DM0565R データシートの表示（PDF） - Fairchild Semiconductor
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DM0565R Datasheet PDF : 20 Pages
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FSDM0565R
Vds Power
on
Fault
occurs
Fault
removed
VFB
6.0V
Over load protection
2.5V
Vcc
T12= Cfb*(6.0-2.5)/Idelay
12V
T1
T2 t
8V
Figure 7. Over load protection
t
Normal
operation
Fault
situation
Normal
operation
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 in order to protect the SMPS. However,
even when the SMPS is in the normal operation, the over
load protection circuit can be activated during the load
transition. In order 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 Sense FET 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.
3.2 Abnormal Over Current Protection (AOCP) : Even
though the FPSTM has OLP (Over Load Protection) and
current mode PWM feedback, these are not enough to protect
the FPSTM when a secondary side diode short or a
transformer pin short occurs. The FPSTM has an internal
AOCP (Abnormal Over Current Protection) circuit as shown
in figure 8. When the gate turn-on signal is applied to the
power Sense FET, the AOCP block is enabled and monitors
the current through the sensing resistor. The voltage across
the resistor is then compared with a preset AOCP level. If the
sensing resistor voltage is greater than the AOCP level for
longer than 300ns, the reset signal is applied to the latch,
resulting in the shutdown of SMPS.
2.5R
R
OSC
PWM
SQ
RQ
LEB
AOCP
Gate
driver
Rsense
Vaocp
2
GND
Figure 8. AOCP block
3.3 Over voltage Protection (OVP) : If the secondary side
feedback circuit were to 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
12
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