Samsung SF4500, Service Manual

Page: 61 / 130

6-30 Samsung Electronics
6-4-3-4 OCP CIRCUIT
(OVER CURRENT PROTECTIVE CIRCUIT)
If a short in secondary load terminal or a short inside
power device causes excess current, power consump-
tion increases. Then it increases current of power
t r a n s i s t o r.
The current of power transistor increases linearly
when it is on. This current is detected by sensing
resistance and is impressed to excess voltage
p rotective circuit of PWM control circuit through RC
integration circuit where over current protection is
built in.
If this voltage becomes bigger than Vbe of excess
c u r rent control transistor, the potential of power tran-
sistor gate falls immediately and it stops driving
power transistor to protect it from excess current.
6-4-3-5 O V P CIRCUIT
(OVER VOLTAGE PROTECTIVE CIRCUIT)
If the main power of power device 11.75V incre a s e s
a b n o r m a l l y, this excess voltage is detected by OVP
detecting circuit and 11.75V output terminal shifts to
a state similar to
a short.
If 11.75V output terminal shorts, over current runs on
over current detecting terminal and OCP c i rc u i t
operates to prevent a fire in power device.
6-4-3-6 +5V OUTPUT TERMINAL
S q u a re wave voltage generated by Ns1 produces DC
voltage rectified and smoothed by half-wave re c t i f i e r.
This voltage put in to a thre e - w i re regulator gener-
ates stable DC power of +5V.
6-4-3-7 +30V OUTPUT TERMINAL
S q u a re wave voltage generated by Ns2 is re c t i f i e d
and smoothed by half-wave rectifier and supplies
load with output voltage without separate
regulation.
6-4-3-8 +11.75V OUTPUT TERMINAL
S q u a re wave voltage generated by Ns3 is re c t i f i e d
and smoothed by half-wave re c t i f i e r, and then detect-
ed by voltage detector. Signals compared by erro r
amplifier are fed back to primary PWM
c o n t rol circuit through photo coupler.
PWM control circuit controls the voltage of power
transistor gate driver according to the voltage to sup-
ply load with stable output voltage.
6-5-1. Paper Sensing Circuit
This circuit senses whether there is paper or not,
using photo interru p t e r. If there is no paper, the lever
will cover its sensor and output “/P_EXIT” will become “High”. If there is paper, the lever will
be lifted and output “/P_EXIT” will become “Low”.
6-5. Others