Valleylab Force FX-8C, Service Manual

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Power Supply/RF Board
4-22 Force FX-8C Service Manual
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 ohm resistors (R148 and R149) in series
with the primary of T13 work with the 100 ohm 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.
Cut Modes
In the cut modes, K2 is set so that diode CR2 is in parallel with the
MOSFET body drain diode, C34 and C41 are across the MOSFET, and the
transformer primary consists of windings 1-2 and 3-4 in series. K14 is
closed so the series capacitor bank (C150–C152, C158, and C159) is across
the output.
In the Low and Pure cut modes, the T_ON\ signal is a continuous pulse
train with a pulse width of 846 ns and a frequency of 390 kHz. In this case,
essentially two resonant circuits operate in tandem. The output L-C filter