Covidien Force FX, Service Manual

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Power Supply/RF Board
4-26
Force FX Electrosurgical Generator C Service Manual
The other input to the comparator is an analog threshold level (VMAX_CLP) that is set by
the main microcontroller on the Control board and depends on the mode and power
setting.
When the peak detected sample of the output voltage exceeds the threshold, one-shot
U14A is fired and generates a 3 ms pulse (SPARK_CON) that is sent to the T_ON ASIC on
the Control board. This pulse is ignored if it occurs during the first 0.2 seconds of
activation. Otherwise, SPARK_CON causes the T_ON\ signal to stop.
The feedback microcontroller on the Control board senses this and realizes that a spark
has been suppressed. The feedback microcontroller waits 100 ms in Pure cut, then re-
initiates T_ON\ with a frequency of 470 kHz. The frequency returns to 394 kHz after
1 second of continuous activation or when the generator is reactivated.
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