FSL136MRT Ver la hoja de datos (PDF) - Fairchild Semiconductor
Número de pieza
componentes Descripción
Fabricante
FSL136MRT Datasheet PDF : 13 Pages
| |||
4. Protection Circuits: The FSL136MRT has several
self-protective functions, such as Overload Protection
(OLP), Abnormal Over-Current Protection (AOCP),
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
VCC
to
fall.
When
VCC B
B
falls
to
the
Under-Voltage
Lockout
(UVLO) stop voltage of 7.5V, the protection is reset and
the startup circuit charges the VCC capacitor. When VCC
reaches the start voltage of 12.0V, the FSL136MRT
resumes normal operation. If the fault condition is not
removed, the SenseFET remains off and VCC 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.
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 CFB
from 2.4V to 7.0V with 2.0µA. This protection is
implemented in Auto-Restart Mode.
Figure 20. Overload Protection
Figure 19. Auto-Restart Protection Waveforms
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
FSL136MRT has overload protection, it is not enough
to protect the FSL136MRT 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.
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 (VOUT) 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 (VFB). If VFB exceeds 2.4V, D1 is
blocked and the 2.0µA current source starts to charge
CFB slowly up. In this condition, VFB continues
© 2012 Fairchild Semiconductor Corporation
FSL136MRT • Rev. 1.0.0
10
Figure 21. Abnormal Over-Current Protection
www.fairchildsemi.com
Share Link: 