Power Supply/RF Board
Principles of Operation
Force EZ-C Series Service Manual
5-21
RF Output Stage
The RF stage consists of the following components:
•
a single MOSFET power switch with associated gate drive circuitry
•
an RF power transformer
•
tuning capacitors
•
an RF output L-C filter
•
output directing relays
•
topology selecting relays
•
RF voltage and current sense circuits
•
a switched damping network for certain operational modes
The MOSFET gets its gating signal from the T_ON ASIC (U9) on the Control
board. The T_ON ASIC also provides the gating signal for the switched damping
network.
The topology selecting relays (K1 and K15) set the RF stage in the following
modes:
Note that cut modes include the low 2 and low 3 settings from the topology
standpoint.
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 in series with the primary of T12
work in concert with the 100 ohm resistor across the secondary to divide the
output voltage down to a manageable level. Depending on the front panel power
setting, one of three relays (K2–K4) switches in to give optimum scaling. The
four op-amps (U18 and U23), along with the associated resistors, capacitors, and
diodes, form a precision full wave rectifier circuit. U23B is a high input
impedance follower to prevent the rectifier circuit from loading down the resistive
divider. U23A is a follower that adds phase delay, which improves balance in the
rectified waveform between positive and negative half cycles of the input signal.
U18A and U18B perform the actual rectification. The R-C filter after the last
op-amp converts the rectified waveform to DC, with the full scale at 5 Vdc.
Warning
High frequency, high voltage signals that can cause severe burns are present in
the RF output stage and in the associated mounting and heat sink hardware
described in this manual. Take appropriate precautions when testing and
troubleshooting this area of the generator.
unenergized
energized
RF stage is in the coag mode
RF stage is in the cut and bipolar
mode.
