CPI CMP 200 Series Service Manual page 288

CPI Canada Inc.
7.2.5 kV control and feedback (Cont)
Refer to MD-0932, sheet 2 and 3:
The inverter board(s) produce the high power drive for the HT transformers (32 and 40 kW generators
use one inverter board, 50 kW and 65 kW units use two inverter boards). The output of the inverter
board(s) drive the primaries of the HT transformers via the resonant / sharing inductors and the resonant
capacitor and a current transformer that samples the HT primary current.
The HT tank has similar anode and cathode sections. The cathode and anode sections each have their
own high voltage transformer and high voltage multiplier board. The anode section generates the anode
voltage, 0 to 62.5 or 75 kV. The cathode section generates the cathode voltage, 0 to -62.5 or -75 kV. The
anode and cathode sections contain voltage dividers that supply kV anode and cathode feedback
voltages. The kV feedback from the HT tank is brought to J9 on the generator control board as described
earlier in this section.
The HT tank is not field-repairable. Defective HT tanks must be replaced as a complete unit with the
factory-specified replacement.
7.2.6 Filament drive and mA control (MD-0934)
Refer to MD-0934, sheet 1:
D/A converter U42 on the generator control board will produce the filament reference voltage (1 volt = 1
amp of filament current) when the CPU receives a valid Prep command. This is buffered by U32B, and
then routed to the filament supply board via J14 on the generator control board.
The filament reference voltage is fed to U1B on the filament supply board. The output of U1B is summed
with the output of current limit clamp U1A. The filament current limit is set at 5.5 or 6.5 amps via JW1; it is
shown set to limit at 5.5 amps on MD-0934.
Next, the filament reference voltage is summed with the filament feedback voltage at the input of error
amplifier U4B. When the reference voltage is greater than the feedback voltage, the output of U4B rises,
causing the pulse width at the output of U3 to increase.
PWM (pulse-width modulator) U3 drives MOSFETS Q6, Q7, Q12, and Q13, which form a full bridge
inverter. The MOSFETS convert the + and - 35 V supplies to high frequency AC to drive the primary of
the filament transformers at the filament switching frequency, approximately 40 kHz, via C22, the primary
of T1, large / small filament select relay K1, J5 on the filament supply board, and J4 on the tank lid board.
The secondaries of the filament transformers provide high voltage isolation, and drive the filaments via
the filament mounting board and the cathode high voltage connector as shown.
The output of filament current sense transformer T1 on the filament supply board is rectified by D12, D13,
D27, and D28, and fed to RMS converter circuit consisting of U7 and associated components. The output
of the RMS converter drives U4A, which is a variable gain amplifier. The filament current feedback is
calibrated such that 1 volt = 1 amp of filament current. The calibrated filament feedback voltage
(representing actual filament current) appears at the input of buffer U2B, at the summing input of error
amplifier U4B as described earlier in this section, and at the input of comparator U2A.
The output of U2B is brought to J2-2 on the filament supply board. The filament current feedback signal is
then converted to a digital signal by A/D converter U45 on the generator control board in order that the
filament current can be monitored by the CPU.
The output of U2A will be low if a filament fault is present, i.e. filament current less than approximately 1.7
amps. This will turn Q1 on the filament supply off, taking its collector high. The output of Q1 is taken to
the CPU via U78 on the generator control board. If low filament current is detected, X-ray exposures will
not be possible, and a filament fault message will be displayed.
Use and disclosure is subject to the restrictions on the title page of this CPI document.
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CMP 200 X-Ray Generator Service Manual Ch # 901476-09 Rev. C
7
Theory of Operation
Page 7-7