Covidien Force FX, Service Manual

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Power Supply/RF Board
Principles of Operation
Force FX Electrosurgical Generator C Service Manual
4-25
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 of 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.
To minimize ringing on the output voltage waveform at light loads, transistor Q7
switches in the 50 watt, 150 Ω heat sink mounted resistor in series with the transformer
primary for part of the RF switching cycle.
In the Desiccate coag mode, K2 is closed and K14 is open. The T_ON\ signal is a
continuous pulse train with a pulse width of 2 µs and a frequency of 39 kHz. The output
resonates with a frequency of 308 kHz.
Output Relays
In all monopolar modes, K13 is closed and routes patient return current through the
Patient Return Electrode receptacle. K15 routes active current through the Monopolar
1/CEM Instrument receptacle. K16 routes the active current through the Monopolar 2
Instrument receptacle.
In bipolar mode, the Patient Return Electrode receptacle relay is open. Relays K12 and
K17 route bipolar current to the Bipolar Instrument receptacle.
All output relays are open when the generator is not being activated.
Spark Control Circuit
The spark control uses the voltage sense circuit to monitor the output voltage. It
interrupts the delivery of power if the output voltage exceeds a preset threshold. This
greatly reduces sparking when an activated accessory is removed from tissue.
Sparking occurs because the RF stage tuning results in a higher natural gain at light loads
than at heavy loads. Thus, during sudden transitions from heavy to light loads, the
output voltage rises faster than the microcontroller can respond. This analog circuit works
outside the microcontroller loop at a much greater speed.
The rectified but unfiltered waveform from the output voltage sense circuit is fed into a
peak detector (U23A, CR29, and C104). A high impedance buffer (U23B) maintains the
integrity of the peak detected signal. The output of this buffer is divided and fed to a
comparator.