FSL206MRBN Fairchild Semiconductor, FSL206MRBN Datasheet - Page 10

• Manufacturer Part Number: FSL206MRBN
• Description: The FSL206MRBN integrated Pulse-Width Modulator (PWM) and SenseFET is specifically designed for high-performance offline Switched-Mode Power Supplies (SMPS) with minimal external components
• Manufacturer: Fairchild Semiconductor

© 2011 Fairchild Semiconductor Corporation
FSL206MRBN • Rev. 1.0.1
Overload Protection (OLP)
Overload is defined as the load current exceeding a pre-
set level due to an unexpected event. In this situation,
the protection circuit should be activated to protect the
SMPS. However, even when the SMPS is operating
normally, the overload protection (OLP) circuit can be
activated during the load transition or startup. To avoid
this undesired operation, the OLP circuit is activated
after a specified time to determine whether it is a
transient situation or a true overload situation. The
Current-Mode feedback path limits the current in the
SenseFET when the maximum PWM duty cycle is
attained. If the output consumes more than this
maximum power, the output voltage (V
below its rating voltage. This reduces the current
through the opto-coupler diode, which also reduces the
opto-coupler transistor current, increasing the feedback
voltage (V
diode is blocked and the 2.7µA current source (I
starts to charge C
increases until it reaches 5V, when the switching
operation is terminated, as shown in Figure 40. The
shutdown delay is the time required to charge C
2.4V to 5V with 2.7µA current source.
Figure 39. Auto-Restart Protection Waveforms
Figure 40. Overload Protection (OLP)
FB
). If V
FB
FB
exceeds 2.4V, the feedback input
slowly up. In this condition, V
O
) decreases
FB
DELAY
from
FB
)
10
Abnormal Over-Current Protection (AOCP)
When the secondary rectifier diodes or the transformer
pin are shorted, a steep current with extremely high di/dt
can flow through the SenseFET during the LEB time.
Even though the FPS has overload protection, it is not
enough to protect the FPS in that abnormal case, since
severe current stress is imposed on the SenseFET until
OLP triggers. The FPS includes the internal AOCP
(Abnormal Over-Current Protection) circuit shown in
Figure 41. When the gate turn-on signal is applied to the
power sense, 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 latch,
resulting in the shutdown of the SMPS.
Thermal Shutdown (TSD)
The SenseFET and control IC being integrated makes it
easier to detect the temperature of the SenseFET.
When the junction temperature exceeds 135°C, thermal
shutdown is activated and the FPS is restarted after
temperature decreases to 60°C.
Over-Voltage Protection (OVP)
In the event of a malfunction in the secondary-side
feedback circuit or an open feedback loop caused by a
soldering defect, the current through the opto-coupler
transistor becomes almost zero (refer to Figure 38).
Then V
situation, forcing the preset maximum current to be
supplied to the SMPS until the overload protection is
activated. Because excess energy is provided to the
output, the output voltage may exceed the rated voltage
before the overload protection is activated, resulting in
the breakdown of the devices in the secondary side. To
prevent this situation, an over-voltage protection (OVP)
circuit is employed. In general, V
output voltage and the FPS uses V
monitoring the output voltage. If V
OVP circuit is activated, resulting in termination of the
switching operation. To avoid undesired activation of
OVP during normal operation, V
to be below 24.5V.
Figure 41. Abnormal Over-Current Protection
FB
climbs up in a similar manner to the overload
CC
CC
CC
is proportional to the
should be designed
CC
instead of directly
exceeds 24.5V,
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