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4-5 Desk-Top Rapid Charger
4-5-1 SMPS
1ST SWITCHING CIRCUIT
The AC current from AC plug transfers to a high-
voltage DC through BD1, C2, and C3. TNR1 is AC
surge protector, and F1 is fuse to protect circuit
from overcurrent . The line filter composed of LF1
and C1 is used to protect from line noise and
switching noise.
The transferred DC voltage is switched by T1. 2nd
switching voltage is flattened into AC through D20
and C22, and filtered through L20 and C30.
The voltage is regulated through shunt regulator
U5, R28, R29.
2ND SWITCHING
The regulated voltage through 1st switching
circuit performs 2nd switching. Step-down
regulator is composed of Q20, U2, D22, L21, and
C25. Q20 performs high-speed switching
according to the output signal from U2. L21 stores
energy when the switching transistor turns on.
D22 is a fly-wheel diode and transfers the stored
energy when Q20 turns off.
Voltage mode PWM IC U2 controls duty cycle
between the switching on and off of Q20. C30 and
R34 are parameter to determine the switching
frequency of U2.
Samsung Electronics
4-5-2 Constant Current Circuit
U2 (TL494) has two error-amps. VREF (+5V) is
supplied to INV terminal (pin 15) of amp 1 and
N.INV terminal (pin 16) is connected to ground. It
causes the amp 1 output to be zero. In amp 2, the
reference voltage 2.0V is supplied to INV terminal
(pin 15) through R37 and R38, and the voltage
obtained in proportion to constant current is
supplied N.INV terminal (pin 16). The constant
current circuit also includes the shunt resistor R40,
and Q21 and U7.
FAST CHARGING (MASTER MODE)
The fast charging formula is shown below.
R39
I
=
CHG
R40 x (R43 + RR)
For standard battery, RR is set with 680 ohm, and
for slim battery, 1 kohm is used.
SLOW (SLAVE MODE) AND TRICKLE
CHARGING
If batteries are installed both on front and rear port
of the desktop charger, the one installed in
advance performs fast charging while the other
installed later performs slow charging. In slave
mode, Q23 turns off. In trickle charging mode, Q24
turns off. The slow charging formula is shown
below.
R39
I
=
CHG
R40 x (R42)
Circuit Description
x V
(mA)
REF
<RR=330ߟ, V
=2.0 V>
REF
x V
(mA)
REF
<V
=2.0 V>
REF
4-7
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