Power Circuit - Xerox WorkCentre XK50cx Service Manual

Circuit

4-5-3 Power Circuit

INPUT
This product is equipped with a three-wire
grounding-type power inlet. Insulation is class 1.
When a power device part breaks down or some
trouble in this circuit causes a short, excess current
runs through a fuse and blows it to protect power
devices and the system from excess current and
prevent 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
conducted or radiated through power cords.
AC input voltage is rectified by bridge diode of full
wave rectifier, is converted to DC voltage by
smoothing 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 Circuit
adjusts the duty of the switching device by sensing
the change of output voltage, and protects this
product from excess current by receiving signals
from excess current detecting circuit.
DRIVING CIRCUIT (START-UP CIRCUIT)
If you insert a power plug into AC outlet, full-wave
rectified DC output voltage is generated at input
electrolysis 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
circuit with power supplied by auxiliary coil Nb.
4-28
SWITCHING CIRCUIT (VOLTAGE
STABILIZATION)
12V of 2nd voltage is detected by voltage detector
and feeds back through photo coupler to 1st PWM
Control Circuit signals compared by error amplifier.
PWM Control Circuit controls the gate of power
transistor according to the feedback voltage so as to
supply load with stable output voltage.
OCP CIRCUIT (OVER CURRENT
PROTECTIVE CIRCUIT)
If a short in secondary load terminal or a short inside
power device causes excess current, power
consumption 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.
OVP CIRCUIT (OVER VOLTAGE
PROTECTIVE CIRCUIT)
If the main power of power device 12V increases
abnormally, this excess voltage is detected by OVP
detecting circuit and 12V output terminal shifts to a
state similar to a short.
If 12V output terminal shorts, over current runs on
over current detecting terminal and OCP circuit
operates to prevent a fire in power device.