25/08/2004 ©
Söring
Service-Manual Arco- and RF-units
1
not valid for A-3000/ -2000
2
not valid for Arco-2000
-42-
3
not valid for MBC600
4
not valid for Arco-1000
8. Description of coagulator module
8.1. General description
The coagulator module provides the necessary high RF-voltage for spray coagulation.
For changing the output power the pulse repetition frequency as well as the pulse energy
is variable. In order to reduce disturbances of OP-equipment, especially endoscope
cameras, not every output pulse is generated with an ignition energy necessary for spray
coagulation. In order to reduce disturbing noise at low pulse frequencies, the ignition
pulses are transmitted irregularly. An automatic spark detection enables the
microprocessor to adjust the output parameters in such a way that the RF-leakage
current is sufficiently low for all cases. A safety device informs the microprocessor about
possible overheating of the output transformer. The module is designed for CF-operation.
8.2. Function of coagulator module
Fig- 9.1 shows the schematic concept of the coagulator module. It receives all the control
information from the micro-controller via the interface board (see description interface
board). Status information is transferred backwards via this board. Essentially the board
contains a programmable oscillator, a pulse width modulator, a MOS-switch and a
storage inductance. The longer the on-time of the MOS-switch, the more energy is stored
in the storage choke, and the higher is the obtained voltage after switching off the
transistors. By means of the output capacitors at the output transformer on the RF-output
board a resonant frequency of more than 300 kHz is assured and a flow of not
permissible low-frequency components through the patient is prevented.
In order not to dissipate the complete stored energy in the choke or the damping resistor
in case of idling, a back transfer of energy into the power supply is made by means of a
free run diode. A rectifier/clamp circuit charges the capacitors to the peak value of the
voltage at the storage choke. If the required peak voltage is exceeded, the pulse length
of the pulse width modulator is reduced by voltage regulators until the output voltage
reaches the desired value.
By switching the clamp circuit by means of a switching transistor two different pulse
voltages -Ignition pulses and standard pulses- can be generated. When the output is
loaded, the pulse voltage drops and the pulse width increases to a "maximum" value.
This maximum value corresponds to the pulse energy which is transferred to the tissue
under load conditions (tissue contact respectively spark-over).
Together with the pulse frequency, also programmed by the micro-computer, the output
power is defined. The spark-over recognition checks after an ignition pulse whether
energy is back transferred via the free run diode to the power supply. If no energy is
transferred back a spark-over is recognized. This is notified to the micro-controller. With
activated frequency-automatic only then the pulse frequency is increased to the value
given by the micro-controller in order to reduce the leakage current under idle conditions.
By module selection the ignition pulse energy can be switched from the internal fixed
value to the value given by the micro-computer. By program selection different ignition
sequences, stored in an EEPROM, are available.
