Samsung DVD-808K, Service Manual

Page: 69 / 165

Circuit Descriptions
7-4
Samsung Electronics
7-1-2(d) Feedback control circuit
Fig. 7-8
- F/B terminal of PIC1 determines output duty cycle.
- C-E(Collector-Emitter) of PIC2 and F/B potential of PIC1 are same.
3 Operation descriptions
a. 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)
b. 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
c. 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 Æ Duty down Æ Vout down Æ Maintain 5.8V
(i.e., the feedback to maintains 5.8V).
- PR35, PR36 : Reduce 5.8V overshoot
- PR37, PC44 : Prevent PIC3 oscillation(for phase correction)
- PC09 : Adjust feedback response rate
1st
2nd