Over Current Protection - NEC MultiSync FP1370 Service Manual

8-9. Over Current Protection

If some failure occurs and the total power of secondary outputs increase, the peak voltage of IC621 pin6
increases proportionally.
And when the peak voltage of IC621 pin 6 reaches -1V, IC621 decreases ON period duty cycle of TR1
(internal IC621).
And also the voltage of secondary outputs and the voltage of auxiliary winding of T601 decrease.
When the auxiliary winding T601 decreases below 5V, IC621 operation stop.
But the charging current flows into the capacitor C623 through R634, R635 and IC621 operation start when
the voltage of IC621 pin 9 reaches 8V by this action.
So IC621 repeats the intermittent oscillation.
AC
9
IC621
DRIVE CIRCUIT
TR1
B
-1V
D601
SW601
C623
Q624
R641
5
DRIVE
Q625
4
+
R623 R622
-
C621
3
B
C
1
C
E
2
GND
R624
6
R626
OCP
(Fig.8-9) Over Current Protection
D602
L601 Q623
R634
R635
+
C623
-
R625
8-48
D601
R636
D624
+
R638
C624
-
D625
ZD622
T601
L621