FSDM0465RB Ver la hoja de datos (PDF) - Fairchild Semiconductor
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FSDM0465RB Datasheet PDF : 18 Pages
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FSDM0465RB
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Figure 6. Auto Restart Operation
3.1 Over Load Protection (OLP): Overload is defined as
the load current exceeding a pre-set level due to an
unexpected event. In this situation, the protection circuit
should be activated to protect the SMPS.
However, even when the SMPS is operation normally, the
over load protection circuit can be activated during the load
transition. To avoid this undesired operation, the over load
protection circuit is designed to be activated after a specified
time to determine whether it is a transient situation or an
overload situation.
Because of the pulse-by-pulse current limit capability, the
maximum peak current through the SenseFET is limited, and
therefore the maximum input power is restricted with a given
input voltage. If the output consumes beyond this maximum
power, the output voltage (Vo) decreases below the set
voltage. This reduces the current through the opto-coupler
LED, which also reduces the opto-coupler transistor current,
thus increasing the feedback voltage (Vfb).
If Vfb exceeds 2.5V, D1 is blocked and the 3.5uA current
source starts to charge CB slowly up to Vcc.
In this condition, Vfb continues increasing until it reaches
6V, when the switching operation is terminated as shown in
Figure 7. The delay time for shutdown is the time required to
charge CB from 2.5V to 6.0V with 3.5uA.
In general, a 10 ~ 50 ms delay time is typical for most
applications.
2.5V
T12= Cfb*(6.0-2.5)/Idelay
T1
Figure 7. Over Load Protection
T2 t
3.2 Over Voltage Protection (OVP): If the secondary side
feedback circuit malfunction or a solder defect caused an
open in the feedback path, the current through the opto-
coupler transistor becomes almost zero. Then, Vfb climbs up
in a similar manner to the over load situation, forcing the
preset maximum current to be supplied to the SMPS until the
over load protection is activated. Because more energy than
required is provided to the output, the output voltage may
exceed the rated voltage before the over load protection is
activated, resulting in the breakdown of the devices in the
secondary side. To prevent this situation, an OVP circuit is
employed. In general, Vcc is proportional to the output
voltage and the FSDM0465RB uses Vcc instead of directly
monitoring the output voltage. If VCC exceeds 19V, an OVP
circuit is activated resulting in the termination of the
switching operation. To avoid undesired activation of OVP
during normal operation, Vcc should be designed to be
below 19V.
3.3 Thermal Shutdown (TSD): The SenseFET and the
control IC are built in one package. This makes it easy for
the control IC to detect the heat generation from the Sense
FET. When the temperature exceeds approximately 150°C,
the thermal shutdown is activated.
4. Soft Start: The FSDM0465RB’s internal soft-start circuit
slowly increases the PWM comparator’s inverting input
voltage along with the SenseFET current after it starts up.
The typical soft-start time is 10msec, The pulse width to the
power switching device is progressively increased to
establish the correct working conditions for transformers,
inductors, and capacitors. The voltage on the output
capacitors is progressively increased with the intention of
smoothly establishing the required output voltage. It also
helps to prevent transformer saturation and reduce the stress
on the secondary diode during startup.
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