Daewoo DTF-2950-100D, Service Manual, Page: 68 / 92

CP-850FX Service Manual
Europe R&D 67
5.10.1.4 Latch Circuit
The latch circuit is a circuit that holds the oscillator output low and stops the power supply circuit
operation when OVP or OLP circuit operates. The holding current of the latch circuit is 150µA
MAX (Ta = 25C) when the Vcc terminal voltage is minus 0.3V to the operation stop.
In order to avoid improper operations caused by noises, etc., the delay-time is provided with a
timer circuit incorporated in the HIC, and thereafter, the latch circuit starts its operation when
OVP or OLP circuit operates for more than the set time. While, the Vcc terminal voltage drops
even after the latch circuit starts its operation because the constant voltage (Reg) circuit of the
control circuit continues its operation with higher circuit current.
Where the Vcc terminal voltage falls lower than the operation stop voltage (9.6V(TYP)), the
voltage starts rising as the circuit current becomes lower than 150µA (Ta = 25C). Where the Vcc
terminal voltage reaches the operation start voltage (18.2V(TYP)), it falls as the circuit current is
increased again. Consequently, the latch circuit prevents the
Vcc terminal voltage from rising abnormally by controlling the
voltage between 9.6V (TYP) and 18.2V(TYP). The Fig.6
indicates the voltage waveform when the latch circuit is under
operation. The latch circuit operation is cancelled by reducing
the Vcc terminal voltage below 7.3V (TYP), and generally, it is
restarted by AC input switch-off of the power supply.
5.10.2 SS/OLP Terminal (Pin 5)
The operation of SS/OLP terminal is classified as Soft-Start
and Overload Protection, and the SS/OLP terminal is generally
connected to a condenser having the value of 0.47µF to 3.3µF.
5.10.2.1 Soft-Start Operation at Start-up of Power Supply
At the power supply start-up, an external
condenser is charged up to the threshold
operating charging voltage (VSSOLP(SS)) by
the Soft-Start operating charging current
(ISSOLP(SS)) flowing from SS/OLP. The
Soft- Start is provided at power supply start-up
by utilizing the changing of SS/OLP terminal
voltage from 0V to 1.0V. The timing chart of
the Soft-Start is shown in Fig.7. Comparing
the oscillation waveforms between OLP
terminal voltage and the oscillation waveform
of the internal control part, the Soft-Start widen
the ON width. Besides, at the burst stand-by,
the Soft-Start is operated every time; so, the
magnetostriction noises from transformers are
controlled with the increase of the drain current
gradually.
Fig.6. V
CC
Terminal Vol. Waveform
at Latch-mode
Fig.7. Soft-Start Operation
Power MOSFET
Waveform
I
SS/OLP
Normal StartUp
SoftStart
OCP
Limit
V
SSOLP(SS)
V
SS/OLP
I
SSOLP(SS)
I
SSOLP(OLP)
I
SSOLP(NOR)
Ｖ
CC
Ｔｉｍｅ
18.2V
（
TYP
）
9.6V
（
TYP
）
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