NEC MultiSync FE700 Serviceanleitung Seite 122

2) The power supply MOS FET Q101 carries out the ON/OFF operation of the control unit, by means of
U101 KA3842A. KA3842A is a PWM (pulse width modulation) IC chip, with 16 V starting voltage and
10 V cut-off voltage.
The following list shows the pin layout of KA3842A pulse width modulation IC chip.
Pin 1:
Pin 3:
Pin 5:
Pin 7:
3) Over current protection
R111 is a sensor resistor, and it has the function of increasing the current of this loop when the output
of the secondary side is either in the overloaded state or is insufficient.
Since the current passing through the R111 sensor resistor has voltage dropping effect, the operation
of the output pulse is stopped when a voltage lower than 1 Volt is detected at the pin number 3 of the
KA3842A 3chip, and the switch of the power supply MOS FET is kept in the "break" state until the
VCC voltage is charged up to 16 Volts, and the operation of U101 KA3842A is resumed. When it is
not clearly known whether there is secondaly voltage's shortage or not, this circuit repeats the
ON/OFF switching, and the power supply LED lights up
4) Starting circuit
The resistor R123 and R148 and the transistor Q112 and diode D119 and resistor R131, R149 and
zener diode ZD105 are for the starting operation. When the circuit starts its operation, the power
supply transformer T101 supplies the auxiliary 12 Volt power to the control IC chip U101 via pins 6
and 7 of the winding transformer T101.
5) Synchronization circuit
The synchronization signal is induced from the fly-back transformer (FBT), and carries out the
synchronization with the power supply frequency. The frequency range is from 31 kHz to 70 kHz, and
the component elements of the synchronization circuit are C114, R116, D109, R115 and R117.
6) Feedback circuit
The feedback circuit loop induces the 12 V voltage through the pin 6 and the pin 7 of the power supply
transformer. That voltage is connected with the pin 3 of the IC chip U101 by passing through D108,
C113 and passing next through R122, VR101 and R120. This is a regular loop.
7) Snubber circuit
The snubber circuit has the function of clamping the ON/OFF spikes of the power supply MOS-FET,
and its component elements D105, C106, R106 make up a snubber that turns OFF the power supply
MOS-FET.
8) Secondary rectifier and smoothing rectifier
The secondary rectifier is a harmonic rectifier consisting of D111, C124 and R128, and it works as a
snubber circuit as well. The capacitors C120 and C138 are the smoothing rectifier working on the 45
Volt DC output. There are also other DC outputs, such as 80 Volt (D110), 13.6 Volt (D112) and 8.8 Volt
(D113).
Feedback
Current sensor
Ground (GND)
VCC
8 - 3
Pin 2: Compensation
Pin 4: Oscillator
Pin 6: Pulse output
Pin 8: VREF (5.1 V)
.