LG L15V26 Training Manual page 26

CIRCUIT DESCRIPTIONS
No power
D
from +18V
No power
E
from +5VS
POWER SUPPLY DESCRIPTION
If the AC Input voltage (1) is confirmed, this voltage is adjusted as a DC wave form through the EMI
filter (2) and the first rectifier/filter (3). This adjusted wave form is input to PFC circuit (4), Power
Factor Correction). The output voltage (+390Vdc) of the PFC circuit becomes a main input voltage of
the MAIN (5) and standby switching part (10). Using elements such as the FET and PWM IC, the
switching parts (5, 10) convert this high DC voltage (390Vdc) to a spherical wave with a high frequency.
To keep the secondary adjusted voltage regular and safe with changing input voltage and output load,
the output voltage is monitored and fed back to the switching part through a control circuit (9).
BLOCK 2 works as a forward converter by receiving input from BLOCK 1 and supplying MAIN DC (+24V
and +12V) voltage and the step down circuit (8) supplies +18V. BLOCK 3 works as a flyback converter
by receiving the input of BLOCK 1 and supplying +5V for standby. A protection circuit is built into the
switching part (5, 10) to protect over current, short, or over voltage of the secondary output. When
overheating occurs, this circuit protects the output and switches the thermal switch (15) to radiator.
The PFC circuit protects against fire or other accidents by turning the AC voltage off. If the OFF signal
is transmitted to the PFC circuit (4). Only the +5V circuit for standby works and others are turned off to
minimize power consumption.
P800
Directview LCD Training
Output terminal U903 Error, no
No
shorted input of L812
Yes Yes
Check/replace
Check +18V
U802
peripheral parts for
(PQ1CG203) &
patternl short
D916 (SB560)
#7 pin bias power
Output terminal
No of U902 is abnormal
shorted
(normal: 15V-25V)
Yes
Yes
Check/replace
Check +5V
R937-R939, U903
Speripheral parts
(FAN7554), Q802,
for patternl short
D802, CR801
BLOCK 1
PFC Circuit
EMI Filter Rectifiers
T801, Q801,
L801, L802
and Fiter
U901, D801,
C801~5,
BD801,
C811, C812,
etc.
C806, etc.
etc.
390Vdc
Thermal
Switch
PT800
Control
Isolation
PH802,
PH803, etc
Problem lies
No
elsewhere
Problem lies
No T802 Error No
elsewhere
Yes
Replace T802
BLOCK 2
Switching
Rectifiers
Isolation
Elements,
and Filters
Power
OVP, OCP
D803~D805
Transformer
Main Circuit
L805, C827,
T803
Q804, U902,
C823, etc
D912, etc
Feedback
Circuit &
Isolation
PH801,
CR802, etc
Switching
Isolation
Elements,
Power
and Filters
OVP, OCP
Transformer
Circuit
T802
C825, etc
Standby
Q802, U903,
etc
Feedback
Circuit &
Isolation
PH804,
CR801, etc
BLOCK 3
26
Output
12V
24V
Step Down
Circuit
18V
U802, D916,
L807, etc
Output
Rectifiers
D802,
Circuit Descriptions