Samsung DVD-812 Service Manual page 54

5-2-2 (d) Feedback Control Circuit
PICF1
GND
#
! @
DRAIN
$ %
F/B
VCC
1st GND
1) F/B terminal of PICF1 determines output duty cycle.
2) C-E(Collector-Emitter) of PICS1 and F/B potential of PICF1 are same.
◆ Operation descriptions
1) Internal OP-Amp '+' base potential of PICS2 is 2.5V and external "-" input potential is connected with PRS33
and PRS34 to maintain Vout of 5.8V. (Vout = ((PRS38 x PRS34) / PRS34) x 2.5V)
2) If load of 5.8 V terminal increases(or AC input voltage decreases) and Vout decreases below 5.8V, then :
PICS2 "P" potential down below 2.5V --> PICS2 A-K of base current down --> PICS2 of A-K current down -->
PICS1 Diode current down --> PICS1 C-E current down --> PICS1 C-E voltage up --> PICF1 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).
ΠPRD31, PRD32 : Reduce 5.8V overshoot
´ PRS32, PCS32 : Prevent PICS2 oscillation(for phase correction)
Samsung Electronics
OCP
C
PICS1
E
PICS2
Trans PDS33
PES33
A
K
PRD31
PRS32
K
2.5V
A
Fig. 5-18
PLS31
PES37
PRD32
PCS32
P
2nd GND
Circuit Descriptions
5.8V
PRS33
PRS34
5-9