Service manual

5-32 Samsung Electronics
5-3-3-4 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 transistor
gate falls immediately and it stops driving power transistor to protect it from excess current.
5-3-3-5 OVP CIRCUIT (OVER VOLTAGE PROTECTIVE CIRCUIT)
If the main power of power device 11.75V increases abnormally, this excess voltage is detected by OVP detect-
ing 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 oper-
ates to prevent a fire in power device.
5-3-3-6 +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 generates stable DC power of +5V.
5-3-3-7 +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.
5-3-3-8 +11.75V OUTPUT TERMINAL
Square wave voltage generated by Ns3 is rectified and smoothed by half-wave rectifier, and then detected 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 supply
load with stable output voltage.