9600 Service Manual - gmecorp-usa.com

6 kV Generation
This corrected pulse (PWM COMP) is then applied to the drive PAL U7-8. Drive PAL U7
will respond by varying the duty cycle of its drive signals (HVDRVA and HVDRVB)
accordingly, resulting in the actual kV (KVP MEASURED) equaling the desired kV (KVP
CONTROL).
KV ERROR LOOP EXAMPLE 1
Lets assume that the actual kV exceeds the desired kV so it needs to be turned down. In
this case the error amp U23 detects that the actual kV (KVP MEASURED) is greater
than the desired kV (KVP CONTROL) and the dc voltage output of the error amp U23-1
will decrease. This causes the trip point to the PWM comparator U34, as compared to
the ramp signal (RAMP), to decrease.
Because the trigger point on the ramp has decreased, the pulse out of the PWM circuit
U12-5 (PWM COMPARATOR) also decreases. This shortened pulse is tracked by the
drive PAL U7-8.
Since the pulse width of the PWM COMP signal has decreased, drive PAL U7
decreases the duty cycle of its drive signals (HVDRVA and HVDRVB). The on-time of
the Darlington driver transistors is therefore decreased resulting in less energy
transferred to the high voltage tank and the actual kV decreasing to equal the desired
kV.
KV ERROR LOOP EXAMPLE 2
Lets assume that the actual kV is less than the desired kV so it needs to be turned up. In
this case the error amp U23 detects that the actual kV (KVP MEASURED) is less than
the desired kV (KVP CONTROL) and the dc voltage output of the error amp U23-1 will
increase. This causes the trip point to the PWM comparator U34, as compared to the
ramp signal (RAMP), to increase.
Because the trigger point on the ramp has increased, the pulse out of the PWM circuit
U12-5 (PWM COMP) also increases. This lengthened pulse is tracked by the drive PAL
U7-8.
Since the pulse width of the PWM COMP signal has increased, drive PAL U7 increases
the duty cycle of its drive signals (HVDRVA and HVDRVB). The on-time of the
Darlington driver transistors is therefore increased resulting in more energy transferred
to the high voltage tank and the actual kV increases to equal the desired kV.