Dell E2009W Manuale di servizio - Pagina 10

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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

U850 is only 20 uA, R841,R842 and R843 are serial circuit to limit current flow. When current flow

through R841,R842 and R843 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 U830 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 5 and Pin 6, with components of D852, R856, 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. The voltage feedback signal is provided from the

TL431 at the secondary side through the photo-coupler to the COMP pin of the LD7552B.

When the voltage on sense pin reaches Vcomp =(Vcomp-2Vf)/3, a switch cycle will be terminated

immediately. Vcomp is internally clamped to a variable voltage around 1.2 V for output power limit.The

LD7552B can be turned off by pulling COMP pin lower than 1.2V.The gate output pin of the LD7552B will

be disabled immediately under such condition .The off-mode can be released when the pull-low signal is

removed.

2.2.3 DC_24V and 5V Output Circuit and Feedback circuit:(fig.2.2.3)

fig.2.2.2

fig.2.2.3

Service Manual