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Valleylab Force FX-C - Page 72

Valleylab Force FX-C
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Power
Supp/y/RF
Board
4-22
When
the
topology
selecting relays (K2
and
K14)
are
unenergized,
the
RF
stage
is
in
the
Fulgurate
and
Spray
coag modes;
when
both
are
energized,
the
RF stage is
in
the
cut
and
bipolar
modes.
For
the
Desiccate
coag
mode,
K2
is
unenergized
and
K14 is energized.
Primary
Sense
Circuits
The
primary
voltage
and
current
sense
circuits
provide
feedback
information
to
the
feedback rnicrocontroller
in
the
bipolar
and
cut
modes.
For voltage sensing,
the
two
10
k
ohm
resistors (Rl48
and
R149)
in
series
with
the
primary
of
T13
work
with
the
100
ohm
resistor across
the
secondary
to
divide
the
output
voltage
down.
Depending
on
the
front
panel
power
setting,
one
of
the
four relays
(K3
to
K6)
is
switched
in
to give
optimum
scaling.
The
four
AD827
high
speed
op-amps,
along
with
the
associated resistors, capacitors,
and
diodes,
form
a precision full
wave
rectifier circuit.
UllB
is a
high
input
impedance
follower
to
prevent
the
rectifier circuit
from
loading
down
the
resistive divider.
UllA
is a follower
that
adds
phase
delay,
which
improves
balance
in
the
rectified
waveform
between
positive
and
negative
half
cycles
of
the
input
signaL
The
actual
rectification
is
done
with
U8A
and
U8B.
The
rectified
waveform
is
converted
to
DC
by
the
R-C filter after
the
last
op-amp,
with
full scale
being5Vdc.
The
current
sense
circuit,
which
uses
current
transformers
T6
and
T8,
works
the
same
as
the
voltage sense circuit. T6 senses bipolar
current
and
T8
senses
monopolar
current. Relay K7 selects
the
appropriate
current.
Note
that
the
current
scaling relays (K8 to
Kll)
switch
at
different
power
settings
than
the
voltage
scaling relays.
Redundant
Sense
Circuits
Redundant
voltage
and
current
sense circuits
provide
dosage
error
monitoring.
T16
monitors
the
current
through
the
output
capacitors (C150, C152,
and
C158). This
current
is
proportional
to
the
output
voltage
of
the generator.
A full
bridge
rectifier
is
formed
by
CR25-CR28.
The
rectifier
output
is
filtered
by
R1l8
and
C96.
Op-amp
U18 buffers
the
DC
signal. R1l9
and
C97
provide
additional
filtering.
T17
and
the
associated circuitry
operate
the
same
as
the
redundant
voltage sense circuit. The
output
is a signal
proportional
to
the
output
current
of
the generator.
Cut
Modes
In
the
cut
modes,
K2
is
set
so
that
diode
CR2 is
in
parallel
with
the
MOSFET
body
drain
diode,
C34
and
C4l
are
across
the
MOSFET,
and
the
transformer
primary
consists
of
windings
1-2
and
3-4
in
series. K14 is
closed
so
the
series capacitor
bank
(C150-C152, C158,
and
C159)
is
across
the
output.
In
the
Low
and
Pure
cut
modes,
the
T_ON\
signal is a
continuous
pulse
train
with
a
pulse
width
of
846
ns
and
a frequency
of
390
kHz.
In
this case,
essentially
two
resonant
circuits
operate
in
tandem.
The
output
Force
F)(TM-C
Service
Manual

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