Covidien Force FX, Service Manual

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Power Supply/RF Board
Principles of Operation
Force FX Electrosurgical Generator C Service Manual
4-21
and T1D form the other.
The output of the power transformer is full wave rectified by a high voltage diode bridge
(CR10, CR13, CR19, and CR23). L1, C33, and C35 filter the rectified power signal. The
regulated DC output from this supply is the input to the RF stage of the generator.
The SYS_ECON signal from the microcontroller controls the output voltage level. This 0 to
5 Vdc signal sets the reference for the PWM control loop.
An external op-amp (U7B) is used for gain and integration, because common mode
voltage limitations in U5’s internal op-amp preclude its use over the full range of 0 to 5 V.
The internal op-amp is connected as a follower.
SYS_ECON is compared to the feedback voltage from the output divider (R34, R35, and
R49), and an error signal (ECON) is sent to the PWM microcontroller. In addition to the
error signal, U7B and the associated R-C networks provide lead-lag loop compensation to
increase the bandwidth of the regulator beyond that of the output L-C filter.
Note:
U7A is used for random gain switching in the Spray mode and is configured for
unity gain in all other modes.
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.