Valleylab Force FX-8C, Service Manual

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Power Supply/RF Board
Principles of Operation
Force FX-8C Service Manual 4-23
is tuned just slightly higher than the RF switching frequency, achieving a
high degree of filtering. The output is very sinusoidal over the full range
of load impedances. Capacitors C34 and C41 are tuned with the RF
transformer primary so that the flyback voltage appearing across the
MOSFET at turn off is a half sine pulse and returns to zero volts before the
next cycle begins. The T_ON\ pulse width is chosen to support this
tuning. This zero voltage switching improves the efficiency of the RF stage
and is effective over a wide range of load impedances.
The circuit topology of the Blend cut mode is the same as the Pure cut mode.
In Blend mode, however, the T_ON\ signal is an interrupted pulse train with
a 50% duty cycle and a pulse train repetition rate of 27 kHz. For a given
power setting, Blend gives a higher peak current, providing better hemostasis
than Pure or Low. To minimize ringing at the beginning of the off period of
the Blend waveform envelope, the damping resistor is switched on just before
switching ends and stays on for part of the off period.
Bipolar Modes
The circuit topology for the bipolar modes is essentially the same as the
cut modes, except the output voltage is tapped off C152 and the switching
frequency is 470 kHz. These differences allow for the higher currents and
lower voltages required in bipolar surgery while still maintaining the
advantages of zero voltage switching in the MOSFET. The T_ON\ signal
is a continuous pulse train with a 423 ns pulse width.
Coag Modes
In the Fulgurate and Spray coag modes, K2 is set so that diode CR2 blocks
reverse current in the power MOSFET, C40 as well as C34 and C41 are
across the MOSFET, and the transformer primary consists of winding 1-2
only. K14 is open, keeping the series capacitor bank (C150–C152, C158,
and C159) out of the circuit.
In the LCF Fulgurate coag mode, the T_ON\ signal is a continuous pulse
train with a pulse width of 1.69 µs and a frequency of 57 kHz. When the
MOSFET turns on, some energy is delivered to the output and some is
stored in the T4 core. When the MOSFET is turned off, the energy stored in
the core rings out with a frequency of 591 kHz. The frequency is set by
C34, C40, C41, and the inductance of winding 1-2 of T4. CR2 blocks
reverse current in the body drain diode of the MOSFET so that the power
waveform can ring negative. This allows high peak voltages to be
achieved at the output. In most cases, all the energy stored in the
transformer core during one switching cycle is delivered to the load before
the next cycle begins. The Fulgurate mode works the same as the LCF
Fulgurate mode, except the T_ON\ signal is a continuous pulse train with
a pulse width is 1.69 µs and a repetition frequency of 30 kHz.
The Spray mode works essentially the same as the Fulgurate mode, except
the T_ON\ pulse frequency is randomized over the range of 21.6 kHz to
35.23 kHz. In addition, amplifier U7A randomly varies the output
amplitude by 10%. The ECON–GAIN signal from the Control board
changes the gain of U7A between 1 and 1.1.