FSD146MRBN Fairchild Semiconductor, FSD146MRBN Datasheet - Page 10

• Manufacturer Part Number: FSD146MRBN
• Description: The FSD146MRBN is an integrated Pulse Width Modulation (PWM) controller and SenseFET designed for offline Switch-Mode Power Supplies (SMPS) with minimal external components
• Manufacturer: Fairchild Semiconductor

© 2011 Fairchild Semiconductor Corporation
FSD146MRBN • Rev. 1.0.0
4. Protection Circuits: The FSD146MRBN has several
self-protective functions, such as Overload Protection
(OLP), Abnormal Over-Current Protection (AOCP),
Output-Short Protection (OSP), Over-Voltage Protection
(OVP),
protections are implemented as auto-restart. Once the
fault condition is detected, switching is terminated and
the SenseFET remains off. This causes V
When V
stop voltage of 7.5V, the protection is reset and the
startup circuit charges the V
reaches the start voltage of 12.0V, the FSD146MRBN
resumes normal operation. If the fault condition is not
removed, the SenseFET remains off and V
stop voltage again. In this manner, the auto-restart can
alternately enable and disable the switching of the
power SenseFET until the fault condition is eliminated.
Because these protection circuits are fully integrated
into the IC without external components, the reliability is
improved without increasing cost.
12.0V
4.1 Overload Protection (OLP): Overload is defined
as the load current exceeding its normal level due to
an unexpected abnormal event. In this situation, the
protection circuit should trigger to protect the SMPS.
However, in normal operation, the overload protection
circuit can be triggered during the load transition. To
avoid
protection circuit is designed to trigger only after a
specified time to determine whether it is a transient
situation or a true 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 more
than this maximum power, the output voltage (V
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 (V
2.5V, D1 is blocked and the 2.0µA current source
starts to charge C
7.5V
V
V
Figure 19. Auto-Restart Protection Waveforms
DS
CC
CC
B
and
Power
this
B
on
falls to the Under-Voltage Lockout (UVLO)
Thermal
operation
undesired
Normal
FB
occurs
Fault
slowly up
Shutdown
situation
operation,
Fault
CC
.
capacitor. When V
removed
In this condition, V
Fault
FB
(TSD).
). If V
the
operation
CC
FB
Normal
CC
overload
drops to
exceeds
All
to fall.
OUT
t
the
CC
FB
)
10
continues increasing until it reaches 7.0V, when the
switching operation is terminated, as shown in Figure
20. The delay for shutdown is the time required to
charge C
50ms delay is typical for most applications. This
protection is implemented in auto-restart mode.
4.2 Abnormal Over-Current Protection (AOCP):
When
transformer pins are shorted, a steep current with
extremely high di/dt can flow through the SenseFET
during the minimum turn-on time. Even though the
FSD146MRBN has overload protection, it is not
enough to protect the FSD146MRBN in that abnormal
case; since severe current stress is imposed on the
SenseFET until OLP is triggered. The FSD146MRBN
internal AOCP circuit is 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 greater than the AOCP
level, the set signal is applied to the S-R latch,
resulting in the shutdown of the SMPS.
Figure 21. Abnormal Over-Current Protection
the
Figure 20. Overload Protection
FB
from 2.5V to 7.0V with 2.0µA. A 25 ~
secondary
rectifier
diodes
www.fairchildsemi.com
or
the