Power Supply Circuit Operation - Epson DLQ-3000 Service Manual

2.2.2 Power Supply Circuit Operation

The AC power source is connected to the inlet, and voltage is provided to the power supply board
via the power switch and fuse. The AC voltage is full-wave rectified using diode bridge DB1 and
smoothed.
The surge cut circuit reduces the rush current at power-on.
For the +35V block, the voltage is applied with AC/DC conversion between the primary and
secondary side via coil T101 and T201. The amount of voltage and current output to the secondary
coil can be adjusted by controlling the gate voltage of the main switching circuit MOS FET's (Q101
and Q201), i.e. by switching it on/off. Control is fed back from each protection circuit and control
circuit to Q101 and Q201.
Voltage Line Overcurrent Protection Circuit (Primary Side)
IC 101 and IC 201 detect the input voltage of the primary circuit. When the input voltage is normal,
the current dose not flow into the shunt-regulator. But the overcurrent flows to the input voltage
line, the shunt-regulator is turned on, Q103 (Q203) is turned on, Q102 (Q202) is turned on, and then
the Q101 (Q201) is turned off.
If the +35VDC line drops down to +13VDC, Q153 (Q253) and Q154 are turned on by detector
circuit that consists of R173 (R273) and R174 (R274), Q154 is turned on, PC102 is turned on, and
then the input voltage cuts off.
At this time, the delay timing creation circuit that consists of the C157 and R174 makes the delay
timing.
When the printer turns on, this protection circuit can not be started by this arcuit, because if this
protection circuit is operated before the +5VDC line raises up, the drivers of the BOARD ASSY.,
C124 MAIN are not active.
The +5V line is supplied with voltage and stabilized by the regulator IC TL494, based on the
stabilized output through the +35V block. Therefore, all of the controlling and stabilizing circuits
for current and voltage are located inside the TL494 (with the circuit constants determined by
external elements). There is no feedback to the primary side as there is for the +35V line.
The only feedback to the primary side is for circuit protection. When the +5V output exceeds the
max. overvoltage level (i.e. 7V), PC102 cuts power to the primary side using Zener diode ZD153
and shortcircuits the gates of the switching FET Q101 or Q201 in order to prevent abnormal
operation of the mechanism.
AC LINE
FUSE
EMI Filter
Rectifier
DB101
Surge Cut
TYI 01
Figure 2-9. Power Circuit Block Diagram
Main Half-wave
Rectifier
Circuit
[PC102 t
Main Half-wave
Switching Rectifier
Circuit Smoothing
Operating Principles
. (CH. B)
CONSTANT
VOLTAGE
CIRCUIT
. (CH. A )
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