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Valleylab Force FX-C - Page 69

Valleylab Force FX-C
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Force
FXT,'lLC
Service
Manual
Power
SupplylRF
Board
input
line voltage. Since
the
voltage
at
pin
8
varies
with
the
line,
it
can
sense
the
line
voltage
zero
crossing
as
well
as
the
peak
voltage.
Pins
2.
and
3
are
inputs
to
an
oscillator
used
for
triac
triggering
timing.
R7
and
CI
set
the
oscillator frequency.
Pin
7 is
tied
to Vss,
which
places
the
circuit
in
the
fail-safe
mode.
11ms,
once
the
circuit
enters
full
bridge
mode,
it
remains
in
that
mode
lmtil
input
power
is recycled. A
power
dropout
cannot
caLise
the
circuit to
accidentally
act
as
a
doubler
,",,,hen
the
higher
input
voltage
range
is
used.
AC/DC
Converter
The
AC
/DC
converter
uses
CR80
as
either
a
doubler
or
full
wave
rectifier,
depending
on
the
input
voltage.
In
either
case,
an
unregulated
nominal
300
Vdc
is
provided
to
the
DC/DC
switching
regulator.
Thermistors
R32
and
R,3
provide
inrush
current
limiting,
and
fuse
F4
provides
protection
against
faults
in
the
DC/DC
switcher.
Capacitors
C3, CIO,
Cll,
and
C22
are
an
energy
storage
reservoir
for
the
DC
/DC
switcher. C29
is
a
high
frequency
bypass
filter. Bleeder
resistors
R5
and
R6
discharge
the
capacitors
when
the
AC
line is
disconnected
or
the
power
switch
is
turned
off.
DC/DC
Switching
Regulator
The
DC/DC
switching
regulator
is a
buck
derived,
pulse
width
modulated
(PWM)
transformer.
It
is
an
isolated,
fixed full
bridge
converter.
The
PWM
IC (US) is
used
in
the
\·oltage
mode.
The
output
of
the
regulator
is
adjustable
from
approximately
zero
(0)
to
180 Vdc.
The
full
bridge
consists
of
four
power
MOSFETs
(Ql,
Q3,
Q4,
and
Q5)
that
operate
at
AC
line
potential.
Transistors
Q3
and
Q5
are
on
while
QI
and
Q4
<He
off,
and
the
reverse.
]n
this
manner,
power
signals
to
the
power
transformer
are
bidirectional,
or
push-pull.
This
allows
full
utilization
of
the
transformer
core
magnetization
capability.
Regulation
is
achieved
by
modulating
the
time
that
each
MOSFET
pair
is
on.
Capacitor
C32
in
series
with
the
power
transformer
T3
primary
prevents
DC
nux
imbalance.
A
snubber
circuit
(C27
and
R31)
absorbs
leakage
energy
spikes.
Another
snubber
circuit
(C49
and
R5I)
reduces
due
to
reverse
recovery
of
the
output
bridge
rectifier.
The
gate
driver
circuitry
for
each
MOSFET is
transformer-coupled
through
TI
to
provide
AC
line isolation.
It
consists
of
a
dual
MOSFET
driver
(U3)
and
various
damping
resistors. Resistors RI2, R18,
R2I,
and
R26
minimize
tum-off
oscillations. Resistors R22
and
R23
damp
ringing
due
to
parasitic
inductances
in
Tl.
Blocking
capacitors
C24
and
C25
prevent
DC
flux
imbalance
in
Tl.
Note:
TI
consists
of
two
transformers
electrically
and
magnetically
isolated
from
each
other
but
assembled
into
the
same
package.
TIA
and
TIB
form
one
transformer;
TIC
and
TID
form
the
other.
The
output
of
the
power
tran.sformer is full
wave
rectified
by
a
high
voltage
diode
bridge
(ClUO, CR19,
and
CR23).
Ll,
C33,
and
C35
filter
the
rectified
pmver
The
regulated
DC
output
from
this
supply
is
the
input
to
the
RF
stage
of
the
generator.
4-19

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