25/08/2004 ©
Söring
Service-Manual Arco- and RF-units
1
not valid for A-3000/ -2000
2
not valid for Arco-2000
-34-
3
not valid for MBC600
4
not valid for Arco-1000
6.3. Oscillator and driver
IC7works as an oscillator with a frequency of approx. 350kHz. The frequency is adjusted
by potentiometers P3 an P4 independently for monoplolar and bipolar operation. This is
necessary because the transformation ratio of the output transformers for mono- and
bipolar mode and the patient protection capacitors influence the series capacitance of the
output resonant circuit. This results in slightly different resonance frequencies. The
oscillator is followed by a 2-stage driver circuit T53, T54, T1 and T2. Now the driving
power is sufficient to drive the final MOS-bridge via the driver transformer TR2.
6.4. Switching stage
The switching stage connects or disconnects the power supply of the MOS-bridge,
thereby switching it on or off. It consists of the MOS-transistor T7. This transistor is
controlled by the driver T9 and T10 which again is controlled by the AND-gate (CMOS)
IC1. Only if the three inputs of this gate are simultaneously high, the final stage is
switched on.
6.5. Short circuit switch-off
This circuit switches off the final stage within microseconds in case a maximum current is
exceeded. The final stage is thereby protected against overload in case of short circuit
and spark development when short circuiting the bipolar tweezers is significantly
reduced. As the current through the MOS-bridge is nearly proportional to the output
current, this current is used for determining the RF-output current. The final stage current
goes through resistor R121 (0.1
Ω
). The voltage drop over this resistor is amplified by
operational amplifier U1a and U1b. The analog switch IC4a serves as a range switch. If
the measured current (Signal I_VAL) exceeds the reference value (I_SET), the output of
comparator U5a goes high and switches off the final stage for a short time. The Maximum
current I_SET is transferred to the cut module in analog form from the interface board.
6.6. Current limiting, current control
The current limiting circuit limits the output current to the maximum value given by the
signal I_SET (see above). The current regulation is performed by reducing the power
supply voltage by the control signal DOWN. This signal is routed to the high-power
supply via the interface board. The current value I_VAL acquired by the operational
amplifiers U1a and U1b is compared by the operational amplifier U1c with half the value
of I_SET (Signal I_SET/2). As long as I_VAL, the actual current value, is less than
I_SET/2, U1c delivers approx. 14V. If I_VAL is larger than I_SET/2, the output voltage
drops more and more. The signal is passed via D49 and R440 to the DOWN output. If the
DOWN- signal falls below a certain voltage (0 to 10 V), the power supply voltage, given
by the micro-controller is down regulated, the RF-output voltage falls and reduces the
RF-output current to the maximum value given by I_SET. Current limiting is indicated by
LED D50. In pulse mode the analog switch IC4 disconnects during the switch-off phases
of the final stage the actual value from the regulator and keeps the regulator output
constant during the current pauses. This prevent an up-regulation of the RF-output
voltage during the current pauses (hold function).If the module is switched off the down-
regulation of the power supply is prevented by switching off the current reference (IC4b).
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