Circuit Description Of Power Supply Pwb - Sharp AM-900 Service Manual

AM-900U

[4] Circuit description of power supply PWB

This power supply unit has the function to convert the AC120V (60Hz)
to DC+24V, and provide these output to the equipment. The following
explains the function of each block. (See Fig. 19)
1. Filter circuit block
This circuit reduces the outgoing noise through the input lines which is
generated in the power supply unit, and prevents the invasion of the
noise from the lines. (The excessive surge such as thunder is pre-
vented by the varistor (Z1).)
2. Rectification and smoothing circuit block
This circuit rectifies and smoothes AC input, and provides the DC volt-
age to the switching circuit block.
3. Switching circuit block
This circuit converts the DC voltage (provided from the rectification
and smoothing circuit block) to the high-frequency pulse voltage by
FET (Q1)'s switching (on/off repeat), and provides the energy to the
transformer (T1). It discharges the energy (charged during the FET
ON time) to the secondary side during the FET OFF time through the
secondary windings. The output voltages on the secondary side pro-
vided by the energy depend on the ratio of the winding turns (primary :
secondary) etc..
4. Control circuit block
This circuit block controls the output voltage by transmitting the
detected +24V voltage to the primary control circuit through the photo-
coupler (PC1). In case of the over-current, this circuit reduces provid-
ing the energy to the transformer. In case of the over-voltage, this cir-
cuit reduces providing the energy to the transformer by letting the
power-zener (D104 : connected between the +24V output voltage and
GND) into the short mode and letting the over-current protection circuit
work.
(1)
Filter
Input
circuit
block
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
5. +24 V SUB output circuit block
This circuit block rectifies and smoothes the high-frequency pulse volt-
age provided by the transformer, and provides the DC+24V output to
the equipment. The output voltage is adjusted by the variable resistor
(VR101).
6. +24 V MAIN output circuit block
This circuit block supplies DC+24V output to equipment through a con-
nector (CN2) from +24V provided by the transformer.
7. +5 V output circuit block
This circuit block rectifies and smoothes the high-frequency pulse volt-
age provided by the transformer and provides about DC+7V output to
the regulator IC, and provides the DC+5V output to the equipment.
8. +3.3 V output circuit block
This circuit block supplies +3.3V output that is stable by the chopper
circuit which considered the above-mentioned +24V as the input. This
system supplies energy to load through L271 during the ON of MOS
FET Q271, and is making the energy accumulated L271 return to load
by D271 during the OFF. Control of constant voltage is performing +5V
output by applying detection and feedback by Integrated circuit regula-
tor IC201.
9. AC output circuit block
This circuit block supplies AC output from AC input to equipment
through the optical-isolator (PC2) and the power-relay (RL1) by sig-
nals (+24VS/HL ON-/PWRLY-).
10. Zero cross circuit block
This circuit block rectifies the AC input, and provides the ZC signal to
equipment through the photo-coupler (PC3).
Power relay
Optical isolator
Transformer
(5)
+24V circuit
block
Photo-coupler
Fig. 19
5 – 21
+24VS
HLON-
PWRLY-
(6)
+24V Main circuit
block
+24V MAIN
CN2
+24V SUB
(7)
+5V circuit +5V
block
(8)
+3.3V circuit
+3.3V
block
GND
/ZC