3-4
3.2.1 RF Power Supply (RFPS)
The RFPS is comprised a line transformer and a
Power Factor Control (PFC) section. The PFC
converts Mains power from the transformer to
approximately 95 volts D.C. using techniques
that ensure the mains current into the supply is
sinusoidal and in phase with the mains voltage.
By doing so, RMS current and harmonic distor-
tion are reduced. Jumpers on the power board are
used to select input line voltage range. A graphic
on the power board and on the schematic indicate
which jumpers are used for each input voltage
range.
WARNING: THE RFPS VOLTAGE TAKES
SEVERAL SECONDS TO DECAY AFTER
THE POWER IS SWITCHED OFF. TO
WARN THE SERVICE TECHNICIAN, A
RED LED (DS1) ON THE POWER BOARD
INDICATES WHEN VOLTAGE IS STILL
ON THE BOARD.
The System Controller can enable or disable the
PFC section of the RFPS. The PFC is normally
enabled during operation to ensure a resistive load
is presented to the Mains.
In addition to the RFPS winding, there is also
another secondary winding on the Mains trans-
former that is rectified and filtered to provide
unregulated +25VDC voltage as the input for a
+12V switching regulator. The +12V output is
then used to supply input voltage to a variety of
low-voltage regulators on the controller board.
3.2.2 RF Amplifier and Transformer
The RF Amplifier and Transformer portions use
a switch-mode resonant hybrid-cascode ampli-
fier to convert the power from the RFPS to the
RF energy necessary for electrosurgery. One may
think of the amplifier as a high-speed switch that
pulses current through a resonant circuit, which is
formed by the monopolar or bipolar transformer
together with capacitors that are connected to
the transformer primary and secondary wind-
ings. The transformers are designed with a good
deal of leakage inductance in order to provide
inductance for resonating with the capacitors.
One Metal-Oxide-Semiconductor Field Effect
Transistor (MOSFET) is connected in series with
three parallel bipolar transistors to provide the
switching. The pulses to drive the gate on the
MOSFET in this arrangement come from the RF
Controller (RFGATE). The base connections of
the three parallel bipolar transistors are also driven
by a signal that originates from the RF Controller
(Vbase_PWM).
The RFGATE drive pulses provide the basic pulse
pattern that is used to form the electrosurgical
waveform, and have a set pulse-pattern and pulse-
width for each mode. A drive of several pulses
at a frequency that closely matches the resonant
frequency of the amplifier characterize Cut and
Blend modes, and the output pulses substantially
correspond to the drive. Spray and Standard
Coag modes, however, are characterized by pulses
that occur less frequently where the amplifier is
allowed to “ring” at its resonant frequency.
Rapid regulation of the output power in this
arrangement is provided by VBASE_PWM; as
VBASE_PWM is increased, the output power
increases. As noted in the RF Controller dis-
cussion, the RF Controller compares the out-
put power with the desired power and adjusts
the VBASE_PWM to minimize the difference.
VBASE_PWM enters the amplifier as a 312 KHz
Pulse-Width-Modulated (PWM) signal that is fil-
tered to become a variable DC base drive signal.
Finally, the RF Amplifier and Transformer provide
capabilities for sensing RF output current and
voltage. The voltage sensors that are used for
power control and power monitoring are indepen-
dent windings on the output transformers. The
current is also measured on the primary side of
the transformers. With proper characterization of
the transformer, the controller obtains an accurate
representation of the voltage, current, and thus the
output power of the system.
3.2.3 Electrosurgical Outputs
Relays are provided to isolate electrosurgical out-
puts and select which outputs are active. The
System Controller selects the appropriate output
relays based upon activation command inputs.
The Monitor utilizes current sensors implemented
on each monopolar electrosurgical output to
determine whether current is flowing only to the
correct outputs. In the event that current flows
in an output that is not selected, the Monitor can
independently disable RF. The monitor uses the
bipolar primary voltage to sense that the bipolar
relay has been activated.
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