Samsung SCH-1900, Service Manual, Page: 21 / 112

Circuit Description
Samsung Electronics 4-11
4-7-1 Input Filter and Regulator
The AC input converts to a high-voltage DC
through overcurrent protector F1 and F2, surge
protector VA1, noise eliminator CO1 and LF1, and
regulator D1 and C3.
4-7-2 Switching Controller and
Transformer
DC voltage is biased to the gate of Q1 through
R3-R7, resulting Q1 to be on. When the current is
supplied to the primary winding of T1, the gate
voltage of Q1 is increased.
The voltage detected by R2 is biased to the base of
Q2 through R10 and C7. It turns Q2 on. The gate
voltage of Q1 is reduced so that Q1 turns off. The
flyback current transfers to the secondary
winding.
4-7-3 Feedback
The transferred current is rectified and flattened
by D20, C21 and C22. If there is no battery
installed or Q2 turns off, the current is biased to
U21 through R26 and R27, then applied to OP1
according to the value set through R27-R29. It
controls the bias voltage of Q2 base. It causes the
output voltage to be 8.0V.
4-7-4 Battery Charging
Without battery installed, Q24 turns on. It turns
LED off. The output status of U23 and U24 are as
follows:
OUT-A OUT-B Q21 Q20
U23 L L
U24 L L OFF OFF
With battery installed, and when the output
voltage of U23A pin 3 is smaller than pin 2, the
output status of U23 and U24 are as follows. Q24
turns off, the LED is lit in red.
OUT-A OUT-B Q21 Q20
U23 L L
U24 H H OFF OFF
When the battery voltage of pin 3 of U23A is
greater than pin 2, Q21 turns off, the output of
U23A becomes low, the output status of U23 and
U24 are as follows:
OUT-A OUT-A Q21 Q20
U23 H H
U24 H H ON ON
When Q20 and Q21 turn on, the battery is charged.
The output voltage detected by R21 is increased
until the output voltages of U22A pin 6 and pin 5
are the same level. When the output voltage of pin
5 becomes greater than that of pin 6, U22 becomes
high, resulting U21 reference voltage to be high.
When the OP1 current increases, Q2 turns on, Q1
turns off, and the output voltage is reduced.
The battery voltage is detected by U23B pin 5.
U23B compares the voltage with reference voltage
in pin 6, and remains low status until the voltage
reaches the reference voltage. It causes U24A to be
remained high.
4-7 Travel Charger