Power Supply Board; Power Supply Operation - Epson Stylus COLOR 440 Service Manual

EPSON Stylus COLOR 440, 640, and 740

2.2.1 Power Supply Board

The Stylus COLOR 440, 640, and 740 use one of two power supply
boards: the PSB, which accepts 120 volts, or the PSE, which accepts
240 volts. The difference between them lies in their primary circuitry,
which includes the power line filter, the main switching transistor, and
the transformer. In the PSE, these components are designed to
withstand higher input voltages than in the PSB. Except for these
differences, the two boards are exactly the same.
The power supply provides filtered output at 5 and 42 VDC. These
voltages power the various printer functions shown in the table below.
Table 2-8. Power Output Applications
Voltage
42 VDC CR motor
PF motor
Printhead PZTs
5 VDC Logic circuitry
Control signals
Sensors
Control panel LEDs
Printhead nozzle selector IC
Built into the power supply is a delay circuit, which keeps the printer fully
powered for about 30 seconds after the printer is turned off. (This works
only when you turn the power off by pressing the Power switch on the
front control panel; the printer will not stay on if you unplug it.) The
delay gives the printer time to eject any paper that may be in the paper
path and to return the carriage to its home position. (The carriage must
return to its home position in order to cap the printhead. If the printhead
remains uncapped, ink dries out and clogs the printhead.)
Application

Power Supply Operation

When the printer is plugged in, electricity is always present in the
primary circuitry of the power supply. This means that certain
components are always "hot," including the power line filter circuit, the
main switching transistor, and the transformer. Note that the heat sink
on the main swtiching transistor (Q1) is not electrically isolated. Never
touch the primary components of the power supply with the printer
plugged in.
The secondary power supply circuitry, as well as the rest of the printer's
electrical components, are electrically isolated from the primary power-
line voltage. Isolation is accomplished by transformer T1 and photo-
coupler PC1.
The power supply consists of a line filter, a ZC-RCC (Zero-Cross
Ringing Choke Converter) switching circuit, and a 5-volt chopper
regulator IC. See Figure 2-16. When AC power enters the printer from
an external power source, the line filter blocks power-line transients
from entering the printer and prevents radio frequency interference
(high-level harmonics) generated by the switching circuit from being
placed back onto the power line. In the PSB board, the primary current
(at 120 VAC rms) then undergoes full-wave rectification and is
smoothed to produce 160 VDC. (In the PSE board, the 220 VAC input
is over 300 VDC after filtering.) This is fed through the RCC switching
circuit and a secondary smoothing circuit to produce a stepped-down 42
VDC; feedback ensures stable output. A second, additional output at 5
VDC is produced by a chopper regulator IC which draws from the 42-
volt supply.
At the heart of the switching circuit is a power MOSFET (Q1), an
efficient type of transistor that dissipates relatively little heat. By using a
transistorized switching circuit instead of a transformer to regulate
Chapter 2 Operating Principles
49