Epson Stylus Photo785EPX, Service Manual

Page: 46 / 192

EPSON Stylus PHOTO 895/785EPX/915/825 Revision B
Operating Principles Electrical Circuit Operating Principles 46
2.2.1.1 Process how 42VDC, 5VDC and 3.3VDC are generated
This section provides the description how +42VDC, +5VDC and +3.3VDC are
generated from the supplied power supply referring to the Figure 2-18, "C408 PSE
Board Block Diagram".
1. Regardless of the state of the power switch (On or OFF), the 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. At this time, F1 plays a role of preventing
AC100V from coming into the F1.
L1 also prevents high harmonic wave noise generated in the RC circuit filter
which consists of C1 from going out, and eliminates the noise from outside here.
2. The AC is full-wave rectified by the diode bridge DB1, and converted to x
AC in voltage by the smoothing electrolytic capacitor C11.
3. The pressured up direct current turns Q1 on through the starting resistor and starts
the primary side of the circuit.
4. When the primary side is On, the energy (current) led by the electromagnetic
induction through the trans (T1) does not flow to the secondary side since the
diode (D51) on the secondary side is installed in the opposite direction.
5. When the energy which is charged in the trans is reaching the saturated state, the
voltage which makes Q1 on becomes weak gradually. At the point that this voltage
drops at the certain voltage, C13 absorbs the current in the opposite direction and
Q1 is quickly shut off by the resulting sharp drop.
6. When the primary side is turned off, the energy charged in the T1 is opened
according to the diode (D51) direction which is installed on the secondary side.
Basically, 42 V DC is output by these circuit operations and the number of T1
spiral coil.
7. +5VDC and +3.3VDC are generated by pressured down this +42VDC as power
supply. The 5V chopper circuit (IC51) and 3.3V chopper circuit (IC151) pressures
down the +42VDC and generates p5VDC and +3.3VDC by chopping off
the output, forming the standard sawtooth wave form by the outer RC integration
circuit.
2.2.1.2 Protection circuit
The C408 PSE board has the various control circuits to stop voltage output if a
malfunction occurs on the power supply board or the main board or while the printer
mechanism is on duty. Following explains each control and protection circuit.
†
+42VDC line constant voltage control circuit:
The output of +42V line is monitored by the ZC-RCC converter detection circuit.
This circuit prevents voltage drop so that the output voltage can be held at a
constant level.
†
+42V line over current protection circuit:
The output voltage extremely drops when the +42V line becomes the over current
condition. The ZC-RCC converter feeds back the occurrence of over voltage to the
primary side via the same IC if it detects the voltage drop in the 42V line. The
primary side stops switching and protects the printer electric circuit and
mechanism. Reset is automatically performed.
†
+42V line over voltage protection circuit:
If the +42V line exceeds 58V, the ZC-RCC converter detects the over voltage and
feeds back the occurrence of over voltage to the primary side via the same IC. The
primary side stops switching and protects the printer electric circuit and
mechanism.
†
+5VDC, +3.3VDC line constant voltage/current control circuit:
+The +5V and +3.3V line output currents and output voltage are monitored by the
self-excited down chopper circuit (IC51, IC151). The monitor information is input
to the internal comparator and stabilization circuit in IC. The self-excited down
chopper circuit (IC51, IC151) stops the output if the over current is detected.
†
+5V, +3.3V line over current protection circuit:
The ZC-RCC converter feeds back the occurrence of over voltage to the primary
side via the same IC when the +5V or +3.3V line becomes the over current
condition. The primary side stops switching and protects the printer electric circuit
and mechanism. It is possible to reset only by turning on the power again.
†
+5V, +3.3V line over voltage protection circuit:
If the +42V line exceeds 9V, the ZC-RCC converter detects the over voltage and
feeds back the occurrence of over voltage to the primary side via the same IC. The
primary side stops switching and protects the printer electric circuit and
mechanism.
2