Power Supply/RF Board
Principles of Operation
Force FX Electrosurgical Generator C Service Manual 4-27
In bipolar mode, the Patient Return Electrode receptacle relay is open. Relays K12 and
K17 route bipolar current to the Bipolar Instrument receptacle.
All output relays are open when the generator is not being activated.
Spark Control Circuit
The spark control uses the voltage sense circuit to monitor the output voltage. It
interrupts the delivery of power if the output voltage exceeds a preset threshold. This
greatly reduces sparking when an activated accessory is removed from tissue.
Sparking occurs because the RF stage tuning results in a higher natural gain at light loads
than at heavy loads. Thus, during sudden transitions from heavy to light loads, the output
voltage rises faster than the microcontroller can respond. This analog circuit works
outside the microcontroller loop at a much greater speed.
The rectified but unfiltered waveform from the output voltage sense circuit is fed into a
peak detector (U23A, CR29, and C104). A high impedance buffer (U23B) maintains the
integrity of the peak detected signal. The output of this buffer is divided and fed to a
comparator.
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.
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