BenQ Q7T4-FP71G, Service Manual

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Q7T4-FP71G LCD Monitor Service Guide
Circuit Operation Theory
6
Confidential
#5 PWM Controller
The PWM controller NCP1200A implements a standard current mode architecture. With an
internal structure operating at a fixed 40KHz. Where the switch time is
dictated by the peak current set-point. When the current set-point falls
below a given value. The output power demand diminishes, the IC
automatically enters the so-called skip cycle mode and provides
excellent efficiency.
Fig. 6
#6 Feedback circuit
PC123 is a photo-coupler and TL431 is a shunt regulation. They are
used to detect the output voltage change and be the primary and
secondary isolation. When output voltage changes, the feedback voltage
will be compared and duty cycle will be decided to control the correct
output voltage. (See Fig.7)