DM0465RS Ver la hoja de datos (PDF) - Fairchild Semiconductor
Número de pieza
componentes Descripción
Fabricante
DM0465RS Datasheet PDF : 19 Pages
| |||
disable the switching of the power SenseFET until the
fault condition is eliminated (see Figure 19).
VFB
Vds Power
on
Fault
occurs
Fault
removed
6.0V
FSDM04565RS Rev: 00
Overload protection
2.5V
T12= Cfb*(6.0-2.5)/Idelay
Vcc
T1
T2 t
12V
Figure 20. Overload Protection
8V
Normal
FSDM0465RS Rev: 00 operation
Fault
situation
t
Normal
operation
Figure 19. Auto Restart Operation
3.1 Overload 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 in the normal operation, the overload
protection circuit can be activated during the load
transition. To avoid this undesired operation, the
overload 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 reduces the opto-coupler
transistor current, thus increasing the feedback voltage
(VFB). If VFB exceeds 2.5V, D1 is blocked and the 3.5µA
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 20. The delay time for shutdown is the time
required to charge CB from 2.5V to 6.0V with 3.5µA. In
general, a 10 ~ 50ms delay time is typical for most
applications.
3.2 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
overload situation, forcing the preset maximum current
to be supplied to the SMPS until the overload protection
is activated. Because more energy than required is
provided to the output, the output voltage may exceed
the rated voltage before the overload protection is
activated, resulting in the breakdown of the devices in
the secondary side. To prevent this situation, an over-
voltage protection (OVP) circuit is employed. In general,
VCC is proportional to the output voltage and the
FSDM0465RS 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
SenseFET. When the temperature exceeds
approximately 150°C, the thermal shutdown is activated.
4.2 Abnormal Over-Current Protection (AOCP): When
the secondary rectifier diodes or the transformer pins are
shorted, a steep current with extremely high di/dt can
flow through the SenseFET during the LEB time. Even
though the FPS has overload protection, it is not enough
to protect the FPS in those abnormal cases, since
severe current stress is imposed on the SenseFET until
OLP triggers. This IC has an internal AOCP circuit
shown in Figure 21. When the gate turn-on signal is
applied to the power SenseFET, the AOCP block is
enabled and monitors the current through the sensing
resistor. The voltage across the resistor is compared with
a preset AOCP level. If the sensing resistor voltage is
© 2008 Fairchild Semiconductor Corporation
FSDM0465RS Rev. 1.0.1
12
www.fairchildsemi.com
Share Link: 