Feedback Circuit; Dc-Dc Circuit - BenQ G2420HD Product Service Manual

#6 Feedback circuit

PC123 is a photo-coupler and KA431 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)

#7 DC-DC circuit

IC851(PWM IC TL1451) is used to convert V-INVERTER to other voltage needed.
F851
32V 2A
V-INVERTER
R852
Q857-B 2.7K
B
Q857
2N3906S
C853
1U K
R856
330K
R853
10K F
10K F
C852
0.1U K
C851 R851
470P K 10K F
R857
10K F
C856
1U K
R858
R859
330K
10K F
IC602
PC123FY 1J00F
C862
+
AUDIO_CTRL R863
220U 25V
330
R860
C860
22K
0.1U
IC851-10
C855
0.01U K
R855
47K
R854
C854
220P K
IC851-14
IC851
TL1451ACN
C859
1U K
C857
IC851-7
220P K
R873
47K
C858
0.01U K
IC851-3
- 64 -
R708
2K
R707
10K
R709
10K F
R
IC702
KA431AZT A_NL
Fig. 7
Q851
2SJ598
Q853-C
S
R864 R865
3.3K 22K
Q853-B
Q853
B
PMBS3904
R861 R866
2.7K J 22
Q854
B
2N3906S
Q852
2SJ598
Q855-C
S
R872
R871
C867
+
22K
3.3K
220U 25V
Q855
B
PMBS3904
Q855-E
R862 R882
2.7K J 22
Q856
B
2N3906S
R711
22K F
C709
0.1U K
R712
3.32K F
L851
D
47U
R876
2.05K F
FD851
C863
SB360-LF
470P K
R877
1.2K F
L852
D
47U
C868
3.09K F
470P K
FD852 FD853
ERB83-004 ERB83-004
L853
Z45/100MHZ
R875
1K F
MCU_3.3V
C864
0.1U
C865
+
220U 25V
PANEL_5V
R874 C869
0.1U
C871
+
220U 25V