Samsung SF4500, Service Manual

Page: 60 / 130

Samsung Electronics 6-29
6-4-2-2. OUTPUT CONDITIONS
6-4-3. Power Circuit
6-4-3-1 INPUT
This product is equipped with a thre e - w i re
g rounding-type power inlet. Insulation is class 1.
When a power device part breaks down or some
t rouble in this circuit causes a short, excess curre n t
runs through a fuse and blows it to protect power
devices and the system from excess current and
p revent a fire .
Input filter of power input terminal removes line
noise flown in from outside and reduces noise which
is made in power circuit and logic part and conduct-
ed or radiated through power cord s .
AC input voltage is rectified by bridge diode of full
wave re c t i f i e r, is converted to DC voltage by smooth-
ing circuit, and switches the power device of this
voltage.
As switching is in RCC(Ringing Choke Converter)
mode, switching frequency changes from about
50KHz to 100KHz.
PWM(Pulse Width Modulation) Control Circ u i t
adjusts the duty of the switching device by sensing
the change of output voltage, and protects this
p roduct from excess current by receiving signals
f rom excess current detecting circ u i t .
6-4-3-2 DRIVING CIRCUIT (START-UP CIRCUIT)
If you insert a power plug into AC outlet, full-wave
rectified DC output voltage is generated at input
e l e c t rolysis condenser and supplied to the gate of a
switching device through driving resistance. This
voltage drives the switching device. Once it is
driven, it continues to switch via PWM control
c i rcuit with power supplied by auxiliary coil Nb.
6-4-3-3 SWITCHING CIRCUIT
(VOLTAGE STABILIZATION)
11.75V of 2nd voltage is detected by voltage detector
and feeds back through photo coupler to 1st PWM
C o n t rol Circuit signals compared by error amplifier.
PWM Control Circuit controls the gate of power tran-
sistor according to the feedback voltage so as to sup-
ply load with stable output voltage.
NO ITEM CH1 CH2 CH3
1 RATED OUTPUT VOLTAGE +5.0V +11.75V +30V
2 RATED OUTPUT CURRENT 0.5A 0.8A 0.5A
3 MAXIMUM LOAD CURRENT 0.8A 1.3A 0.76A
4 LOAD CHANGE RANGE 0.1 ~ 0.8A 0.03 ~ 1.3A 0.05 ~ 0.76A