Samsung DVD-A500 Service Manual page 49

Circuit Descriptions
6-1-2 (d) Feedback Control Circuit
PIC1
STR-G6153T
OCP
GND
#
! @
DRAIN
% $
F/B
VCC
1st GND
1) F/B terminal of PIC1 determines output duty cycle.
2) C-E(Collector-Emitter) of PIC2 and F/B potential of PIC1 are same.
Operation descriptions
1) Internal OP-Amp '+' base potential of PIC3 is 2.5V and external "-" input potential is connected with PR38 and
PR39 to maintain Vout of 5.8V. (Vout = ((PR36 x PR39) / PR39) x 2.5V)
2) If load of 5.8 V terminal increases(or AC input voltage decreases) and Vout decreases below 5.8V, then :
PIC3 'P' potential down below 2.5V --> PIC3 A-K of base current down --> PIC3 of A-K current down -->
PIC2 Diode current down --> PIC2 C-E current down --> PIC2 C-E voltage up --> PIC1 F/B voltage up -->
Out Duty up --> Transformer 1st current up --> Transformer 1st power up --> Vout up --> Maintain Vout 5.8V
3) If load of 5.8 V terminal decreases(or AC input voltage rises) and Vout rises above 5.8V, then :
Reverse sequence of the above description --> Duty down --> Vout down --> Maintain 5.8V (i.e., the feedback
to maintains 5.8V).
ΠPR35, PR36 : Reduce 5.8V overshoot
´ PR37, PC44 : Prevent PIC3 oscillation(for phase correction)
ˇ PC09 : Adjust feedback response rate
6-4
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Trans
C
PIC2
E
PC09
PIC3
PD33
PL34
PE33
PR35
A
K
PR36
PR37
K
P
2.5V
A
Fig. 6-8
5.8V
PC34
PR38
PC44
PR39
2nd GND
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