1.02

1.57 1.57 1.21

3.14 1.57 2.09

2.82 1.41 2.45

1.41 1.41 1.41

1

.oo

1

.oo

1

.oo

0.500

0.333

0.785 0.785 0.587

0.707 0.707 0.578

1.57

1.21

1

.oo

1

.oo

1

.oo

3.14 3.14 3.63

2.00 2.00 3.00

48% 48% 18.3%

2 2 3

Inductive Load or Large Choke Input Filter

1.11 1.11 0.855

(to center tap) (total)

(to

neutral)

2.22

1.11

1.48

0.707 1

.oo

0.578

1.57 1.11 1.48

1

.oo

1

.oo

0.471

1.11

1.21

1.34 1.11 1.35

1

.oo

1

.oo

0.817

0.900

0.826

*

The

data assume zero forward

drop

and zero reverse current in rectifiers and no alternating-current line or source reactance.

14-1

5

CIRCUIT

CHART

*

Three-phase Bridge Six-Phase Star Three-phase

Double

Wye

(Three-phase Diametric) With Interphase Transformer

Ih

To

Determine Actual

Value of Parameter in

rrTyyy',

Any Column, Multiply

0

Factor Shown

by

Value

of

6

1

.oo

1.05

2.45

1.41

6

1

.oo

1.05

2.09

2.83

1.41

6

1

.oo

1.05

2.42

2.83

I

.41

(diametric)

1

.oo

0.167

X

Average dc voltage output

X

Average dc voltage output

X

Average dc voltage output

X

Rms

secondary volts per

transformer leg

X

Rms

secondary volts

line-to-line

X

Average dc output current

X

Average dc output current

1

.oo

0.333

1

.oo

0.167

0.579

0.578

0.409 0.293

0.408 0.289

X

Average dc output current

X

Average dc output current

1.05

1

.oo

1.05

0.525

1

.oo

0.500

X

Average dc output current

X

Average dc output current

3.15

3.00

6.30

6.00

3.15

3.00

4.2%

6

4.2%

6

4.2%

6

Line frequency,

f

X

Inductive Load

or

Large Choke Input Filter

0.428

0.740

(to neutral) (to neutral)

0.855

(to neutral)

1.71

(max-no load)

0.289

Average dc voltage output

X

0.740

0.816

1.48

0.408

(max)

Average dc voltage output

Average

dc

output current

Dc

watts output

Average dc output current

Dc

watts output

Dc

watts output

(Avg. load current

X

sec.

leg

voltage)/primary

line voltage

X

I

.05

1.81

0.816 0.577

1.05

1.28

1.48

0.408

1.05

1.26

0.707

X

1.05

I

.41

1.55

0.817

X

0

955

0.955

14-16

2%

Eo

REFERENCE

DATA

FOR

ENGINEERS

-

/

------4-n--x

VOLTAGE ACROSS

C1

VOLTAGE ACROSS

Cs

(E)

Waveform.

Fig.

17.

Conventional voltage doubler.

(A)

Basic circuit.

VOLTAGE

ACROSS

C2

(E)

Waveform

Fig.

18.

Cascade voltage doubler.

these circuits provide electronic regulation of

the

dc

output voltage.

Figs. 21 through 26

are

basic circuits of thyristors

used

as

controlled rectifiers.

In

many applications, it is

not necessary for all rectifier elements

to

be controlla-

ble, and it is common to find bridges composed of thy-

ristors and ordinary diodes

in

equal numbers; such

circuits are called

halfcontrolled,

whereas those con-

taining only thyristors

are

termed

full-controlled.

Figs.

21

and 22 show a full-controlled single-phase

thyristor bridge, the thyristor circuit being the same

whether

a

resistive or inductive load is being driven.

Fig.

19.

Bridge rectifier or voltage doubler.

2"'E,

t

I

.I

Fig.

20.

Circuit for high-order voltage multiplication.

0

Fig.

21.

Full-controlled single-phase thyristor bridge. The

broken lines indicate the path of

the

normal waveform at

full

conduction, i.e., where

a

=

0"

and

the

circuit behaves

like

a

diode rectifier. The quantity

a

is

the

firing

angle delay. Delay

range required for this circuit is

0"

to

180".

The voltage waveforms are different, however,

in

the

two cases. The principal difference of practical impor-

tance is that the range of firing-pulse phase control is

required to be different

in

the two cases. For the resis-

POWER

ELECTRONICS

+

--

I-

<

-

14-1

7

I--

1

CYCLE

---I

0

Fig.

22.

Full-controlled single-phase thyristor bridge working

into

an inductive load where

wL

>>

R.

Delay range required

is

0"

to

90".

tive load, phase control over the range

0"

to

180"

is

necessary to obtain full control from maximum output

voltage down

to

zero; for an almost pure inductive

load

(ie.,

a very high

wLiR

ratio), full-phase control is

obtained with a range of only

0"

to

90".

Fig.

