Main Switching Circuit; Circuit Operation - Epson LQ-1050 Technical Manual

The voltage across C6 increases after the power is applied, and the current is accordingly stabilized.
Until the voltage across C6 becomes stable, the surge current is limited by a resistor in series with
capacitor C6. When 120 VAC is applied, the voltage across C6 increases to about 160 VDC. In this
After C6 is fully charged, resistor R1 is shorted to keep its temperature low. The circuit includes TRIAC
TY 1 for this purpose, which shorts T1 to T2 by passing current
power is applied to coil 14-12 of transformer T1 so that a voltage is induced in coil 9-10. When this
voltage reaches 1.5 V,

2.2.4 Main Switching Circuit

This circuit uses a ringing choke converter (RCC) AC input switching power circuit. This system has
the merits of using fewer parts and a smaller transformer, and is often used when a smaller power supply
is required.

2.2.4.1 Circuit Operation

Figure 2-12 shows the main switching circuit. When the power is applied, drive current Is flows to the
base of switching transistor Q 1 via starting resistor R 14. Diodes D20, D21, D22 on the secondary side
of T1 prevent current flow in the secondary side. Therefore, as shown in Figure 2-13, collector current
from Q1 reaches the same level of current which flows in the coils, and starts from O A so that a small
base current can switch Q 1 on. Once Q 1 is turned on, the primary side of transformer
an input voltage which induces voltages in windings Tll~ and
respectively).
and eB releases base current
as shown in the following formula:
Therefore, when collector current
and DC
amplification rate hm can be expressed by the fOrmUla, hFE X iB
and hFE is insufficient because of carrier saturation so Q 1 is turned off. As a result, the voltage at
and Tll~ drops and base current runs out, and Q 1 is quickly switched off.
When Q 1 is switched off, the back electromotive force which has the opposite polarity from the previous
momentarily flows through the windings of the transformer. Since the amount of energy
to that stored in the transformer before Q 1 is turned off, P is:
L , "
SURGE CURRENT$
INPUT VOLTAGE
Fig. 2-11. Surge Current
flows to switch on TY 1.)
in the direction which causes the gate of the TRIAC (Refer to 2.2.3.) to conduct,
in the direction which causes Q 1 to remain on. The value of
increases as shown in Figure 2-14, the relationship between
L, (~ 2
ton)
I
A
POWER ON
V
I N
R 4 + R59(1 + hFE)
2 - 1 3
to the gate. (When C6 is charged,
(represented by eB and eG
= where
of Coil14-,2
Peak current of
receives
is constant
is fixed
equal
P [W] is