Covidien Force FX, Service Manual

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Power Supply/RF Board
4-28
Force FX Electrosurgical Generator C Service Manual
The phototransistor, which is connected to an input on the FPGA U16-B, through the
U24 Bus Transceiver, 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.
At temperatures below 40° C, the output of U2A is low. When the thermistor exceeds
the threshold, the voltage comparator changes state causing the output at U2A to go
high, turning on FET Q2. This applies 12 V to the cooling fan. When the temperature
drops to 35° C, the circuit reverts to its low temperature state, and the cooling fan
switches off.
Thermal Sensing (High Temperature Limit)
A reference voltage is applied to the non-inverting input of comparator U26A. Resistors
R111 and R110 determine the reference voltage. When the temperature of NTC
thermistor R109 is 50° C, the reference voltage is set to be equal to the voltage at the
inverting input of U26A. Resistor R94 provides positive feedback causing approximately
10° C of hysteresis.
When the thermistor exceeds the threshold, the voltage comparator changes state,
causing the output at U26A to go high. The main microcontroller reads this signal
(TEMP_HI) and flashes number 451 alternately with the power settings. When the
temperature drops to 40° C, the circuit reverts to its low temperature state, and the
generator allows activation for an unlimited duration.