Epson Stylus PHOTO 890 Service Manual page 45

EPSON Stylus Photo 890/1280/1290
Figure 2-12 shows the block diagram for the C378PSB/PSE board.
Figure 2-12. C378PSB/PSE Board Block Diagram
OPERATING PRINCIPLES
The C378PSB/PSE Board produces the +42VDC and +5VDC using AC power as
described below:
1. Regardless of the power switch's on/off condition, voltage is always applied to the
primary side of the power supply board from the moment or at the state that AC-
plug is plugged in. F1, a fuse, prevents AC100V from flowing into the circuit. A
power thermistor TH1 also protects the circuit from rush current after power-on.
The filter circuit composed of L1, C1, and C2 prevents high harmonic wave noise
generated in the switching circuit from going out, and eliminates the noise from
outside.
2) The AC voltage is full-wave rectified by the diode bridge DB1 and smoothed by
C11.
3) Switching FET Q1 turns on via starting resistors R18 and R28 that are located on
the AC side to begin switching operation. By arranging the starting resistors on the
AC side, half waves of the AC voltage are only applied, and power used for this
operation is reduced compared with usual serial layout.
4) When the primary side is on, because the diode (D51) on the secondary side is
installed in the reverse direction, energy (current) led by the electromagnetic
induction through the trans (T1) does not flow to the secondary side.
5) When the energy charged in the transformer reaches a saturated state, the voltage
which keeps Q1 on becomes weak gradually. At the point this voltage drops to a
certain level, C13 absorbs the reverse current and Q1 quickly shuts off.
6) When the primary side is turned off, the energy charged in T1 is opened according
to the diode(D51) direction on the secondary side. +42 V DC is output by these
circuit operations and the number of T1 spiral coil.
7) +5VDC is generated out of this +42VDC. Forming reference sawteeth waveforms
with an external RC integrating circuit, IC51 produces stable +5VDC with a
chopping circuit.
Electrical Circuit Operating Principles
Revision C
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