Epson LQ-2550 Technical Manual page 84

The relationships between the error amplifier
and the dead-time control voltage are shown
High
Deed time control voltage
Low
(+< –)
The on-time of TR 1 and TR2 becomes short when the outputs of EA1 and
becomes long when the outputs are inverted. The maximum on-time, however, is limited to 45% of one
cycle by the dead-time control.
Current Limiting
Figure 2-30 shows the +35 V current limiting circuit. Current limiting is performed mainly by error
amplifier EA2.
Reference voltage V,,nw which is obtained by dividing the +35 V by R36 and R37 is supplied to the
minus
side of EA2 via resistor R35.
V o l t a g e which is proportional to current 1035 flowing to current detection resistor RO (
As the load at 1035 increases. When the condition shown above is met, the output of EA2 changes from
LOW to HIGH. When the output of EA2 is HIGH, TR 1 and TR2 turn off, and Q3 in main switching circuit
2 turns off.
In this way, the voltage induced in the secondary side of T3 is dropped.
Voltage Regulation
Figure 2-30 shows the +35
amplifier EA 1.
R e f e r e n c e v o l t a g e
V R E F (+5 V) output from IC2 (pin 14) via resistor R27 is supplied to the minus side
of EA 1.
V o l t a g e which is obtained by dividing the +35 V by R33 and R34 {see the expression below) is supplied
to the plus side of EA 1.
outputs, sawtooth
in Figure 2-31.
Sawtooth
w a v e f o r m
1
I
.
1
Figure 2-31. PWM Comparison Sequence
V voltage regulator circuit.
=
waveform output from the oscillator,
T
ON
Voltage regulation is performed mainly by error
2-37
(max. )
One cycle
=