CPI CMP 200 Series Service Manual page 287

7
Theory of Operation
7.2.5 kV control and feedback (MD-0932)
Refer to MD-0932, sheet 1:
When there is no X-ray request by the CPU, the RAD INHIBIT line from MD-0931 will be pulled high. This
circuit is described in 7.2.4. When the RAD INHIBIT line is high, the junction of R131 / R123 on the
generator control board will be held high (+12 V). This ensures that the error amplifier cannot generate
any kV demand.
When a valid X-ray request is received, the RAD INHIBIT command is removed and the D/A converter
U43 will produce the required kV reference voltage. The kV reference voltage is buffered by U38A,
inverted by U48A, and then summed with the positive going kV feedback signal from U51B at the input of
error amplifier U25A. Error amplifier U25A will regulate the kV by producing a DC output that is
proportional to the difference between the kV reference voltage and the kV feedback voltage.
The kV demand signal and the kV feedback signals are also processed by U72A, U72B, and Q25. The
result is an HV ON signal at the output of Q25 that is high when the actual kV is greater than
approximately 75% of the demanded kV.
The HT primary current is sampled by T2 and rectified by D33, D36, D38, and D42 to produce a current
limit signal (a negative voltage proportional to the primary current). This is applied to the input of U25B,
where it will limit the kV demand if the primary current exceeds safe limits. This current limit signal is also
fed to comparator U69, which generates a fault pulse if excessive primary current is sensed. This fault
pulse is fed to the CPU via U78, immediately causing the gate drive to be inhibited, preventing inverter
damage due to the over-current condition.
The output of U25B swings increasingly negative for increased kV demand. This kV demand voltage is
buffered and inverted by U33A, and then applied to the VCO (voltage controlled oscillator).
The VCO generates complementary output pulses that vary in frequency. The frequency of these pulses
is inversely proportional to demanded generator output power. The current sense feedback from T1
synchronizes the start of the pulses.
The gate-drive pulses from the VCO are fed to AND gates U26A and U26B. The control inputs of U26A
and U26B are held high if all logic conditions to allow an X-ray exposure are satisfied. The drive pulses
are inverted and level shifted by U27A and U27B, and then applied to the gates of the MOSFET inverter
consisting of Q16, Q17, Q19, and Q20 on the generator control board. These MOSFETS form a full
bridge inverter circuit that outputs high frequency gate pulses to the power MOSFETS on the inverter
board(s) via J5 and / or J13 on the generator control board.
The differential kV feedback voltage from the HT tank is brought to J9 on the generator control board.
This is applied to the inputs of U51A and U56B, where the kV feedback scaling is precisely set by R218.
The differential feedback signals are then summed by U51B. The output of U51B supplies a kV feedback
signal to error amplifier U25A as described earlier in this section. The kV feedback signal is also supplied
to the CPU via A/D converter U45, which monitors the output voltage during an exposure. Lastly, the kV
feedback signal is fed to the kV over-voltage comparator U55, which will send a fault pulse to the CPU via
U78 if a kV over-voltage condition is detected.
The generator control board also contains circuits that guard against an inverter "shoot-through" fault. If a
shoot through fault is detected, T3 and / or T4 on the generator control board will supply a current pulse
that is detected by comparator circuits U70 and U75. The relevant comparator will then generate a fault
pulse that is passed to the CPU via U78, causing the gate drive to be inhibited immediately, thus
preventing inverter damage due to this fault.
Use and disclosure is subject to the restrictions on the title page of this CPI document.
Page 7-6
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CMP 200 X-Ray Generator Service Manual
CPI Canada Inc.
Ch # 901476-09 Rev. C