Dell E1709W, Service Manual

Page: 10 / 62

Service Manual
9
used to discharge the waves that L850 produced. High frequency waves are damped by C852.
2.2.3) High Voltage to Low Voltage Control Circuit :( fig.6)
fig.6
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. R853 is a fuse resistor to protect the following
circuit when inrush current is too large. U850 is a highly integrated PWM controller. During the
startup, a high-voltage current source sinks current from the bulk capacitor to provide the startup
current as well as charge the Vcc capacitor C878, during the startup transient, the Vcc is lower
than the UVLO threshold thus the current source is on to supply a current with 1mA. Meanwhile,
the Vcc supply current is as low as 100uA thus most of the HV current is utilized to charge the
Vcc capacitor.
Whenever the Vcc voltage is higher than UVLO (on) to power on the LD7575 and further to
deliver the gate driver signal, the high-voltage current source is off and the supply current is
provided from the auxiliary winding of the transformer (T850). Pin8 (GATE) of U850 will output
square wave to drive Q850, and then the main current flow HV to GND by passing through T850,
Q850. Because of the change of current flow, wires in the other side of T850 will induct current.
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.