Xerox Phaser 3450 Service Manual page 142

11.2.3.3 Circuit
1) AC Input and DC
AC input unit is composed of FUSE, VARISTOR, NOISE
CAPACITOR and LINE FILTER. DC unit is composed of
BRIDGE DIODE and ELECTRIC CAPACITOR. The
FUSE(F1) has specification of 125V/8A for 110V and
250V/5A for 220V. FUSE contains the function to prevent
damage due to SMPS fire in the event of malfunction of 1
st. PWM Control and parts causing excess voltage.
NOISE CAPACITOR is EMI counter part, which is
composed of CAPACITOR(C1, C14) for the normal mode
noise and CAPACITOR(C3, C4, C9, C34) for common
mode noise. Larger the capacity of the CAPACITOR,
greater the NOISE absorption effect. But, adequate
capacitor should be carefully selected since leakage
current increases with the increased capacity in capacitor,
which has high potential of electric shock. The Line Filter
connected to the Capacitor has the EMI Noise eliminating
function. Especially, the filter and the capacitor not only
absorbs the noise being transferred out of the printer
through power line, it also absorbs the opposite noise
coming from the exterior.
2) SWITCHING Control Section
SMPS is driven by OSC. It has uniform switching
frequency and the range of 'On Time' Pulse may vary
depending on the surrounding condition. Basic operation
procedure is as following.
Turn on the AC power. AC input voltage will pass through
the Noise Filter circuit, Bridge Diode(BD1) and capacitor
(C6). The rectified DC voltage supplies the bias voltage to
the pin6 of PWM contract IC IC1 (2BS01) through R2 ad
R3. Then, turn on the Gate in the FET. At this point the
current passed through FET will flow through the 1st
trance coil T1. Pin #4 and the supporting current in the
trance #7 and 8 will be abandoned.
The current flowing through the 1st trance coil Q1(IIN60)
will rapidly increase current in #2 Drain and this process
will block the abandoned current in the 2nd trance coil of
the transformer to flow out. Followed by this operation,
due to the surrounding condition change, Drain current
reaches the critical rigging point of Q1(hfe). This is caused
by the relative decrease in Drain current compared to
time. This indicates that Q1 is unable to maintain On
status. Such condition is the area of Insufficient Gate
Current.
According to the FET characteristics. The drain voltage of
Q1 operate in switching models which are composed of
turn on state and turn off state. When Q1 is turned off, the
created electrical power in 1st trans (#2~#4) will be
transfered to the 2nd trans. (#13, 14, 15, 9~#10, 11, 12)
The current then flows into PC1(PHOTO COUPLER) to
bypass the IC1 feedback. Then, U1 turns on. Through
such repetitious process, the current flowing through 2nd
trance coil is filtered to create necessary DC voltage. DC
output and feedback circuit are created by +24V of D5
and C19. +5V is created by abandoned voltage from the
2nd. trance coil due to D5 and C19. In order to maintain
uniform DC output level, detect the output level from R11
and R12 to maintain uniform level, then enabled feedback
by disconnecting the Shunt Regular IC(IC2) and used
PHOTO COUPLER (PC1).
3) Fixed Temperature Control
To control the fixed lamp, the control signal of "FUSER
ON" in Logic section and the supply of DC voltage in the
SMPS section is necessary. The circuit turns on when the
"FUSER ON" sends the operation signal and the DC
power is supplied. The operation principle of control circuit
of fixed lamp is as following. Trigger current flows to Triac
Driver IC6 LED due to "FUSER ON" of Logic. Then the
Photo detector of IC6 will detect the infrared ray and the
Triac of IC5 will activate. The current will then send the
Trigger input to the Gate of Triac IC5.
At this point IC5 will be activated and AC power will be
supplied to the fixed lamp, then the lamp will turn on and
the temperature will rise. Because the fixed lamp circuit
uses AC voltage as it's voltage supply, bidirectional Triac
(IC5) is applied, since it has advantage in price, size and
reliability compared to unidirectional SCR.
The Gate terminal of Triac can be triggered as ordered or
opposite direction signal. Once the triac turns on, it cannot
be controlled by the Gate signal. It maintains until the level
of current flowing in the main terminal falls below the level
of maintaining current. In other words, it cannot be turned
off by counter current as that of SCR. Such characteristics
is categorized as Critical Rigging Time Rate ( Commutation
: dv/dt). In the application of electric power exchange,
Triac either turns off the activation or switches twice in
each period at Zero Crossing. Such switching movement
is called Commutation.
It is able to turn off the triac at the end of the half period
by deleting the Gate signal. It is possible when the level
of (IL) current is below the level of maintaining current.
When the Triac commutation is Off, the voltage will
instantly change to opposite direction and the VAC will
increase to the maximum level. At this time, the range of
increase rate is decided by dv/dt and the Overshoot
voltage is decided by the circuit. Also, the voltage of Triac
will have same level of voltage when the commutation of
Triac is Off.
CIRCUIT DESCRIPTION
Service Manual
11-11