Epson LQ-2550 Technical Manual page 76

The starting sequence is described below.
When the printer power is turned on, DC input voltage V
2. Drive current Is flows to the base of switching transistor Q1 via starting resistor R4, and Q1 turns
on.
3. At the same time as step 2, V 1
Tl, voltage VIA is induced in coil T1 M of TI at the primary side, and positive bias current ]B flows.
Therefore, Q1 turns on quickly.
4. Since the valUe of current IB is constant as shown in the expression below, current IL applied to coil
T1 m-s increases continuously in proportion to time.
R5 + R2 (1 + hFE)
where, Voltage drop across D2
Voltage drop between the base and emitter of Q 1
At this time, voltages V1.S, V1O+12, and V1O-12 are induced in coils T13-5, T1 3-4, and T1 2-3 at the
secondary side. Since the current flows in the reverse direction to diodes D7, D8, and D9, no voltage
is output to the circuits at the secondary side.
5.
Therefore, voltage V 1 IN across coil T110.E drops. At the same time, a reverse voltage is applied to
coil T1 7-6, the value of IB lowers, and current flows in the reverse direction via R 1 and D 1. Since the
potential at point @ becomes higher than that at point@, speed-up capacitor C 11 absorbs current
which flows from point @ to @. Therefore, Q 1 turns off quickly.
6. When Q 1 turns off, the energy induced in coils T1
4 is released in the reverse direction and the forward current flows to diodes D7, D8, and D9 at the
secondary side. In this way, voltage is applied to the secondary side of the circuit.
7. The energy being released decreases linearly with time. When the energy release is completed, the
voltage at each coil of T1 reaches zero momentarily, but switching current
direction again due to R5, and Q1 switches on again. The potential at point@ becomes higher than
that at point @, and the energy accumulated in C 11 is released to keep
8. The operating sequence then returns to step 3. The circuit continues switching by repeating this
sequence.
The above described sequence is generally known as a self-excited ringing choke converter (R. C. C.)
system.
When Q1 turns off in step 5, reverse voltage is induced at point @ momentarily, but D4, R7, and C 13
(snubber circuit) act as a limiter.
applied to coil T1
is
IN
3.5,
2-29
is input to this circuit.
IN
at the primary side of pulse transformer
and T1 z-a at the secondary side in step
3-4,
flowing to Q1.
is limited.
flows in the forward