Valleylab Force FX-8C, Service Manual

Page: 81 / 218

Power Supply/RF Board
Principles of Operation
Force FX-8C Service Manual 4-25
RF Leakage Reduction Circuit
Fulgurate and Spray Coag Modes
For the Fulgurate and Spray coag modes, the high voltage RF output pulse
repetition period varies with changes in spark and patient tissue
impedance to limit the RF leakage current to a desired level. The VSENSE
signal is obtained from the divider (R58, R25) located on the primary side
of T4. VSENSE is input to a negative peak detector (U24A) that generates
the analog signal (VPEAK–). Then U25A amplifies and inverts the signal.
The averaged signal (now called VPK+) is input to the feedback
microcontroller on the Control board and added to the ECON value at the
selected power setting. The sum of these signals, with the proper gain
factors, varies linearly with load impedance at the patient site. This sum is
input into a pulse width modulator that sends its output (WAK\) to a
NAND gate. Thus, the T_ON\ signal is inhibited for up to four
consecutive cycles. Without the leakage control, the pulse repetition
period is 17 µs. With the leakage control fully activated, the total pulse
train repeats every 84 µs with a maximum dead time of 60% or 51 µs.
REM Circuit
Components U17 along with R95, R96, R97, and C79 form a precision
oscillator. R96 is adjusted for the frequency that will produce maximum
voltage amplitude (80 ± 10 kHz) at the REM connector (J17). R98 is a
temperature compensating thermocouple that cancels temperature drift
from the components forming the oscillator.
The REM transformer (T10) provides isolated reflected impedance sensing
for tissue impedance across the REM patient return electrode terminals
(connected to J17, pins 1 and 2). In addition to tuning the REM circuit,
capacitors C155, C156, C169, and C170 provide a return path for high
frequency RF signals through C157 to the RF output transformer. The
REM transformer (T10) and capacitors C155, C156, C169, and C170 form a
resonant circuit with a nominal operating frequency of 80 kHz.
Pin 1 of T10 clocks the active synchronous rectifier formed by CMOS
switch U28A. This device is closed during the positive period of the
REM_AC signal and open during the negative period. When the switch is
closed, C122 is charged to the peak positive value of REM_AC. Then,
U31B amplifies, filters and buffers the charge on C122 to produce the
R_SEN signal. The microcontrollers monitor the R_SEN signal (which is a
DC voltage proportional to impedance) to determine the patient return
electrode status.
IsoBloc Circuit
The IsoBloc circuit provides a means of detecting a switch closure in an
output accessory while maintaining electrical isolation between the
generator output and ground referenced circuitry. The IsoBloc circuit
consists of an isolated DC power supply, a comparator to detect switch
closure, and an optoisolator link from the output connected circuitry to