Circuit Description Of Power Supply Pwb - Sharp FACSIMILE FO-2950M Service Manual

[4] Circuit description of power supply
PWB
1. Noise filter circuit
The filter part removes noises generated from the power unit to avoid
noise release outside and prevent external noises from entering.
Excessive surge such as thunder is prevented by varistor Z1.
2. Rectified smoothing circuit
The rectified smoothing circuit rectifies AC input at diodes D10, 11, 12,
and 13, and then smoothens it at capacitor C5 to supply DC voltage to
the switching part.
3. Switching part
This circuit adopts the ringing choke converter system of self-excited
type.
By repeating ON/OFF of MOS FETQ1, this system converts DC voltage
supplied from the rectified smoothing part into high-frequency pulse,
stores energy in the primary winding of transformer T1 during ON pe-
riod, releases energy to the secondary winding during OFF period, and
supplies power.
Frequency changes according to output load; As load increases, ON
period becomes longer.
Constant voltage is controlled by applying feedback to the control circuit
via photo coupler from 24 V output.
The overcurrent protective circuit detects prolonged ON period caused
by excessive output load, lengthens Q1 OFF period by using the control
circuit, and restricts energy stored in the primary winding of transformer
T1.
F1
CN1
AC IN
F3
NOISE
RECTIFIED
FILTER
SMOOTHING
CIRCUIT
CIRCUIT
ZERO CROSS CIRCUIT
HEATER CIRCUIT
CN2
Increase of the secondary output voltage 24 V is led to the overcurrent
condition by turning on power zener diode D202 between 24 V output
and GND.
Thus overvoltage is protected by operating the overcurrent protective
circuit of the control circuit.
4. 24 V circuit
To supply output, transformer T1 output is rectified and smoothened
with the use of diode D101 and capacitor C101. Voltage is controlled by
Volume VR101.
5. +5 V circuit
Transformer T1 output is rectified and smoothened with the use of diode
D301 and capacitor C301 to stabilize +5 V output by using 3-terminal
regulator IC301.
6. Heater circuit
To maintain the optimal temperature, the heater lamp is controlled by
HLON signal from the control panel.
This HLON signal is to switch ON/OFF the heater lamp. If this signal is
input LOW, PC2 is switched ON, resulting TRIACK TRA1 ON.
Accordingly, AC power is supplied to the heater lamp to switch the heater
lamp ON.
7. Zero cross circuit
When AC input reaches the zero cross point (0 V), PC3 is switched ON.
When Q501 is switched ON, the zero cross signal is output to the con-
trol panel.
TRANSFORMER
SWITCHING
CIRCUIT
PC1
PC2
Fig. 27
5 – 27
FO-2950MGR/GH/MY/TH
T1
+5V
CIRCUIT
+24V
CIRCUIT
SW101
+5V
+24VH
+24V
ZC
+24VS
HLON