Service Manual
11
C849
1nF/50V NPO
Signal_GND
U850
LD7575
1
4
5
2
8
3
7
6
R
T
GND
OUT
COMP
HV
I sense
N
C
Vcc
Signal_GND
C848
100nF/50VX
R849
220R
R842
20K
Signal_GND
C846
220pF/50V NPO
D849
NC
R841
10K/NC
CN852
CON8PIN
1
8
3
4
5
6
7
2
Gate
HV
GND
FB
GND
VDD
NC
Isense
Signal_GND
R846
100K/ 1%
Signal_GND
Signal_GND
R843
20K
R847
NC
R848
20R
C847
Open
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. R852 is a fuse resistor to protect
the following circuit when inrush current is too large.
U850 is a current-mode PWM controller with excellent power-saving opration,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 U850 is 100 uA, When current flow from the bulk capacitor C854 through R8
4
3
and R842 gets to HV pin
to start up U850, Meanwhile,the VCC supply current is as
low as 100 uA thus most of the HV current is utilized to change the VCC capacitor
C855, Whenever the Vcc voltage is higher than UVLO (16V),the GATE pin will
output signal to drive the power MOSFET(Q850), the high-voltage current source is
off and the supply current is provided from the auxiliary winding of the trasformer
PIN5.
When U850 begins to operate Pin8 of U850 will output square wave to drive
Q850, 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. In the same time, the current inducted by wires which connected T850 Pin 1
and Pin 3, with components of D852, R856 and C855 ,will be supplied to U850 for
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