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g. Overcurrent protection circuit
The d line of the primary side is connected to the detecting resistor
of the primary side current. When an overcurrent is generated, the
current in the switching FET (Q701, Q702) is detected and a signal is
sent to the control IC (IC701) to reduce the ON pulse width of the
switching FET to reduce the output voltage. In this circuit, the switch-
ing FET is intermittently operated.
R707
0.1/5W
R708
0.1/5W
R717
100(H)
100(H)
To IC701
Fig. 9 Overcurrent protection circuit
h. Series regulator circuit (–5.2V series, –20V series)
IP701
ICPN25
L720
-8.2VDC
C736
B01A
470P/1KV(HA)
D729
(L)
(B)
ERB9102
Fig. 10 Series regulator circuit (–5.2V)
IP702
ICPN25
L721
-25.2VDC
C740
B01A
470P/1KV(HA)
D731
(A)
(L)
ERB9102
Fig. 11 Series regulator circuit (–20V)
This is also called the dropper system. The high frequency pulses
from the choke coil (L715) are rectified and smoothed, and the volt-
age is dropped by the regulator IC to stabilize.
i. Chopper regulator circuit (5V series, 12V series)
RT
6
OSC
CT
5
DEAD
0.2
time
4
control
Error amp1
+IN
1
+
A1
PWM comparator
-
-IN
2
+IN
16
+
A2
-
-IN
15
Error amp2
Feed
3
back
i (From Q701, Q702)
R718
D728
1SS178
IC703
P030RV11
C737
C738
22/35V(PF)
330/35V(PF)
D730
1SS178
IC704
NJM70M20FA
C741
C742
22/35V(PF)
330/35V(PF)
Output control
13
8
Q
T
9
Q
11
Pulse steering F/F
10
12 VCC
Reference
14 VREF
regurator
7 GND
+IN1
-IN1
F.B.
GND
Fig. 12 IC705 (IC706) block diagram
The switching frequency is determined by the CR connected to the
IC705 (IC706) 5pin and 6pin, and triangle waveforms of about 50KHz
are generated.
The output voltage of the output voltage detecting circuit and the
reference voltage (5V) at IC705 (IC706) 14pin are inputted to the
error amplifier in each IC to control ON/OFF time of the output tran-
sistors TR1 and TR2 (PWM control), driving the switching FET Q706
RET
(Q708) and stabilizing the output voltage.
-5.2V
CN703-16
RE T
Fig. 13 Switching waveform of each section
-20V
CN703-18
C1
TR1
E1
C2
TR2
E2
9 – 25
TOP VIEW
1
+
+
16 +IN2
A1
A2
2
-
-
15 -IN2
3
14 VR
D.T
4
13 O.C
CT
5
12 VCC
OSC
TR2
RT
6
11 C2
7
10 E2
TR1
C1
8
9 E1
TR1, TR2 (8)(11) < > (9)(10)
voltage
0V
Switching FET Q706(Q708)
Drain < > Source voltage
0V
Switching FET Q706(Q708)
Drain voltage
0V
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