Epson Stylus PHOTO 1270 Service Manual page 44

EPSON Stylus Photo 870/1270
Figure 2-11 shows the block diagram for the C298PSB/PSE board.
Figure 2-11. C298PSB/PSE Board Block Diagram
OPERATING PRINCIPLES
The C298PSB/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 B
44