FSD176MRT Fairchild Semiconductor, FSD176MRT Datasheet - Page 10

• Manufacturer Part Number: FSD176MRT
• Description: The FSD176MRT is an integrated Pulse Width Modulation (PWM) controller and SenseFET specifically designed for offline Switch-Mode Power Supplies (SMPS) with minimal external components
• Manufacturer: Fairchild Semiconductor

© 2011 Fairchild Semiconductor Corporation
FSD176MRT • Rev. 1.0.0
4. Protection Circuits: The FSD176MRT 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 a
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 FSD176MRT
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.
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, even when the SMPS is in normal
operation, the overload protection circuit can be
triggered during load transition. To avoid this
undesired operation, the overload 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
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
blocked and the 2.0µA current source starts to charge
C
Figure 19. Auto-Restart Protection Waveforms
FB
slowly up
CC
B
and
B
falls to the Under-Voltage Lockout (UVLO)
Thermal
.
In this condition, V
FB
). If V
Shutdown
CC
FB
capacitor. When V
exceeds 2.5V, D1 is
(TSD).
OUT
FB
) decreases
CC
CC
continues
drops to
All
to fall.
the
CC
10
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
from 2.5V to 7.0V with 2.0µA. A 25 ~ 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. Overload protection
is not enough to protect the FSD176MRT in that
abnormal case; since severe current stress is
imposed on the SenseFET until OLP is triggered. The
FSD176MRT 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
secondary
rectifier
diodes
www.fairchildsemi.com
or
the
FB