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REV.-A
2.3.3 Rectifier and Smoothing Circuit
The AC IN voltage from the filter circuit is full-wave rectified by diode bridge DB 1, and converted to
approximately 2 X AC IN voltage by smoothing capacitor C7. The
+
24 VDC,
+
5 VDC, and
±
12 VDC
voltages are converted from this DC voltage.
DB1
+
*
AC IN
+
C7
DC OUT
3301£
( 1501£)
( ): 220/240 V Version
Figure 2-8. Rectifier and Smoothing Circuit
2.3.4 Starting Circuit (Main Switching Circuit)
Figure 2-9 shows the starting circuit. The operation sequence is as follows.
(1) When the main power source is connected, the AC input passes through the input filter, and is then
rectified and smoothed. The resulting DC voltage VIN is input into the circuit.
(2) PVIN is applied to starting resistance R7 and passes through point
@.
Base current IB flows through
transistor
Q
1, causing the transistor to conduct.
(3) At the same time, VIN is applied to coiI0-@of pulse transformer T 1, causing a voltage of (7 /60T)(VIN)
at
@-@,
so that the positive feedback current of switching circuit IB flows in the direction of @.
This causes a sharp rise, and
Q
1 quickly switches on.
(4) Current IL through coil 0-@ increases linearly over time.
During this time, a voltage of (1 0/60T) VIN is induced through coils
0-0
and
0-0),
and a voltage
of (19/60T) VIN through coil
@-@.
For all these coils, then, current attempts to flow in the direction
of@; however, this direction opposes the direction of diodes D22, D23, and D20, so that no current
flows through the secondary side of the circuit.
(5) Current IL in the primary winding increases, but because the potential at point
@
of coil
@-@
is
fixed,
Q
1 base current IB cannot surpass a specified level. Because of this, the value of current le
(= IL) flowing from coil 0-@ to the
Q
1 collector cannot surpass a maximum of (hfe)(IB).
Therefore, the 0-@ current value stops changing, and the coil voltage drops. At the same time,
a reverse voltage is applied to coil
@-@,
IB drops, and current flows through D3 opposite to direction
@. At this time, the potential at point
@
is higher than that at point
@,
and C 10 absorbs the current
flowing in the
@-@
direction.
Q
1 is quickly shut off by the resulting sharp drop.
2-13
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