23

shows a half-controlled single-phase recti-

fier driving an inductive load. For such loads a bypass

diode," CR,, must be added at the output; at the end of

a voltage half-cycle, current still flows

in

the choke,

but in

this

circuit the current is transferred at the end of

the voltage half-cycle to the bypass diode. The two

important effects

of

this diode are

(A)

that the output

voltage is clamped to zero while this inductively main-

tained current flows,

so

that the output waveform is the

same as that of Fig.

21

(having the characteristic flat

portion), and

(B)

that the transfer of load current from

the thyristor to the diode turns the thyristor

off.

If CR,

were

not

present, the waveform would still be the

same, since a zero voltage clamp would exist through a

series combination CR,-Q, or CR,-Q,, depending

on

which thyristor was conducting during the previous

half-cycle. However,

this

flow

of

uncontrolled current

through the thyristor, bypassing the supply, is undesir-

able; for example, should it be required that

the

output

voltage be turned

off

by removal of gate pulses,

this

action may prove

to

be impossible. The thyristor can

be held

on

continuously through the negative half-

cycle by inductive circulating current and is then ready

to conduct

on

the next positive half-cycle.

Thus

the

gate has lost control, and a

continuous

half-wave out-

put is produced. The bypass diode overcomes these

difficulties by ensuring thyristor turn-off at the end of

each voltage half-cycle.

Fig.

24

is a push-pull controlled rectifier circuit. Fig.

25

is a three-phase full-controlled rectifier circuit. Fig.

26

shows the circuit of a three-phase half-controlled

rectifier, the bypass diode being necessary only for

inductive loads.

In

all the cases illustrated in which a bypass diode is

used, this diode must be rated continuously for a maxi-

mum average current equal to the load current if the

full load current is

to

be drawn when

the

average

out-

put voltage is reduced almost to zero.

In

practice, a

larger diode

is

often used in this position than in the

bridge

arms,

or several diodes may be used in parallel.

For each of the circuits illustrated in Figs.

21

through

26,

Table

2

gives equations for the average dc

output voltage,

V,,,

at any angle

a

in

terms of the

maximum average dc output voltage,

V,,,

obtained at

a

=

0".

The table also shows the value

of

V,,

for each

circuit in terms of the peak sinusoidal input voltage,

Ep.

For the single-phase push-pull circuit, the peak

input voltage is

E,,-O-Ep,

and for all the three-phase

circuits

Ep

is

defined as the peak value of the line-to-

line voltage.

L

Fig.

24.

Full-controlled single-phase push-pull thyristor

rectifier working into inductive load. With diode connected,

waveform and delay range

are

as for Fig.

21.

Without diode,

waveform and delay range

are

as for Fig.

22

(if

WL

>>

R).

Addition

of

diode reduces critical inductance

of

L

for contin-

uous current.

14-18

REFERENCE

DATA

FOR ENGINEERS

I

0

Fig. 25. Full-controlled three-phase thyristor bridge working

into resistive load; waveforms of voltage output are

shown

for three values of

a.

Waveform develops characteristic flat

portion

when

a

2

60". Delay range required is

0"

to

120".

Linear Transistor Circuits

A

block diagram

of

a series voltage regulator is

shown

in

Fig.

27.

The regulator is composed of three

basic parts, a comparator, an amplifier, and a series

pass transistor. The purpose of the comparator is

to

compare the actual load voltage with a reference volt-

age and develop an error signal proportional to the dif-

ference between the two. This error voltage is

i

I

01

=

450

U

0

Fig.

26. Half-controlled three-phase thyristor bridge. The

bypass diode is necessary only when feeding an inductive

load. The waveforms are the

same

for

resistive

or

inductive

load. Delay range required

is

0"

to

180".

amplified and level shifted

to

provide base current to a

series transistor operating

in

its linear region. The

advantages

of

this system are its simplicity, its current

availability as a complete integrated circuit from sev-

eral manufacturers, its fast response, and its low

switching noise. Its disadvantage

is

its power dissipa-

TABLE

2.

MEAN

DC

OUTPUT

VOLTAGE FOR THYRISTOR-CONTROLLED

RECTIFIERS

Circuit

vdo

vda

Fig. 21

2EJa

Vd0(

1/2)(l+cosa)

Fig. 22

2EJr

v,,

cosa

Fig. 24, no diode and inductive load 2EJa

v,,

cosa

Fig. 24, all other cases 2EJr Vd0( 1/2)(1+cosa)

Fig. 23 2EJa vdo( 1/2)(1+cOsa)

Fig. 25

Fig. 26

14-19

SERIES

TRANSISTOR

+O

*

REFERENCE

VOLTAGE

Fig.

27.

Block

diagram

of

linear

series

transistor voltage

reg-

ulator.

tion, since the series transistor operates as a linear

resistive element. Series pass transistors can be pro-

vided external to available integrated circuits for

higher-voltage and higher-current applications. These

integrated control circuits can be provided with load-

current limit, remote start-up and shut-down, and over-

temperature limits.

