FSD156MRBN Fairchild Semiconductor, FSD156MRBN Datasheet - Page 10

• Manufacturer Part Number: FSD156MRBN
• Description: The FSD156MRBN 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
FSD156MRBN • Rev. 1.0.0
4. Protection Circuits: The FSD156MRBN 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 FSD156MRBN
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, 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 the 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
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
transistor
.
In this condition, V
FB
). If V
Shutdown
current,
CC
FB
capacitor. When V
exceeds 2.5V, D1 is
(TSD).
increasing
OUT
FB
) decreases
CC
CC
continues
drops to
All
to fall.
the
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. Even though the
FSD156MRBN has overload protection, it is not
enough to protect the FSD156MRBN in that abnormal
case; due to the severe current stress imposed on the
SenseFET until OLP is triggered. The 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
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or
the
FB