Universal APX 525 - Page 214

APX 525/625 Generator Revision C Diagnostics
© 2005 Del Medical Imaging Corporation
7/5/2005 Page 178
mA feedback voltage is received from the mAs regulator board and used on
this board to provide closed loop regulation of the actual tube current.
Fault detection circuits on the kV Regulator and mA/Rotor boards are used to
monitor the operation of the generator during the exposure and to send Fault signals
to the Logic Board to stop the exposure if an error is detected.
kV Control Functions: (see illustration X4)
The Inverter Power Supply is a three-phase full wave rectifier circuit, fed by the
output of the line-matching transformer, which develops approximately 400 VDC
across the filter capacitors. This voltage is passed through a large air coil inductor to
make the voltage source appear more inductive, and then through a 60-ampere dual
element fuse to the inverter bridge assembly.
kV and mAs settings from the Operator’s Console are decoded on the Data Board,
and used to provide the kV reference voltage to the kV Regulator Board and to
select the appropriate combination of commutation capacitors, based upon the
power requirements of the selected exposure technique. Different parallel
configurations of the commutation capacitors are used to vary the resonant
frequency of the high voltage primary switching circuit, providing optimum operation
for various power output levels.
On the kV Regulator Board, the core frequency oscillator begins to run, sending
pulses to U5, a 4013 flip-flop, which divides the pulse rate in half and delivers
symmetrical pulses to the on-board switching transistor network, which then
provides drive pulses to the pulse drive transformers in the inverter assembly. Each
pulse that is sent to U5 is also buffered and sent to U21, where an adjustable dead
band is created, which temporarily disables the SCR drive pulses, in order to prevent
cross-conduction firing of the SCRs in the inverter bridge assembly.
The inverter assembly consists of four high speed SCRs with external reverse
current diodes and RC snubber networks, are constructed in an H-bridge
configuration. The pulse transformers fire the SCRs in opposing pairs, switching the
400 VDC inverter supply back and forth across the output load, which consists of the
commutation capacitor(s) connected in series with the high-voltage transformer
primary windings, in a series-resonant configuration. The resonant circuit is self-
commutating, and requires no additional extinguishing to terminate conduction.
Inside the high-voltage transformer assembly, two separate transformers, configured
in series, are used to develop high voltage AC, which is then rectified and multiplied;
producing two filtered DC supplies for the anode and cathode voltages. Two one
hundred eighty Megohm resistive divider networks on the output sides of the DC
supplies provide proportional kV feedback voltages, developing 1 volt per 10 kV,
which are then sent to the kV Regulator board.