Dell S1909WXf Service Manual page 10

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. R852 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 U830 is only 8uA; R838, R839 and R840 are serial circuit to limit current flow.
When current flow through R838, R839 and R840 gets to Pin 3 of U830, with VDD hold-up
capacitor C857, U830 is enough for starting up.
When U830 begins to work, Pin8 (GATE) of U830 will output square wave to drive Q850, and
then the main current flow get to GND by passing 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, C857
and C839, will be supplied to U830 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, C880, R880
and D853. This will prevent Q850 from being damaged under large current impulse and voltage
spike.
2.2.4) DC_16V and 5V Output Circuit and Feedback circuit :( fig.7)
fig.6
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Service Manual