Power Supply/RF Board
Principles of Operation
Force FX Electrosurgical Generator C Service Manual 4-23
The output of U5 is a pair of 180° out-of-phase signals that are pulse width modulated
by comparing ECON with the internal oscillator ramp waveform. At the start of an
oscillator cycle, an output is turned on. It turns off when the ramp voltage crosses the
ECON level. The two output signals from U5 (pins 11 and 14) feed the MOSFET drivers
(U3A and U3B).
R36 and C42 set the U5 oscillator frequency to approximately 170 kHz. C45 controls the
ramp-up of the pulse width at power on for slow start control. Transformer T2 limits the
power transformer primary current, protecting against faults in the DC/DC switcher
power stage and faults in circuitry downstream of the switcher.
The output of T2 is rectified (CR3–CR6), filtered (R30 and C30), and fed to the current
limit pin (pin 9 of U5). During an overcurrent condition the U5 current limit function
resets the slow start circuit, resulting in the output cycling from on to off until the current
falls.
Pin 9 of U5 is also used for remote shut down of the DC/DC switcher through U6A and
CR8. The shut down signal comes from the main microcontroller on the Control board.
The resistor divider on the high voltage DC output formed by R52 and R53 is used for
dosage error sensing.
Low Voltage Power Supply
The low voltage power supply is rated for 40 W. It delivers a regulated +5 Vdc and
±12 Vdc output. This power supply automatically adjusts for both input voltage ranges. It
also has internal current limiting. The pinouts between the low voltage power supply and
the Power Supply/RF board are listed below:
PIN Voltage Test Point
1 +5 Vdc TP8
2-12VdcTP5
3 +12 Vdc TP6
4GROUNDTP9
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