Dell 2209WA - UltraSharp - 22" LCD Monitor, Service Manual

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Service Manual
16
As shown in Fig.7, peak-current-mode control is utilized for Flyback PWM.
The U850 (SG6901A) inserts a synchronized 0.5V ramp at the beginning of
each switching cycle. When the IPWM voltage, across the sense resistor,
reaches the threshold voltage, 0.65V or 0.7A selected by RANGE, the
OPWM will be turned off after a small propagation delay Tpd-pwm. When
RANGE is shorted to GND, the PFC output voltage is high and the
corresponding threshold is 0.65V. When RANGE is opened, the PFC output
voltage is low and the corresponding threshold is 0.7V.
When VDD is lower than the turn-off threshold such as 10V, U850
(SG6901A) will be totally shut down. Due to the start up resistor, VDD will be
charged up to the turn-on threshold voltage 12V until U850 is enable again. If
the over loading condition still exists, the protection will take place repeatedly.
This will prevent the power supply from being overheated under loading
condition.
2.2.4)DC_24V ,DC 12V and DC 5V Output Circuit and Feedback circuit:(fig.7)
D854
SRF10-20CT
1
2
3
+ C864
470u/35V
C861
10nF/50V
+ C863
470u/35V
+ C862
470u/35V
L860
Bead
1 2
F851
4A/125V
1W
+24V
R856
20 3W
+ C865
470u/35V
.
.
T850
SPW-128 PQ3220
7
2
8
5
10
11
+24V_DC
R865
4.7K
C858
1000p/1KV
D853
SRF10-20CT
1
2
3
L854
CHK BR3X20/2.2uH
F852
4A/125V
○
A