FSL156MRBN Datasheet, PDF (10/16 Pages) Fairchild Semiconductor – Green-Mode Fairchild Power Switch (FPS™)

4. Protection Circuits: The FSL156MRBN has several
self-protective functions, such as Overload Protection
(OLP), Abnormal Over-Current Protection (AOCP),
Output-Short Protection (OSP), Over-Voltage Protection
(OVP), and Thermal Shutdown (TSD). All the
protections are implemented as auto-restart. Once the
fault condition is detected, switching is terminated and
the SenseFET remains off. This causes V_CC to fall.
When
V_CC B
B
falls
to
the
Under-Voltage
Lockout
(UVLO)
stop voltage of 7.5V, the protection is reset and the
startup circuit charges the V_CC capacitor. When V_CC
reaches the start voltage of 12.0V, normal operation
resumes. If the fault condition is not removed, the
SenseFET remains off and V_CC drops to 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.
Power
VDS
on
Fault
occurs
Fault
removed
VCC
12.0V
7.5V
t
Normal
operation
Fault
situation
Normal
operation
Figure 19. Auto-Restart Protection Waveforms
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_OUT) decreases
below the set voltage. This reduces the current
through the opto-coupler LED, which also reduces the
opto-coupler transistor current, increasing the
feedback voltage (V_FB). If V_FB exceeds 2.5V, D1 is
blocked and the 2.0µA current source starts to charge
C_FB slowly up_. In this condition, V_FB 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_FB
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.
Figure 20. Overload Protection
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 minimum turn-on time. Even though the
FSL156MRBN has overload protection, it is not
enough to protect 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
© 2012 Fairchild Semiconductor Corporation
FSL156MRBN • Rev. 1.0.0
10
www.fairchildsemi.com