Samsung SCH-470, Service Manual, Page: 21 / 66

Circuit Description
Samsung Electronics 5-7
5-4 Desk-Top Rapid Charger
The Desk-top rapid charger(DTC21) is largely divided by two parts. One part generates secondary static
voltage and current from AC power source, and the other part detects the battery pack, the battery type,
and charge voltage, and controls the charging status.
5-4-1 Power Supply
AC POWER PROTECTOR AND REGULATOR
The AC power is regulated through BD1, C2 and
converted to the high DC voltage.
TNR1 is used for surge protector, F1 is fuse to
protect from overcurrent, and C1 and LF1 are
filters to eliminate the noise of the switching
circuit.
SWITCHING CONTROLLER AND
TRANSFORMER
U1 as a switching controller supplies static voltage
and current to the secondary through U2 (photo
coupler).
Transformer PTF1 is combined with the 4 winding
coils. The primary winding is linked to the
primary side and the secondary winding is linked
to the secondary side so that it supplies power.
The fourth winding is used to supply power to U1.
This SMPS circuitry uses a flyback method, so the
secondary1, 2 and fourth coils are wound
reversely against the primary. When the power
applies to the primary, the secondary and third
will be off. When the primary is power off, the
saved power will apply to the secondary and U1.
D1, D2 is a snubber circuitry, and absorbs the
counter-voltage which comes out when the
primary winding is off.
STATIC ELECTRICAL CURRENT CIRCUITRY
The electric current which flows on the secondary
winding is detected by R25. The current will be
converted into proportional voltage through U23-
A and Q21.
The proportional constant is changed according to
the ON/OFF status of Q22, Q23, and Q24, so that
it finally change the value of the static electrical
current.
The Vi is added to the U24-A pin 2, and the
voltage is compared with the reference voltage
(Vr) of pin 3. When the Vi is greater than the Vr,
Q36 turns on and the IC2-2 is activated.
At this time, IC2-1 becomes on. It makes IC1 be off,
as a result, the primary will be off and limit the
electric current output.
Assumes the static current on the secondary is lc,
the Vi will be obtained by following below.
STANDARD :
•·
=
°ƒ
E X T E N D E D :
•·
=
°ƒ
•‚ = 1 •‚ = °ƒ
•„
=
°ƒ •„
= 1
The Vi is maintained as the same level as Vr of the
comparator U24-B, so Vi is Vr. That is:
R68 and C49 are used to compensate the phase
difference occured due to the time delay for the
circuit.
V = Ic [R26//(
•·
R27)//(
•‚
R28)//(
•„
R29)]
(R23//R24)
(R25//R77)
IC =
(R25//R77) [ R26//( •· R27)// ( •‚ R28)//( •„ R29)]
(R23//R24) °§ Vr
Vr = Vcc
R47 + R48
R48