Output Section - ConMed Sabre 2400 Operators & Service Manual

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The collector voltage may rise to many times the
value of the RF supply voltage RFSUP after turn
off. Since the emitters are now disconnected, the
collector-base voltage can take on the highest
value which that junction will sustain with little
chance of second-breakdown. The base bypass
capacitors, are sized to ensure that they can
absorb all of the stored turn-off charge without
allowing Q5's drain voltage to approach its break-
down limit. Further, this charge is now available
to charge the base on the next cycle, thereby sig-
nificantly reducing the net current drain from the
VBASE supply.
Each bipolar transistor base has its own current
control network which is driven from a common
VBASE supply. Each collector and each base is
separately fused, allowing a failed part to dis-
connect itself from the circuit without seriously
affecting performance. Voltage snubbing net-
works protect BGATE, VBASE, and the power
MOSFET drain from being damaged in the event
of any transistor failure. This limits the extent of
failure damage. Each collector is equipped with a
diode which allows the voltage on the output bus
to swing negative with respect to ground, as it
does in all monopolar modes of operation at suf-
ficiently high power and load resistance.
In Cut modes, BGATE is a fixed frequency rect-
angular pulse and VBASE is varied from about
0.3 to +8.5 Vdc to control output power. The
same is true in Blend except that BGATE is fur-
ther modulated to produce dead time with no
output.
Bipolar Coag Mode uses a fixed VBASE and var-
ies the number of pulses per cycle period to con-
trol output power. See Figure 3.2 for representa-
tive BGATE drive waveforms.
3-8
3.1.7 RF Output Section
CAUTION: Because of the high peak-to-peak
amplitudes of these waveforms, use oscillo-
scope probes that can withstand 2 KVpp mini-
mum for cut, 10.0 KVpp minimum for coag,
and 500 Vpp minimum for bipolar.
Refer to Figure 4.6 for the schematic diagram.
RF output power may be supplied through one
of two RF isolation transformers, as selected by
the relay, K4. When deenergized, K4 connects the
power amplifier collector bus to the Monopolar
output transformer, T3, which is resonated by
C27 and C28 and damped by the chassis mount-
ed resistor A5R1. The secondary of T3 is capaci-
tively coupled by C26 and C29 to the return
monitor circuit, and via high-voltage reed relays,
to the user selected monopolar active accessory
connectors. An auxiliary single-turn T3 secondary
supplies a replica of the power amplifier collector
voltage to the VSENSE circuit. This rectifies and
peak-detects this signal for use by the control cir-
cuitry to limit output voltage.
When K4 is energized, it disconnects T3 and
supplies the bipolar output transformer, T1, with
power from the power amplifier. The primary of
T1 is resonated by C62 and is damped by A5R1.
This transformer is designed to meet the particu-
lar requirements of bipolar electrosurgery which
are characterized by much lower impedances and
permissible voltages than those in monopolar
operation. Its secondary is capacitively coupled
to the appropriate output connectors. Output
waveforms under various conditions are shown in
Figure 3.3.