Power Supply (Circuit Daigrams Main Pwb) - NEC MultiSync 20WGX2 Series Service Manual

7. Power Supply (Circuit daigrams MAIN PWB)

7.1 Line filter consists of C802, T802, C808, C805, C836, T803. It eliminates high frequency interference to
meet EMI's requirement.
7.2 Rec & Filter :
Bridge diode D802 converts AC source into pulsed DC. This pulsed DC is smoothed and filtered by
C801.
R801 is NTC ( negative thermal coefficient ) resistors, used to reduce inrush current to be within safe
range.
7.3 Power transformer :
T801 converts energy for square wave from power source C801 to secondary side to generate +24V.
T806 cibverts energy for square wave from power source C801 to secondary side to generate +5V
and +12V.
7.4 Output :
The square wave from T806 is rectified by D817 and D815, then filtered by C845, C857 and C848,
C842 generate +5V, +12V respectively.
The square wave from T801 is rectified by D805, then filtered by C811, C814 generate +24V
respectively.
7.5 Driver :
Q802 drive T806 from PWM control of I808 for power converted respectively.
Q801 drive T801 from PWM control of I801 for power converted respectively.
7.6 FB :
Negative feedback CKT consist of photo coupler I807 and adjustable regulator I806. It can maintain
output voltages +5V at a stable level.
7.7 PWM :
7.7.1 Start (5V): When power is turned on, Q802 conducts due to bias from C801 and R868. I808 starts to
oscillate and outputs a pulse train through pin 8 to drive Q802.
7.7.2 OPP (5V): When Q802 turns on, C801 supplies a linearly increasing triangle current through the
primary inductance of T806 to the driver Q802, once the peak value of this current multiplied by
R840 exceeds 0.85 volt, pulse train will be shut down immediately to protect Q802, T806 from being
burned out.
7.7.3 Regulation (5V): If output voltage +5V goes up, the R terminal of I808 gets more bias, accordingly
photo transistor and photo diode flows more current. The voltage of pin 2 goes down, making the
pulse width of pin 8 to become narrower. So the output voltage +5V will be pulled down to a stable
value.
7.7.4 SCP (5V): If output terminal is short to ground, photo transistor does not conduct, then oscillation of
I808 is stop, shuntting Q802 off immediately.
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