Sharp FO-4500, Service Manual

Page: 113 / 192

[6] Circuit description of power supply PWB
DC power circuit
The DC power circuit directly rectifies AC power, changes the voltage
through switching with the DC/DC converters, and rectifies and
smoothens the current again in order to produce the DC voltage. See
Fig. 13.
1. Block diagram
AC IN
NOISE
FILTER
CIRCUIT
SURGE CURRENT
PREVENTING CIRCUIT/
RECTIFYING AND
SMOOTHENING CIRCUIT
INVERTOR CIRCUIT
(FLY-BACK
CONVERTER SYSTEM)
RECTIFYING/
SMOOTHING
CIRCUIT
+24V
CONTROL
CIRCUIT
OVERCURRENT
PROTECTION
CIRCUIT
T701
PC701
CONSTANT
VOLTAGE
DETECTION
CIRCUIT
3-TERMINAL
REGULATOR
CHOPPER
CIRCUIT
+12V
+5V
HEATER
ON/OFF
PC702
HEATER
CIRCUIT
HEATER OUT
Fig. 13
2. Noise filter circuit
The filter circuit is composed of L and C in order to reduce the
common noise and normal mode noise which flows into and out of the
AC line.
The common mode noise is called the noise which generates in cach
line against GND, and C703, C704, C722, C723 and C725 flows the
noise component to GND.
Moreover, the normal mode noise is called the noise which is super-
imposed on the AC line or output line and is attenuated with C701,
L701, C702, and L702. See Fig. 14.
D701
C725
L702
C703
C723
C704
C702
L701
C701
C722
Fig. 14
3. Surge current preventing and
rectifying/smoothening circuit
As shown in Fig. 15, AC is rectified and smoothcned with D701 and
C705 which include four diodes.
To prevent surge current, it is mode to be effective using the power
thermistor TH701.
D701
C705
TH701
Fig. 15
FO-4500H
5 – 23