Feedback Circuit - BenQ Q7C3 Service Manual

Q7C3 LCD Monitor Service Guide
#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.

#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)
Circuit Operation Theory
Fig. 6
Fig. 7
6