Covidien Force FX, Service Manual

Page: 103 / 246

Power Supply/RF Board
Principles of Operation
Force FX Electrosurgical Generator C Service Manual
4-23
The RF stage consists of a single MOSFET power switch with associated gate drive
circuitry, an RF power transformer, tuning capacitors, an RF output L-C-C filter, output
directing relays, and topology selecting relays. Also included in this section are the RF
voltage and current sense circuits and a switched damping network for certain
operational modes.
The MOSFET gets its gating signal from the T_ON ASIC on the Control board. The T_ON
ASIC also provides the gating signal for the switched damping network.
When the topology selecting relays (K2 and K14) are unenergized, the RF stage is in the
Fulgurate and Spray coag modes; when both are energized, the RF stage is in the cut and
bipolar modes. For the Desiccate coag mode, K2 is unenergized and K14 is energized.
Primary Sense Circuits
The primary voltage and current sense circuits provide feedback information to the
feedback microcontroller in the bipolar and cut modes.
For voltage sensing, the two 10 k Ω resistors (R148 and R149) in series with the primary
of T13 work with the 100 Ω resistor across the secondary to divide the output voltage
down.
Depending on the front panel power setting, one of the four relays (K3 to K6) is switched
in to give optimum scaling. The four AD827 high speed op-amps, along with the
associated resistors, capacitors, and diodes, form a precision full wave rectifier circuit.
U11B is a high input impedance follower to prevent the rectifier circuit from loading
down the resistive divider. U11A is a follower that adds phase delay, which improves
balance in the rectified waveform between positive and negative half cycles of the input
signal.
The actual rectification is done with U8A and U8B. The rectified waveform is converted to
DC by the R-C filter after the last op-amp, with full scale being 5 Vdc.
The current sense circuit, which uses current transformers T6 and T8, works the same as
the voltage sense circuit. T6 senses bipolar current and T8 senses monopolar current.
Relay K7 selects the appropriate current. Note that the current scaling relays (K8 to K11)
switch at different power settings than the voltage scaling relays.
Redundant Sense Circuits
Redundant voltage and current sense circuits provide dosage error monitoring.
T16 monitors the current through the output capacitors (C150, C152, and C158). This
current is proportional to the output voltage of the generator. A full bridge rectifier is
formed by CR25–CR28. The rectifier output is filtered by R118 and C96. Op-amp U18
buffers the DC signal. R119 and C97 provide additional filtering.
T17 and the associated circuitry operate the same as the redundant voltage sense circuit.
The output is a signal proportional to the output current of the generator.