Over Current Protection - NEC MultiSync FP1350 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
6
OCP
(Fig.8-9) Over Current Protection
D602
L601
R634
R635
+
C623
-
R625
R624
R626
8-47
D601
R636
Q623 D624
+
R638
C624
-
D625
ZD622
T601
L621