Line Output Voltage Limiting - Epson LQ-1050 Technical Manual

REV.-A
2.2.4.3 +35 V Line Output Voltage Limiting
The back electromotive described in 2.2.4.1 causes diode D20 on the secondary side of transformer
T1 to conduct, which supplies power. Consequently, the amount of energy stored by the transformer
during a unit~l~riod of time becomes equal to the power output. That is:
t o n )
—
(
The output voltage is detected by R22 and R23. In an AC input switching power circuit, the AC line
and switching circuit are not isolated, but the transformer isolates the DC output (secondary side).
Therefore, photo-coupler PC 1 is used to feedback the fluctuating output voltage to the switching circuit
while still isolating it. Refer to Figure 2-15.
This circuit employs a programmable shunt regulator TL431 (Q20) as a constant voltage element to
keep the output voltage (Vo) at 35 V. When Vo goes up, cathode current
the photodiode receives
photo-transistor side which becomes a constant voltage signal for Q4, and then the current from TII-ls
to the base of Q 1 is shorted to the O V line to turn off switching transistor Q 1. Consequently, the period
of time
that VIN is applied to winding
in winding T1-s on the secondary side is reduced.
The switching circuit includes C26 between the cathode terminal and gate terminal for phase correction
(of the amplifier in theTL431, in this case), and prevents an inductance element from entering the feed
back loop and making it oscillate,
=
2
" f Vo " 10
more current. This action produces an increase in the collector current in the
Fig. 2-15. Constant Voltage Generation Circuit
f:
switching frequency of Q 1
ton: on-time of Q 1
on the primary side is cut down, and the voltage induced
020
,
Pc I
2 - 1 6
V o
Vref (l+% I
Vo=