Sharp FACSIMILE FO-4500 Service Manual page 114

FO-4500H
4. Inverter and control circuit
(fly-back converter system)
This circuit is called the fly-back converter which is the one-chip type
separately excited DC-DC converter as shown in Fig. 16. In the cir-
cuit, IC starts oscillating to make Q701 conductive if the start voltage
of IC is first applied to IC701 through R726, R704 and R705.
As the result, the voltage is applied to the primary winding of the
converter transformer (T701), and the voltage is also generated in the
winding which drives IC701. Thus IC701 is put into operation. Then,
IC701 alternately turns on and off Q701 at the frequency (approx. 80
kHz) which is determined by C709 and R714.
When it is on, any current does not flow in the secondary winding of
T701 since the voltage of the secondary winding is applied in the
direction opposite to the diode D707.
When it is off, the direction in which the current flows to the primary
winding becomes the pole direction of the secondary winding, and
D707 becomes conductive to transmit energy to the secondary side.
See Fig. 16.
R726
C705
R704
R705
Q701
R706
R707
C709
R710
IC701
R714
D707
T701
IC702
PC701
Fig. 16
The control circuit is a circuit which the negative feed-back is given
from the secondary side as shown in Fig. 16.
The photocoupler (PC701) is used for insulation between the primary
and secondary sides in order to feed back the control signal to the
primary side.
When the output voltage rises due to the energy transmission from
T701, the voltage detected by R721 and R722 are compared with the
reference voltage of IC702. As it is higher than the reference voltage,
the current of IC702 (that is, the current of the photo diode of PC701)
is increased to be transmitted to the primary side in order to reduce
the potential of the feed-back terminal (2 pin) of IC701. Thus, Q701 is
controlled. Accordingly, the varying rate of the output voltage on the
primary side is passed through IC702 and PC701 in order to control
IC701 and Q701. Thus, the output voltage is stabilized.
Q701 VDS
R721
R722
Q701 VGS
Q701 ID
D720 IO
T701
VOLTAGE OF
SECONDARY
WINDING
5 – 24
TON TOFF
Fig. 17
0
0
0
0
0
t