Valleylab Force FX-8C, Руководство по обслуживанию

Страница: 80 / 218

Power Supply/RF Board
4-24 Force FX-8C Service Manual
To minimize ringing on the output voltage waveform at light loads,
transistor Q7 switches in the 50 watt, 150 ohm 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. The 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. The
other input to the comparator is an analog threshold level (VMAX_CLP)
that is set by the main microcontroller on the Control board and depends
on the mode and power setting.
When the peak detected sample of the output voltage exceeds the
threshold, one-shot U14A is fired and generates a 3 ms pulse
(SPARK_CON) that is sent to the T_ON ASIC on the Control board. This
pulse is ignored if it occurs during the first 0.2 seconds of activation.
Otherwise, SPARK_CON causes the T_ON\ signal to stop. The feedback
microcontroller on the Control board senses this and realizes that a spark
has been suppressed. The feedback microcontroller waits 100 ms in Pure
cut, then re-initiates T_ON\ with a frequency of 470 kHz. The frequency
returns to 394 kHz after 1 second of continuous activation or when the
generator is reactivated.