Circuit Description Of Power Supply Pwb - Sharp FO-DC635U Service Manual

[6] Circuit description of power supply PWB

This unit supply input AC 120 V/60 Hz, +24 V, +5 V as block diagram
shows. (See Fig. 12)
(1)
Filter
Input
circuit
block
1. Noise/Rectification and smoothing circuit
Filter part reduces the outgoing noise that generated in the Power
Supply unit, and prevents the invasion of the noise from outside. The
excessive surge such as the thunder is prevented by Varistor X701.
Rectification and Smoothing circuit is supply DC voltage to switching
part by AC input rectify by Diode D701, D702, D703, D704 and
smoothing Capacitor C706.
2. Switching circuit
This circuit is adopt Ringing Choke Converter system that self-excited.
This system supplies power as follows;
AC voltage that supplied by Rectifying and smoothing part is change
high-frequency pulse by NO/OFF repeat of MOS FET Q701. And while
Q701 ON, accumulate energy in primary winding of transformer T701.
And while OFF, eject energy to secondary winding.
Control of constant voltage is carry out by as follows;
Feedback put on control circuit through photo coupler from +24 V Out-
put. Over Current Protection Circuit detect spread of ON period by
Output Load is heavy, and extend OFF period by control circuit. And
turn down energy that accumulating in primary winding.
Over Current Protection is make secondary side of output voltage +24
V's rising over voltage condition by conducting Zener Diode D711 that
between +24 V output and GND. Then protection is carrying out by
Over Current Protection Circuit of Control Circuit working.
AC OUT
(9)
AC output
circuit block
(2) (3)
Rectification Switching
and smoothing circuit
circuit block block
(4)
Control
circuit block
(10)
Zero-cross
circuit block
Power relay
Optical isolator
Transformer
(5)
+24V circuit
block
Photo-coupler
Fig. 12
3. +24 V circuit
+24 V Circuit is supplied by Transformer T701 Output Rectification and
Smoothing by Diode D710, Capacitor C715.
4. +5 V circuit
+5 V Output is carry out stabilization by step-down Chopper Circuit
that inputted above +24 V. When MOS FET Q705 ON, this system
supply load energy through L705, and when OFF, accumulated energy
in L705 Rectification to load by D715. Control of constant voltage is
carry out by detect +5 V output by Shunt Regulator and put on Feed-
back.
5. HEATER ON signal
HEATER ON/OFF is carry out by send a signal through Trigger TR701
of Photo Triac Coupler PC702 by (High/Low).
6. HEATER-RELAY OFF signal
ON/OFF of Relay RL701 is carry out by (High/Low). But when
HEATER-RELAY OFF Signal change High to Low, Thyristor SR701
keep ON condition. So RL701 keep OFF condition till Power Supply
re-inter.
7. Zero cross circuit block
This circuit block rectifies the AC input, and provides the ZC signal to
equipment through the photo-coupler (PC3).
5 – 19
FO-DC635U
+24VS
/HEATER_ON
/PWRLY
(6)
+24V Main circuit
block
+24V MAIN
CN2
+24V SUB
(7)
+5V
+5V circuit
block
(8)
+3.3V circuit
+3.3V
block
GND
/ZC