Hologic DISCOVERY A, Technical Manual

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Discovery QDR Series Technical Manual
2-18
2.8.2 X-Ray Controller Board
The X-ray Controller Board consists of five sections: Power, Timing Generator, Pulse
Generator, Filament Control, and H-Bridge.
The Power section converts the split 240 VAC input to the controller into the DC voltages
necessary for the board. The voltages produced in the Power section are: +5VDC (VDD –
used for analog circuitry), +5VDC (VCC – used for digital circuitry), and +/- 15 VDC.
The LED, D25, indicates the presence of voltage on the board.
The Timing Generator section produces the timing signals required by the control circuits.
The timing is based on a half cycle of the AC waveform and is, therefore, reset at each
zero crossing. An 8 MHz clock circuit provides pulses to a counter. This counter in turn
provides its output count to an EPROM as its address inputs. The pulse trains produced by
the EPROM are clocked into a register to clean up the signals that are then distributed to
various sections of the board. The ACHIGHLOW signal produced by the timing circuits is
sent to the C-Arm Interface Board where it is used to synchronize X-ray generation with
the Filter Drum position.
The Pulse Generator regulates and shapes the pulses delivered to the X-ray Tank. Sense
lines from the tank set the current threshold for the regulator. A reference voltage is
selected and fed to one of four circuits that control the high voltage peak potential. The
circuits adjust the reference voltage through four potentiometers, R55, R54, R39, and R40
that is used to establish the 80, 100, 120, and 140 KVp. The outputs of these circuits are
sent to a pulse width modulator circuit to produce the control pulses to the H-Bridge.
These pulses are optically coupled to the FETs in the H-Bridge.
The H-Bridge produces the actual pulses fed to the primary of the HV transformer in the
X-ray tank. AC is brought to the circuit, rectified, and filtered. The H-Bridge operates as
both an inverter and a pulse width modulator. The H-Bridge uses two pairs of FETs. Each
pair conducts 256 times for a total pulse width of 4.096 milliseconds. Each of the FETs is
individually biased with the biasing network acting as a bleeder for the large storage
IBEAM+
IBEAM-
Value of current pulses at the X-ray
source. This is a frequency modulated
diagnostic signal whose frequency is
proportional to the quantity being
monitored. JP12-23
JP12-24
JP7-23
JP7-24
JP3-12
JP3-31
kVp+
kVp-
Value of voltage pulses at the X-ray
source. This is a frequency modulated
diagnostic signal whose frequency is
proportional to the quantity being
monitored. JP12-25
JP12-26
JP7-25
JP7-26
JP3-13
JP3-32
XRFAULT-
States whether or not any fault
condition exists in the X-Ray Controller
Assembly. JP12-27
JP12-28
JP7-27
JP7-28
JP3-14
JP3-33
Signal Description C-Arm
Pins
I/O
Logic
Pins
XRC
Pins