Circuitdescriptionof Powersupplypwb - Sharp FO-155 Service Manual

FO-155A
FO-355A
[4] Circuit description of power supply PWB
1. Block diagram
Noiea +
Rectifying~ Switching
Regulater
Circuk
* 12V
AC IN
Filter Smoothing
Cimuit d Circuit
- Circuit
— Regulater* 5V
Circuit
(RCC avstem)
I
. . .
II
() vMrvni
t
&l
$+
Phot*pler
Fig. 1. Block diagram
2.
General description
Theinputvoltageis 187VAC - 276VAC conforming to ULstandards.
The outputs
are+24V(VM), +5V,+12V.
The overvoltageprotection
functionfor protectionof the load in case of powerabnormality and
the overcurrent p rotectionfunctionfor protection of the powersupply
itselffromoverloadareadded.
3.
Operational description
When230-240V/ACpoweris supplied,it is passedthrougha noise
filter to the rectifiersectionwhereit is smoothed to about300Vthen
suppliedto the invertersection.The invertersectionemploysone-
transistorON/OFFself-excitedinvertor(R.C.C.system)and a stable
DCvoltageis suppliedto the secondary side.
Theoperationof eachsectionis described below:
3.1. Invertorsection
ZDI
RD27
R4
18K
,,
R3
470
2S::W2
2S%18 '
+
a
R9
103
~
1S2471
+
—
7
I[
6
Whenthe voltageacross R4 reachesthe gate ON voltagethrough
R5, R6,and R4,QI beginsto turn ON,flowinga currentfr;m pinl ;f
T1 primarywindingto pin3.Thena voltageis generated frompin5 of
auxiliarywindingto p~n4, t urningQ1 gateON completely. T he drain
current increaseslinearly to store energy in the primary winding.
However, voltage across R3 turns ION Q2 thorough R9 when the
drain current reaches a certain level. As a result, Q1 ga~evoltage falls
below the threshold voltage (about 4V) of the gate and Q1 turns off.
Simultaneously when QI turns off, the
energystoredin the primary
winding is inducedin the secondarywinding to bias the rectifier
diodes D4, D5, and D6 forwardly,smoothingeachoutputcapacitor.
Thusa DCvoltageis obtained.
3.2.
(1)
(2)
(3)
(4)
(5)
Controlsection
+24V (VM)
control
A voltageis generated in the secondary sideby repeatedopera-
tions of 3.1, and the outputin the secondary side is dividedby
R14,VR1,and R15to be inputtedto Q4. The dividedvoltageis
adjustedto about6.2Vby VR1.Q4 alwaysmonitorsthe divided
outputvoltage.Whenthe outputvoltageexceeds+24V,the di-
vided voltage also exceeds6.2V and Q4 judges it as an in-
crease in the output voltage. Then photocouplerPC1 is lit
throughR13to turn on the transistorin the light receivingside,
supplyinga current to the base of Q3, turning off Q1. (The
current which is to be passedthroughthe additionalline, R8,
and C8 to R4 is bypassed by Q3). Resultantly, O Ntimeof Q1 is
shortenedand the energystored in the primarywindingis de-
creased,limitingthe increasein the output voltage.When the
output voltagebeginsto decrease,the light quantityof PCI is
decreased to lengthenthe ONtimeto Q1. As a resultthe energy
in the primarywindingis increasedto compensate for the de-
creasein the outputvoltage.
The negativefeedbackcontrolis repeatedto stabilizethe output
voltage.
+5V,+12Vcontrol
The outputsof +5V and +12Vare stabilizedby the three-termi-
nal regulatorICS(IC1and IC2).The overcurrent p rotection func-
tion protectsthe regulatorICSthemselves.
NoiseFilterCircuit
The input noise filter section is composedof L and C, which
reducesnormalmodenoiseandcommonmodenoise.
Rectifying/Smoothing Circuit
The AC inputvoltageis rectifiedby diode
D1 and smoothed by
capacitor C5 to supply DC voltage to the switching circuit sec-
tion. Power thermistor TH1 suppresses inrush current at
power
switch-on.
SwitchingCircuit
(6)
Thesystemappliedin this circuitis the self excited ringing choke
convertor(RCC).In this system,the DC voltagesuppliedfrom
the rectifying/smoothing sectionis convertedintohighfrequency
pulses by ON/OFF repetition of MOS FET QI. Energy is
charged in the transformerTI during ON period of Q1, and
discharged to the secondary circuitduringOFF period.The out-
put voltageis controlledby automaticadjustmentof ON period
of QI. The ON periodis determinedby C9 chargetime changed
according to 24V output through operation of photo-coupler
PC1.The overcurrent p rotectionis performedby dringingQ1 to
OFF state throughdetectionof voltageincreasein the auxiliaty
windingof T1 by ZD2 and R11. The overvoltageprotectionis
performedby operating the overcurrent p rotection circuitthrough
destruction of zenerdiodeZD3.
+5V,+12VCircuit
EachDCvoltagesuppliedby rectifyingthe outputof transformer
T1 with diodeD4 or D5 is stabilizedby 3-terminalregulatorICI
or IC2.
3.3.Overcurrent p rotection function
Whenthe outputcurrentin the secondaryside increasesto become
an overcurrent o r shortR2/R4detectsthe draincurrentto turnon Q2.
The gate voltageof Q1 is controlledto shorten ON time of QI to
protectthe circuitfromtheovercurrent.
3.4.Overvoltage protection function
When the output voltage is abnormallyincreasedand ZD3 zener
voltage exceedsabout 30V, ZD3 is shorted to operate the same
procedure as the overcurrent p rotection function.To reset,turnoff the
AC switch,removethe cause,andreplaceZD3witha newone.
3.5. Hightemperature protectioncircuit
Thermal fuse F3 included in the power circuit is blown when the
surface
temperature of rectifier D1 exceeds about 115
degreesCenti-
grade.(about239°F)
5-10