uniphy Phyaction 740 Series, Service Information

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Service Information Phyaction 740 & 790-series Page 20
2.4.3 Secondary switch mode power supply
This is a regular down-converter powered by the +29V supply. The only peculiarity being that its output
voltage is controllable by
V
control
. In this way the microcontroller can set the ultrasound power by setting
V
control
proportional to the square-root of the required power.
2.4.4 Power stage
The power stage is a balanced switched half-bridge. It is supplied by the SSMPS mentioned above to
control the momentary output power. The switching operation being controlled by the modulator as
described above. The switches are formed by power-MOSFETs driven from buffers that insert a switch-
on gap to prevent both sides from conducting simultaneously.
2.4.5 Impedance matching
The nature of the load of the transducers in the treatment head is essentially capacitive and varies with
the surface area and the operating frequency. Therefore a collection of inductors and capacitors is
available from which the microcontroller inserts the required combination for the selected type of the
treatment head and operating frequency.
2.4.6 Ultrasound treatment head
The most important part is of course the transducer, a piezoelectric crystal with a surface area
appropriate for the type of treatment head. This piezoelectric crystal is mounted on the base of an
aluminum cylinder, with the thickness of the base enabling it to use two considerably different operating
frequencies. The aluminum cylinder also functions as the second electrode for the patient current when
applying combined therapy. A temperature sensor is mounted on the other end of the cylinder to monitor
the temperature of the treatment surface. The type of the treatment head is encoded by a resistance
value. Also provided are LED-lamps for signalling purposes. A green colour signals the treatment head
being stand-by or in normal operation and a red colour signals an interruption of the treatment because
of overheating of the treatment surface or insufficient acoustical contact. The LED-lamps are driven by a
single conductor by controlling the direction of the current.
2.4.7 Safety circuit
As described above a lock-out of the PLL will force the modulator to switch off. The supply voltage of the
power stage is also monitored for voltages beyond the intended operating range. This is done
independently from the microcontroller as it senses this voltage already to adjust
V
control
for tolerances in
the SSMPS. The resulting Error
us
signal inhibits the modulator and is also sent to the microcontroller.
The Error
us
signal can also be activated the other way round by the microcontroller if it detects another
error.
2
.5 Infrared laser unit
The circuit for infrared laser therapy consists of two main parts. The part that delivers the actual laser
pulse is located in the laser probe. The control-circuit and the measuring facility are in the base
appliance. Only with the switchlock turned on the power supply is connected to the laser probe and the
I/O-buffers. This warrants the operational safety as it is therefore impossible to release laser radiation
without using the key.