Dell S1909WNf Service Manual page 10

2.2.3) High Voltage to Low Voltage Control Circuit :( fig.6)
D850
BL4-06
4 1
- +
C854
68uF/450V
Signal_GND
Signal_GND
D850 is a rectifier in which there are 4 build-in diodes, inverting AC to DC.
C854 is used to smooth the wave from rectifier. R839 is a fuse resistor to protect the following
circuit when inrush current is too large. U850 is a highly integrated PWM controller. Typical
start-up current for U850 is only 30 uA, R841, R842 and R843 are serial circuit to limit current
flow. When current flow through R838 and R839 gets to Pin 3 of U850, with VDD
hold-up capacitor C855, U850 is enough for starting up.
When U850 begins to work Pin8 of U850 will output square wave to drive Q850, and then the
main current flow get to GND bypassing through T850, Q850. Because of the change of current
flow, wires in the other side of T850 will induct current. At the same time, the current inducted by
wires which connected T850 Pin 1 and Pin 2, with components of D852, R855, C855 and C849,
will be supplied to U850 for normal operating.
When the sense voltage across the sense resistor R859, reaches the threshold voltage around
0.85V, the output GATE drive will be turned off. Every time when the output of power supply is
shorted or over loaded, the FB voltage will increase, the build-in PWM output will then be turned off.
Both will prevent the power supply from being overheated under over loading condition. The PWM
duty cycle is determined by this current sense signal and VFB, the feedback voltage. When the
voltage on sense pin reaches Vcomp = (VFB-1.0)/3, a switch cycle will be terminated immediately.
Vcomp is internally clamped to a variable voltage around 0. 85 V for output power limit.
When Q850 are turned off, the main current flow will be consumed through D851, C875 R853 ,
R854 and R855. This will prevent Q850 from being damaged under large current impulse and
voltage spike.
2.2.4) DC_12V and 5V Output Circuit and Feedback circuit :( fig.7)
R839 0.2R/1W FUSE
R841
499K
R842
499K
R843
499K
C849 C855
0.01uF/50V X7R 10uF/50V
Signal_GND
3 8
VCC OUT
U850
LD7552B
5 4
NC RT
C846
NC/47pF/50V X7R
2 6
COMP CS
C848
1nF/50V X7R
Signal_GND Signal_GND
R853
27K
C875 D853
NC/P6KE170A
472P R854
27K
R855
27K D851
FR10-10
R856 D852
10R
SF10-04
R844 47R
D841
Q850
NC/1N4148
R848 AP2762
100K 1%
2
NC/100P/1KV Y 5P
Signal_GND
R884
10K
R840
47R
R859
C847 0.75R/2W 1%
1nF/50V X7R
fig.6
9
Service Manual
T850
SPW-119
1
2
3
4
5
6
C860
C872 332 Y 1
T851
NC
1
2
3
4
5
6
12
11
10
9
8
7
12
11
10
9
8
7