AEG Electrolux ERN 2922 User Manual download pdf (Page 22)

unwinding.

Identify

the two starting

wires

with

a dot of

nail enamel

or

modeler's enamel

paint.

The dot cor-

responds

to the dots shown

near T1

in Fig.

4. If you already

have a com-

mercial

pulse transformer,

so much

the better.

With proportional

controller

cir-

cuits,

an order

of

magnitude better

heat

control

can

be achieved

than

is

possible

with simple on

/off control-

lers.

The

disadvantage

of a phase

control circuit,

however,

is

that,

like

a lamp dimmer

or the

circuit

just de-

scribed,

large

amounts of

r -f

noise

are

normally generated.

This noise

can disturb

communications

and

even disrupt

the operation

of

some

computers.

It is a byproduct

of the

waveform chopping

action,

as

illus-

trated

in Fig. 5.

R -f

filters can be

used to

lower the

level of

noise, but

when kilowatt

loads are

involved,

the

filter becomes

too large

and ex-

pensive

to be practical.

Zero- Voltage-

Switching

Proportional Controller

Zero

voltage

switching,

or ZVS, is a

technique used to control

the

amount of power delivered

to the

load by conducting

integral ac cycles

within

a timebase.

By

controlling

the

ratio of

fully "on"

cycles

to fully

"off" cycles,

the amount of

power

can be adjusted, as shown

in Fig. 6.

While

phase control

generates

noise

as a

result of

waveform

chop-

ping, ZVS conducts

full cycles that

are triggered at

zero

voltage, which

results

in negligible

generation

of rfi

(radio- frequency

interference). There-

fore, this method of control

is rec-

ommended

for high -power

loads and

sensitive environments.

The

sample

waveform shown in

Fig.

6

has a timebase of

only three cy-

cles.

Best resolution available

is in

in-

crements

of 33 percent

of rated pow-

er.

Actual circuits

have timebases

of

hundreds of cycles,

with

resolutions

that

can exceed

1 percent.

For all

practical purposes,

this is considered

to be continuous.

a

El

Ramp

Cerro

rs

for Sync

Comp

Frill Have

logic

Supply

Rel

rence

Vol

leye

Pirl se

Amp

a

0

Fig.

7.

Telefunken's

U217B controller

IC has many functions normally supplied

by several

ICs and discrete components.

Traditionally,

ZVS circuits

have

been

difficult

to implement.

Now,

however,

a new breed

of

integrated

circuits

from

Telefunken offers

a so-

lution.

These ICs

have all the

fea-

tures of

the CA3059

plus

line -syn-

chronized

ramp generation,

dc

offset

correction,

an accurate

reference

voltage

and

much

more.

These chips

represent

a prime

example

of

how

semiconductor

manufacturers

can

incorporate

into a single

IC complete

functional blocks

that

heretofore

re-

quired

a board

filled

with discrete

components

and

ICs.

Simplest

of

the

Telefunken

family

of controller

ICs, the

U217B

inte-

gral -cycle

zero -voltage

switch

is

powerful enough

to handle almost

any task put to it. A block diagram

with

pinouts

of this

integrated cir'icit

is shown in

Fig. 7.

In

this

IC, the

ramp generator

is

the

key to the proportional

control-

ler's

characteristics.

By superimpos-

ing a zero

voltage on bias pin

3, the

comparator

modifies

its

pulse

width

as shown

in Fig. 8.

A high tempera-

ture condition

is shown

in (A),

where

the "on"

periods

are shorter

than

in

a

low-

temperature

condition,

which

is

illustrated in

(B).

Triac

gating

pulses

are generated

only

if pin

3

is positive

with

respect

to

pin 4

of

the

IC. The ramp's period

is

long compared

to the 60

-Hz power

Fig.

8.

In

integral

control generation

with

a U217B,

the thermistor

voltage

at

pin

4 (dashed

lines)

is

compared

with

a dc-

shifted

ramp

at

pin

3.

This modifies

the

power delivered

to the load,

as illustrated by

the blocks at

the

bottom.

28

/ MODERN ELECTRONICS

/ February 1988

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