Second Over Voltage Protection; Second Over Current Protection - NEC MultiSync FP1370 Service Manual

8-16. Second Over Voltage Protection

If some failure occurs and the output voltage of the secondary windings of T651 increase, the voltage of
the auxiliary winding of T651 also increase proportionally. When the voltage of C651 increase about +13V.
This action will stop the oscillator with in IC651.

8-17. Second Over Current Protection

If some failure occurs and the total power of secondary outputs increase, the peak current of IC651 pin3
increases.
And when the peak current of IC651 pin3 reaches about 1.5A,the voltage of IC651 pin 1 decrease.
When the the voltage of IC651 pin 1 decreases below 4.7V, IC651 operation stop.
Next,the charging current flows into C653 and the voltage of IC651 pin 1 increase about 5.7V.
IC651 continues the movement of decreasing and increasing voltages 8 times.
After IC651 repeats the intermittent oscillation.
D601(+)
C610
CONTROL
1
IC651
R652
SOURCE
2
Q652
(Fig.8-17) Second Over Voltage Protection
+ +
- -
C609
C
1
R657
+
C653
-
(Fig.8-18) Second Over Current Protection
PC651
ZD651
Q651
R653
C660
D
IC651
3
PWM
Control
Circuit
2
S
8-53
T651
+
C651
-
R660
T651
D653
R659
C656