uniphy Phyaction 740 Series, Информация о сервисе

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Service Information Phyaction 740 & 790-series Page 18
2.3.3 Voltage to current converter
The voltage-to-current-converter (V/C-converter) converts, as the name indicates, the applied input
voltage into a linear proportional output-current. As it is conceived as a balanced stage it is easy to
determine the polarity of the output current by applying the controlling voltage to one of the two inputs.
The primary output current flows through a low-ohmic resistor. The voltage drop across this resistor is
proportional to the absolute magnitude of the actually generated output current and is fed to the safety
circuit.
In order to reduce the power dissipation in the actual V/C-converter it is powered through a so called
tracing series regulator. This regulator lowers the supply voltage to a level that just accommodates the
converter.
2.3.4 Safety circuit
The voltage drop sensed in the V/C-converter is used in this circuit to monitor whether the output current
oversteps the limits as stated in IEC 601-2-10 or 150% of the set value, whichever is smaller. In addition
the intensity control is monitored for interruptions of the ground terminal. This all is achieved fully
independent of the microcontroller by comparators that, in case of a fault, switch off the safety relay,
which interrupts the patient current circuit. This happens so fast that even in case the full supply voltage
is suddenly applied to one of the primary windings of the output transformer, the energy contents of the
resulting pulse will be well below the allowed limit. Once the safety relay has been switched off it remains
in this state and one must switch on the appliance after the cause has been removed, e.g. turned up
intensity control at switch-on.
Because on switch-on of the appliance a selftest is performed which of course includes a test of the
safety circuit, a way is provided for the microcontroller to switch on the safety relay. This is necessary as
the safety relay will be in the off state after switch-on and should also switch off a couple of times in the
course of the selftest. This possibility to switch on the safety relay is time limited however to 3 seconds
from power-on. When this time has expired the microcontroller only can switch off the safety relay, e.g.
when an error of another nature has been detected. In this way it is guaranteed for the safety circuit to
operate independently from the generation of the patient current and the software.
2.3.5 Current mode selection
As mentioned before it is also possible to generate rectified currents like diadynamic, Ultra Reiz,
galvanic, etcetera. To that object a rectifier is switched into the patient current circuit. The choice
between rectified and alternating currents is also sensed by the microcontroller to select the adherent set
of waveform options and waveform synthesizer control-signals. The rectifier circuit is only present in the
Phyaction 793.
2
.4 Ultrasound therapy unit
Figure 3 shows the functional blocks for the ultrasound therapy unit. As the ultrasound power is
generated by switching, it appeared to be necessary to encapsulate the committed circuitry, from the PLL
(Phased Locked Loop) up to the impedance match and including the SSMPS (Secondary Switch Mode
Power Supply), in a tin case to meet the emission-limit requirements. This shielding extends itself over
the inner shield conductor of the treatment head cable up to the transducer which is mounted in a closed
aluminum cylinder.