Philips DVD712 Service Manual page 41

via D6133, R3135 which will takeover the supply voltage of
T7145 at pin 7.
9.2.3 Secondary Voltage Sensing
The secondary voltage regulating circuit comprise of the
opto-coupler 7131 which isolate the error signal from the
control IC7145 ,on the primary side, and a reference
component 7201 ( TL431 ). The 7201 can be represented by
two components:
• A very stable and accurate reference diode
• A high gain amplifier
R
2.5V
CL 96532065_071.eps
Figure 9-4
When the output voltage increases, due to a reduction in the
load, the voltage across R3205 and R3206 increases to
above the internal reference voltage of about 2.5V then
TL431 conduct. The current through the opto-coupler 7131
will increase due to the fact that the series resistor in 7201
decreases. This result in a increase of voltage to pin 2 of
IC7145, thus reducing the on-time of FET 7125.
In the event of a decrease in output voltage (increase in load
),the control circuit will operate in the opposite way to the
explaination above.
9.2.4 Primary Current Sensing
The current through the FET 7125 resulting in a voltage drop
across R3126,R3127,R3128 which is couple to pin 3 of
IC7145,current sense input.The higher the input voltage, the
more the primary current is limited. In this way the maximum
output power of the power supply is limited.
9.2.5 Undervoltage Protection
Two undervoltage lockout comparators have been
incorporated to guarantee that the IC7145 is fully functional
before the output stage is enable. The supply voltage at pin
7 and reference voltage at pin 8 of IC7145 are each
monitored by separate comparators with built-in hysteresis.
If the supply voltage at pin 7 of IC7145 drops below 10V (
typical ), due to a secondary voltage is short-circuit or
excessive load, the drive pulse at pin 6 of IC7145 will be
disabled and the controller will switch off the complete
SMPS.
Remarks : In the event of the overvoltage situation remaining
present, the SMPS will go in sequence of protection,start- up
cycle, protection and the cycle repeats. This effect is highly
audible.
9.2.6 Overvoltage Protection
The overvoltage circuitry comprising of D6141,R3139,
R3150, R3141,T7141, T7150 which is used to detect an over
voltage situation on the secondary side of the transformer.
After start-up, when the voltage across C2135 exceeds
18V,the overvoltage circuit will trigger the internal latch
circuit, pin 1 of IC7145 and the output buffer is disabled and
Circuit Descriptions And List Of Abbreviations
K
A
130799
DVD712-722
it goes into the overvoltage protection and a complete restart
sequence is required.
9. GB 57