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Theory
of
Operation
-
494[l4g4Ap
Servlce,
Vot.
1
Flgure
7-35.
Tirning
waveforms
for
soft-start
clrcuit
Inverter
Driver.
The
inverter
driver
consists
of
transistors
Q6072
and
e6079,
transformer
T60g1,
and
related
components.
This
is
a
push-
pull
amplifier
with
diode
protection
in
the collector
circuits
to
pr€vent
dam,
age
frorn
voltage
transients
during
operation.
The
drive
signal
is
induced
into.the
two slcondary
wlndings
of
T6081
and
coupted
to
the
output
stage.
^ 9utput
Stage.
This
circuit
consists
of
transistors
Q.2071
and
Q2061,
series
LG
tank
L10g1/C1063,
and
transformer
T4071
. The
output
transistors
are
con-
nected
in
a
half-bridge
configuration.
The
two
transis-
tors
drive
the
series
tank,
which
acts
as
an
energy
storage
element
and.
.
an
averaging
circuit.
Outp-ul
transfonner
T4071
is
driven
by
tne
tanf
circuit,
anct
it,
in
turn,
drives
the
secondary
circuits.
Primary
regulation,-as
discussed
previously,
occurs
when
the
duty cycle
of
the inverter
driver
main
switch_
ing
transistors
is
varied.
Maximum
duty
cycle
occurs
at
low
input
line (90
V)
and
fulty
toaded
output.
At
max-
imum
duty cycle,
both
transistbrs
are
off
for
only
1Ao/o of
the
period,
or
l.Sps.
This
short
interval
allbws
any
stored
base
charge
to
deplete,
so
there
is
no
chance
both
transistors
will
conduct
at
the
same
time.
Minimum
duty cycte
occurs
at
high
input
tine (132
V)
and
minimum
loaded
output.
At minimum
duty cycle,
each
transistor
is
off
for approximately
6
ps,
or
4O;/o
ot
the
total
period.
Soft'Start and
Prlmary
Over-Current
Clrcuits.
The
soft-start
circuit
consists
of
U6053 and
associated
com-
ponents.
Soft-start
gradually
increases
the
switching
transistofs
duty
cycle at
turn-on
or
after over-current
shutdown
to
prevent
excessive
translstor current
due to
charging
output
capacitors.
R€fer
to
Figure
7-35 for
timing waveforms.
The
primary
over-current
circuit
protects
against
secondary
shorts
that could
destroy
the switching
transistors.
T2080
s€nses
the
collector
current
in
Q2071
and creates
a vottage
on
pin
5
of
U60468.
lf the
bias on
pin
5
surpasses
the 2.5 V
reference
on
pin
6, at
approximately
6
A
through
e2OT1,
the output
of
U60468
sets
U6063A.
U6063A is
a
D-type
flipflop
used
as a
timer
to
shut
down
the inverter
logic for
approxi-
matety
1 s and
to
reset
the
soft-start
circuit.
TRIGGER
IN
(PIN
2I
lm-
CONTIOL
VOLTAGE
=
6.6 v
tt(v1(
f
=140k.95
pF
l-----J
F
.,o*.,P
oR
AFIER
CuRRENT.,,.,
__I
ll
l.-
"o.*ro.
o"enanon
*l
2727-161
7-105