9600 Service Manual - gmecorp-usa.com

kV Generation 5
KV ERROR CORRECTION
To correct for the difference between the desired and actual kV by sending an error
correction signal to the kV drive circuit. This causes the drive duty cycles to change
accordingly, forcing the actual kV to match the desired kV.
A logic low on the signal KVEREN from the high voltage drive PAL U7-19 holds switch
U17 closed during non-exposure time disabling the error amp U23. During exposures,
the KVEREN signal goes high, opening switch U17, enabling the error amp U23. The
error amp is an inverting device with unity gain. The error amp compares the desired kV
(KVP CONTROL) to the actual kV (KVP MEASURED) generating a dc error correction
voltage on U23-1 representing the difference between the desired kV (KVP CONTROL)
and the actual kV (KVP MEASURED).
This error correction voltage is applied to comparator U34-2 of the pulse width modulator
(PWM) which is made up of comparator U34 and flip flop U12. This error correction
voltage is compared by U34 to a reference ramp signal applied to U34-3.
The RAMP signal (approximately 0 - 10V) is generated from the ramp generator Q6 with
its supporting components. The ramp generator is controlled by a square wave from the
drive PAL U7-15 (DRV ST*).
The result of the comparison of the error correction voltage and the reference ramp is a
pulse on TP2 (PWM COMP) of the PWM circuit. The width of this pulse is determined by
where the error correction voltage triggered the comparator on the reference ramp signal
(RAMP). See diagram below:
U34-2
CORRECTION
VOLTAGE
U34-3
RAMP
0V
TP2
PWM COMP
10V
TP2
PWM COMP
Figure 2 - TP2 PWM COMP Waveform
10V