Epson LQ-2550 Technical Manual page 86

2.2.8 Over
V o l t a g e P r o t e c t i o n C i r c u i t
As described in Section 2.2.7, the voltage from the +35 V line is monitored by the voltage regulator
circuit, and fed back to main switching circuit 2 to keep the voltage constant. This circuit is employed
to protect main switching circuit 2 from being damaged if the voltage regulator circuit operates
abnormally.
Figure 2-32 shows the over voltage protection circuit.
Reference voltage VREF (5 V) is supplied to the plus side of IC 1. Voltage
is obtained by dividing the +35 V by R45 and R46, is supplied to the minus side (pin 6) of IC 1.
When the output voltage of the +35 V line meets the condition shown in the expression below, the
7) of IC 1 changes from
output (pin
Therefore, 5 V is applied to the dead-time control terminal (pin 4) of IC2, the dead-time becomes 100%,
and TR 1 and TR2 turn off.
Then, Q3 in main switching circuit 2 is cut off, the voltage applied to coil TIo.12 of T3 turns off, and
the voltage induced in coil T3z-s at the secondary side drops,
+35
15.4 Kfl
1%
2.15 Kfl
1%
GH
6 5
—
+
7
+ 5
T2
5 0
3
8
2
HIGH (1 2 V) to LOW (O V), transistor Q5 turns on, and R28 is shorted.
16
EA1
1
1%
105"C
— C 2 9
Figure 2-32. Over Voltage Protection Circuit
15 14 12
2
1%
2-39
X V035), which
9