ConMed ExcaliburPLUS PC - Page 47

[ce
of ZI allow Çİ to saturate over a greater propor-
tion of the conduction cycle as VBASE is
increased. Once saturation occurs, excess stored
charge accumulates, extending the time required
to remove the charge on turn-off and effectively
increasing the duty cycle of the amplifier. At
lower Cut power settings, VBASE is too low to
allow saturation, so Q1] conducts only partially,
absorbing some of the power that could other-
wise to supplied to Z1. This mode is not as effi-
cient as saturated operation because Q] sees
simultaneous voltage and current. However, the
current at this point is low enough to limit Os
dissipation to that Which can be dissipated by the
heat sink without excessive junction temperature
rise. When VBASE drops to below about 0.6 V,
no power is delivered to the load since this volt-
age is too low to cause base current to flow.
The PA consists of two separate Hybrid Cascode
sections connected at the collector bus. Each sec-
tion is made up of a single power MOSFET dri-
ving the emitters of three bipolar transistors.
Each bipolar base has its own base current control
network which is driven from a common VBASE
supply. Each collector and each base is separately
fused, allowing a failed part to disconnect itself
from the circuit without seriously affecting per-
formance. Failure of either power MOSFET will
reduce the RF power available by about half,
since the working half will continue to operate.
Voltage snubbing networks protect VGATE,
VBASE, and the power MOSFET drains from
being damaged in the event of any transistor fail-
ure. 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 sufficiently
high power and load resistance.
In Cut modes, VGATE is a fixed frequency rec-
tangular pulse and VBASE is varied from about
0.3 to +8.5 Vde to control output power. The
same is true in Blend except that VGATE is fur-
ther modulated to produce dead time with no
output. In Monopolar Coag Modes, Standard
and Spray, VBASE is fixed while VGATE is var-
ied in length. This variation in conduction time
controls the amount of energy stored in the
inductive part of Z1 every cycle and therefore
varies the output power level. Bipolar Coag Mode
uses a fixed VBASE and varies the number of
pulses per cycle period to control output power.
3.7 RF Output Section
Refer to Schematic 4.6. RF output power may
be supplied through one of two RF isolation
transformers, as selected by the Bipolar relay,
A5K6. When de-energized, K6 connects the
power amplifier collector bus to the Monopolar
output transformer, T3, which is resonated by
C28 and C29 and damped by A9R2. The main
secondary of T3 is capacitively coupled by C41
and C30 to the patient plate output jack and via
high-voltage reed relays, to the user selected
monopolar active accessory connectors. An auxil-
iary 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 circuitry to limit
output voltage in Cut.
When K6 is energized, it disconnects T3 and sup-
plies the Bipolar output transformer, T1, with
power from the power amplifier. The primary of
TI is resonated by C27 and is damped by A9R2.
This transformer is designed to meet the particu-
lar requirements of bipolar electrosurgery which is
characterized by much lower impedances and per-
missible voltages than those in monopolar opera-
tion. Its secondary is capacitively coupled to the
appropriate output connectors. Output wave-
forms under various conditions are shown in
Figure 3.6.
Balun transformer T6 operates to balance active
and return currents and thus attenuate common-
mode RF leakage in all monopolar modes.
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, 12.0 KVpp minimum for coag,
and 500 Vpp minimum for bipolar.
3.8 Aspen Return Monitor (A.R.M.) Circuitry
and
Software
The A.R.M. Circuit converts the electrical resis-
tance appearing in the return electrode circuit into
a digital value which can be processed by the
microprocessor A3U3. Software processes use
this value in conjunction with the SINGLE
re) 3-13