Samsung DVD905 Service Manual page 27

Reference Information
6. Maximum power limit circuit
* Circuit configuration : PIC1 F/B, CS, IS terminal PR23, PR21,
* Role : Switch overcurrent protection
* The current on switch inputs to IS and outputs as voltage via
PR23, PR21, PR22.
* If the current on switch increases and IS terminal input
voltage exceeds 0.24V, duty is determined by F/B terminal.
Accordingly, the current on switch is limited less than specific
current.(If 2d load rises, the switch current increases. Refer to
clause 5. in circuit description).
* If I/S terminal is open : Switch and other parts are damaged
because the switch is not limited during abnormal status (for
example, 2d ouput terminal short).
1) Intermittent operation mode
- Definition : PIC1 canÕt deliver normal output, and repeats Vcc ON/OFF operation(Vcc voltage is swing
between 16V(ON) and 8.7V(OFF).
- Operation description : for example. 2d output terminals shorted, then :
Specific 2d terminal short --> 2d power is concentrated on short terminal --> 5.8V(PC39)
voltage down --> F/B voltage up (Refer to clause 5.) --> Duty up --> Switch current up --> Limit switch
current to IS terminal(Duty limit) --> Apply 1st supply power in comparison with 2d power -->
Concentrate on 2d power short terminal --> PIC1 Vcc terminal TRANS. output voltage down below
8.7V --> PIC1 OFF --> PIC1 ON via driving resistance --> Above operations repeated ® Maintain
intermittent operation mode (See Fig. 2-19 Waveform).
At short release : normal operation
2) Short OFF mode
- Main examples : Feedback circuit Open, Max, Power
excess(Design spec.), 5.8V Terminal short
- Operation description :
Ex - 2d load is high abnormally;
1st supply power is low in comparison with 2d output power.
5.8V down --> Duty up --> Switch current up -->
IS terminal duty limit --> 1st supply power limit --> 5.8V
voltage down --> F/B potential above 2.8V(Shut OFF voltage)
--> OS terminal potential up (Refer to Clause 4.)F/B -->
OS potential above 7V --> Shut OFF mode
- The output voltage of SMPS is zero because PIC1 doesnÕt work
during Shut OFF mode.
- Shut OFF mode release : OS potential decreases below
7V.(Charge OS terminal forcely)
- Feedback terminal open : Shut OFF (Refer to clause 5.);
PC13A : For IS terminal input noise removal
2-8
PR22, PC13A
Duty Limit sector
CS Voltage(3.6V)
DT Voltage
OSC OUT
FB Voltage
OUT Voltage
COM.C4(0.24V)
IS(+) Voltage
Blas voltage
Over current protection
Fig. 2-18
Time : 100ms / div
Volt : 5V / div
Fig. 2-19
Samsung Electronics
Over current protection
16V
8V
GND