Samsung SF-4500 Service Manual page 61

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
transistor.
The current of power transistor increases linearly
when it is on. This current is detected by sensing
resistance and is impressed to excess voltage
protective 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
current 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 OVP CIRCUIT
(OVER VOLTAGE PROTECTIVE CIRCUIT)
If the main power of power device 11.75V increases
abnormally, 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 circuit
operates to prevent a fire in power device.
6-5. Others
6-5-1. Paper Sensing Circuit
This circuit senses whether there is paper or not,
using photo interrupter. If there is no paper, the lever
will cover its sensor and output "/P_EXIT"
6-30
+5V
Lever
Photo
Interrupter
6-4-3-6 +5V OUTPUT TERMINAL
Square wave voltage generated by Ns1 produces DC
voltage rectified and smoothed by half-wave rectifier.
This voltage put in to a three-wire regulator gener-
ates stable DC power of +5V.
6-4-3-7 +30V OUTPUT TERMINAL
Square wave voltage generated by Ns2 is rectified
and smoothed by half-wave rectifier and supplies
load with output voltage without separate
regulation.
6-4-3-8 +11.75V OUTPUT TERMINAL
Square wave voltage generated by Ns3 is rectified
and smoothed by half-wave rectifier, and then detect-
ed by voltage detector. Signals compared by error
amplifier are fed back to primary PWM
control 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.
will become "High". If there is paper, the lever will
be lifted and output "/P_EXIT" will become "Low".
+5V
/P_EXIT
Samsung Electronics