Dell 2209WA Service Manual page 15

Service Manual
following circuit when inrush current is too large.
U950 (LD7575) is a current-mode PWM controller with excellent
power-saving operation, It features a high-voltage current source to directly
supply the startup current from bulk capacitor and further to provide lossless
startup circuit. Max start-up current for U950 (LD7575) is 100 uA, When
current flow from the bulk capacitor C854 through R835 and R836 gets to HV
pin to start up U950 (LD7575), Meanwhile, the VDD supply current is as low
as 100 uA thus most of the HV current is utilized to change the VDD
capacitor C952, Whenever the VDD voltage is higher than UVLO (16V),the
GATE pin will output signal to drive the power MOSFET(Q950), the
high-voltage current source is off and the supply current is provided from the
auxiliary winding of the transformer PIN5.
When U950 begins to operate Pin5 of U950 (LD7575) will output square
wave to drive Q950, then the main current flow get to GND bypassing
through T850, Q950. Because of the change of current flow, wires in the
other side of T900 will induct current. In the same time, the current inducted
by wires which connected T900 Pin 1 and Pin 3, with components of D952,
Q951, D953, C951 and C952, will be supplied to U950 for normal operating.
The typical current mode PWM controller feedbacks both current signal
and voltage signal to close the control loop and achieve ragulation.U950
detects the Q950 current from Isense pin which max voltage is set as 0.85V,
then the Q950 current can be calculated as: I peak=0.85V/R959,When the
sense voltage across the sense resistor R959, reaches the threshold voltage
over 0.85V, the output GATE drive will be turned off. R957 and C953 is used
to avoid the Isense pin damaged by the negative turn-on spike.
The voltage feedback signal is provided from the TL431 (I951) through the
I950 to the COMP pin. When the voltage on COMP pin is lower than 1.2V,
the IC will turn off.
When Q950 is turned off, the main current flow will be consumed through
D951, C955, R837, R838,
damaged under large current impulse and voltage spike.
RT pin is to program the switching frequency, By connecting R958 to
ground to set the switching frequency, f =(65.0/R958)*100(KHz).
and R839, This will prevent Q950 from being
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