Switch-Mode Power Supplies

To achieve higher output power capability, transistor

switch-mode power circuits have been developed. The

transistor is operated either in its saturated state or its

cutoff state. Switching frequencies can range from

2

kHz up to several megahertz. Load-voltage control is

provided by adjustment of the repetition rate, pulse

width, or on-to-off ratio.

A

functional block diagram

of a switching regulator is shown in Fig.

28.

An

input

and output filter

are

shown to reduce the ripple current

in the source and load, respectively. The transistor

either is on, connecting the source and the load, or is

cut

off,

disconnecting the source and

the

load.

Basic Converters

There are three basic dc-to-dc switching converter

configurations: the step-down (buck), the step-up

COMPARATOR,

ERROR AMPLIFIER

TO

PULSE WIDTH

CIRCUIT

Fig.

28.

Block

diagram

of

switch-mode power

supply.

(boost), the step-dowdstep-up (buck/boost), and two

additional nonisolated converter topologies: the

SEPIC

and the

Cuk

converter. These five circuits do not pro-

vide ohmic isolation between the input and the output.

Several other circuits have been developed that do pro-

vide transformer isolation between input and output.

The typical examples are the flyback converter, the

forward converter, the half-bridge, the full-bridge, the

push-pull, and the

Cuk

converter with isolation trans-

former. Table

3

shows these circuits, their schematic

diagrams, transfer functions, device rating equations,

circuit waveforms, and advantages and disadvantages.

The vast majority of contemporary designs of both

isolated and nonisolated converters is based upon the

application of two basic concepts: the forward-mode

and a flyback or boost-mode converter.

Forward-Mode Converter

Basics

The

buck

or

step-down converter,

which can be seen

in

Fig.

29,

is the basic forward-mode converter. Its

operation period-one switching cycle-has two dis-

tinct parts, which occur when the series power switch

is on and when it

is

off.

When the power switch is

on,

the

input voltage is connected to the input of the induc-

tor. The output of the inductor is the output voltage,

and the rectifier

is

back-biased. During this part of the

switching period, since there is a constant voltage

source connected across the inductor, the inductor cur-

rent begins to linearly ramp upward, which is

described by

where

ton

is the on-time of the switching cycle when

the power switch

Q

is

on.

During the on-time, energy is being stored within

the inductor core material in the form of magnetic

flux.

In

steady-state operation, there is sufficient energy

stored to carry the requirements of the load during the

next off-time, when the input voltage source

(V,)

is

not connected with the load. The next off-time is the

"off"

period of the power switch. When the power

switch turns off, the input voltage

of

the inductor flies

below ground and is clamped at one diode drop below

ground by the

catch diode.

The energy stored in the

inductor core causes the current continue to flow in its

previous direction through the inductor and the catch

diode,

thus

maintaining the load

current

loop.

This

removes the stored energy from the inductor. The

inductor current during

this

time

is

where

tOfi

is the off-time of the switching cycle when

the power switch

Q

is off.

This period ends when the power switch is once

again turned

on.

Regulation is accomplished by vary-

ing the

on-to-off

duty cycle of the power switch. The

Circuit

Configuration

Type

of

Converter

Ideal

Transfer

Function

Collector

Current

(i,)

t

Collector

Voltage

Rating

t

~

Diode

Currents

i

Diode

Voltages

(VRM)

t

TABLE

3.

POWER TRANSISTOR

AND

DIODE

WQUIREMENTS

FOR

SWITCH-MODE POWER

SUPPLIES

*

(A)

Buck (Step-Down)

Boost (Step-up) (B)

VOlV,

=

T/(T-T)

vCEO>vO+

ICRl

=

IRL

VCRI

=

VO

QI

d

e

N

0

a

rn

n

m

a

rn

2

0

rn

:

3

B

a

rn

2

G)

5

rn

a

v,

+

t-'-4

If

t

Voltage and

Current

Waveforms

Advantages

Disadvantages

t

For reliable operation, it is suggested and recommended that all voltage and current ratings be increased

to

125%

of the required maximum. Continued on next page.

d

High efficiency. Simple.

No

transformer. High frequency

operation. Easy

to

stabilize

regulator loop.

No

isolation between input and

output. Requires a crowbar if

QI

shorts. C, has high ripple

current. Current limit difficult.

Only one output

is

possible.

t

t-T-7

I

t

High efficiency. Simple.

No

transformer. High frequency

operation.

No

isolation between input and

output. High peak collector

current. Only one output is

possible. Poor transient response.

Regulator loop hard

to

stabilize.

t

2

hnr

t

Voltage inversion without

using a transformer. Simple,

High frequency operation.

QI must carry high

peak

current.

No

isolation between

input and output. Only one

output

is

possible.

Poor

transient response.

Middleton W.M. (ed.) Reference Data for Engineers: Radio, Electronics, Computer and Communications

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