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After the control circuit starts its operation, power is
supplied by smoothing and rectifying the voltage of the
drive winding D which is provided in the transformer.
An example of the waveform of the V
voltage is shown in Fig. 3.
The drive winding voltage does not increase to the
set voltage immediately after the control circuit starts
its operation.
Although the V
terminal voltage starts decreasing,
IN
the drive winding voltage reaches the set value
before the V
terminal voltage drops to the shutdown
IN
voltage which is set at such a low voltage as 10V.
Consequently, the control circuit can continue its
operation.
The turn number of the drive winding must be
determined in a manner that the C2 voltage can
be above the shutdown voltage [V
and below the OVP operating voltage [V
20.5V
].
(MIN)
However, in the actual power supply circuit, the
V
terminal voltage may vary subject to the secon-
IN
dary side output current, as shown in Fig. 4.
This is because peak charging of C2 is performed
by surge voltage generated instantaneously at
MOS FET due to small circuit current of the IC.
In this case, by adding a resistor with several ohm
to several tens ohm in series with the rectifier diode,
this problem can be eased.
As to the optimal resistance value of the additional
resistor, as the variation ratio of the V
voltage is not same due to the construction of the
transformer used, it must be determined in accor-
dance with the transformer actually used.
The variation ratio of the V
output voltage becomes worse in proportion to the
worsening of the primary-secondary coupling of the
transformer and the coupling between the drive
winding and output stabilizing winding(winding for
a circuit regulating the constant voltage).
In designing the transformer, the above should be
taken into account.
terminal
IN
11V
IN(OFF)
(MAX)
IN(OVP)
terminal
IN
terminal voltage to the
IN
26
Fig. 3 Waveform of V
voltage at start-up
]
Fig. 4 Output current Iout - V
terminal voltage
Fig. 5 Stabilized back-up power supply
circuit against output current Iout
terminal
IN
IN
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