Power Supply/RF Board
Principles of Operation
Force FX Electrosurgical Generator C Service Manual 4-29
Power Supply
The three 4081 buffers (U29) drive three TN2404KL FETs connected to transformers
which are operated in a quasi-resonant flyback mode with their associated 6800 pF
capacitors. The voltages at the secondaries of the three transformers are half-wave
rectified and referenced to three separate isolated grounds to provide -8 V for operating
the isolated activation circuitry.
Optoisolators
The isolated power supply voltages produced by the IsoBloc power supplies are
connected to the active output terminals of the generator. Handswitch activation is
accomplished by sensing active to CUT or active to COAG switch closure in a handheld
accessory.
Switch closure is detected by comparing the voltage across the switch to a divider
reference with comparators U32, U33, and U34. Current limiting resistors, in series with
the LEDs in the optoisolators, cause the LEDs to light. The phototransistor in the
optoisolator detects this light.
The phototransistor, which is connected to an input on the 82C55 expansion port in the
main microcontroller circuit, turns on, pulling the associated input low. This is interpreted
by the software as an activation request, and the generator is activated accordingly.
Temperature Sense Circuits
The Force FX Electrosurgical Generator C features two temperature sense circuits.
The first temperature sense circuit measures the air temperature adjacent to the RF
output FET heat sink. When the temperature at this location reaches approximately
40° C, a cooling fan switches on to minimize heat buildup. The fan switches off when
the temperature drops below approximately 35° C.
The second temperature sense circuit measures the air temperature near the Control
board, which is located in the forward third of the generator enclosure. When this
temperature reaches approximately 50° C, the main microcontroller flashes number 451
alternately with the power settings to indicate that the generator is too hot. Generator
operation reverts to normal when the temperature decreases to below 40° C.
Thermal Sensing (Fan Control)
A reference voltage is applied to the noninverting input of comparator U2A. Resistors
R15 and R16 determine the reference voltage. When the temperature of NTC thermistor
R19 is 40° C, the reference voltage is set to be equal to the voltage at the inverting input
of U2A. Resistor R10 provides positive feedback causing approximately 5° C of hysteresis.
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