Mitsubishi Electric Diamond Pro 930SB, Руководство по обслуживанию

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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: Feedback Pin 2: Compensation
Pin 3: Current sensor Pin 4: Oscillator
Pin 5: Ground (GND) Pin 6: Pulse output
Pin 7: VCC Pin 8: VREF (5.1 V)
3) Overcurrent protection
R120 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 R120 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 voltage shortage or not, however, this circuit repeats the ON/OFF switching, and
the power supply LED lights up.
4) Starting circuit
The resistor R102 and R103 and the transistor Q102 and diode D105 and resistor R104, R105 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 choke (L501), and carries out the synchronization with the
power supply frequency. The frequency range is from 31 kHz to 110 kHz, and the component elements of
the synchronization circuit are D116, R125, D115, R108 and R118.
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 T101 - pin1,
R107, D110, C112, R114, R116, VR101, U101 - pin2 and passing next through 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 D118, C109, R122 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 D151. The capacitors C151 and C157 are the
smoothing rectifier working on the 80 Volt DC output. There are also other DC outputs, such as 80 Volt
(D151), 27 Volt (D152), 14 Volt (D153), 7 Volt (D155) and -14 Volt (D154).