5 • Circuit Descriptions
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Art.-Nr.: 80116-271
04.06
HF generator
The high-frequency generator consists of the "HF generator" circuit
board with the power components, and the programmable logic de
-
vice (CPLD), which is responsible for transistor control but is located
on the "CPU+Sensors" circuit board.
The VIO system is only equipped with one generator module. To
achieve the individual surgical effects there are widely differing re
-
quirements on the types of voltage and current to be generated. Both
the HF generator and the downstream sensors have therefore been
designed for a very wide dynamic range.
The alternating current is generated by using transistors to control a
resonant circuit in the right frequency. This parallel resonant circuit in
-
cludes a transformer which has three taps on its secondary winding:
for HF output voltages up to approx. 1000 V (Rel35), voltages up to
approx. 2500 V (Rel37) and voltages up to approx. 4000 V (Rel39).
Depending on the operating ranges of the high-voltage power sup-
ply unit, a pair of switching transistors is available for DC input voltag-
es up to 250 V and 450 V respectively. Rel43 is used for switching
here.
The zero crossings of the resulting alternating voltage are detected
by a comparator and signaled to the control logic. It can then be de
-
cided here, depending on the type of modulation selected, whether
there should be another actuation pulse for the switching transistors
or not.
With very high-resistance loads the energy stored in the resonant cir-
cuit can only dissipate slowly, so that the generator would also con-
tinue to oscillate without actuation. However, this would mean that
modulation would be determined by the external load and not by the
control system. The transformer is therefore equipped with another
secondary winding, which can be short-circuited via a transistor. This
results in discharge of the resonant circuit and thus a defined dying
out process. This transistor is also controlled by the CPLD; another
comparator circuit indicates when generator oscillation has died out.
When the contact monitor is activated, the generator produces a rel-
atively low HF voltage which is used to produce a measurement cur-
rent. Depending on size of this current, it can be decided whether
there is tissue contact.
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