Valleylab Force FX-8C, Руководство по обслуживанию

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Power Supply/RF Board
4-26 Force FX-8C Service Manual
the ground referenced low voltage circuitry. Each handswitching output
of the generator is associated with its own IsoBloc power source and
isolated signal paths.
Oscillator
The oscillator circuit consists of a 4060 CMOS oscillator/divider (U30)
using a 5 MHz ceramic resonator (XT1) as the frequency determining
element. The output of the oscillator is connected internally to the input of
a counter/divider chain. The output of the divider yields a 78.125 kHz
square wave that is applied to the input of three 4081 buffers (U29).
Power Supply
The three 4081 buffers (U29) drive three VN0808L FETs connected to
transformers which are operated in a quasi-resonant flyback mode with
their associated 6800 pF capacitors. The voltages at the secondaries of the
three transformers are half-wave rectified and referenced to three separate
isolated grounds to provide -8 V for operating the isolated activation
circuitry.
Optoisolators
The isolated power supply voltages produced by the IsoBloc power
supplies are connected to the active output terminals of the generator.
Handswitch activation is accomplished by sensing active to CUT or active
to COAG switch closure in a handheld accessory. Switch closure is
detected by comparing the voltage across the switch to a divider reference
with comparators U32, U33, and U34. Current limiting resistors, in series
with the LEDs in the optoisolators, cause the LEDs to light. The
phototransistor in the optoisolator detects this light. The phototransistor,
which is connected to an input on the 82C55 expansion port in the main
microcontroller circuit, turns on, pulling the associated input low. This is
interpreted by the software as an activation request, and the generator is
activated accordingly.
Temperature Sense Circuits
The Force FX-8C generator features two temperature sense circuits.
The first temperature sense circuit measures the air temperature adjacent
to the RF output FET heat sink. When the temperature at this location
reaches approximately 40° C, a cooling fan switches on to minimize heat
buildup. The fan switches off when the temperature drops below
approximately 35° C.
The second temperature sense circuit measures the air temperature near
the Control board, which is located in the forward third of the generator
enclosure. When this temperature reaches approximately 50° C, the main
microcontroller flashes number 451 alternately with the power settings to
indicate that the generator is too hot. Generator operation reverts to
normal when the temperature decreases to below 40° C.