Mitsubishi Electric Diamond Pro 930SB, Руководство по обслуживанию

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7-2
1. Power Supply Circuit
1.1 Outline
This power supply unit adopts the switching mode technology, and is an off-line mode type unit that provided
several different DC outputs. The scanning frequency is available in different values ranging from 31 kHz to
110 kHz. Moreover, it is capable to operate at an AC input voltage of 100V ~ 240V and an AC frequency of
50-60 Hz
±
3Hz.
The block diagram is the functional construction schematics, that shows the major functions of this power
supply unit.
1.2 Harmonics
L131 is a harmonic choke circuit that reduces the harmonic peak currents, for the purpose of fulfilling the
requirement items of IEC 1000-3-2.
1.3 EMI
The EMI circuit has a 2-stage construction, with the first stage consisting of the common mode choke unit
and one X-capacitor, and the second stage consisting of the common mode choke unit and four
Y-capacitors.
R101 is the bleed resistor of the X-capacitor. When the power supply switch is turned OFF, this resistor
carries out the emergency charging of the capacitor C101.
The EMI is the circuit that prevents the monitor switching noise from being generated, there by minimizing
the negative influence on the other electronic equipment.
1.4 AC Rectifier and Smoothing Capacitor
The AC input is rectified by means of the full-bridge rectifier, that consists of the diodes D101 to D104.
The AC voltage is converted into the DC voltage by passing through the next stage, that consists of the
smoothing capacitor C107.
TH101 is a NTC thermistor for the power supply at the in-rush current limit.
1.5 Degaussing circuit
The degaussing circuit consists of the PTC thermistor TH102, the degaussing coil and the relay RL181.
The relay is controlled by means of the +14V control signal coming from the CPU.
1.6 Transformer and energy induction
1) When the PWM controls IC KA3842A chip, a driving pulse is generated at the gate of the transistor Q101,
and Q101 turns ON. The current returns from the “plus” (+) side of the energy-supplying capacitor C107 to
the “minus” (-) side of the same capacitor C107, passing through the transformer Q101 DS.
During the ON cycle, the energy is stored in the transformer T101. The transistor Q101 turns OFF when
the driving pulse disappears from Q101. As a result, all voltages of the dot ends of the winding flow to the
positive direction and reach the fly-back rectifier. At that point of time, the diodes of the rectifier of the
secondary side turn ON, a temporary energy is induced at the secondary side, and the ON cycle of the
driving pulse is repeated.