FSL206MR Fairchild Semiconductor, FSL206MR Datasheet - Page 10

• Manufacturer Part Number: FSL206MR
• Description: The FSL206MR integrated Pulse Width Modulator (PWM) and SenseFET is specifically designed for highperformance offline Switch Mode Power Supplies (SMPS) with minimal external components
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FSL206MR • Rev. 1.0.2
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 LED, which also reduces the
opto-coupler transistor current, thus increasing the
feedback voltage (V
feedback input diode is blocked and the 2.7µA current
source (I
condition, V
switching operation is terminated, as shown in Figure
19. The shutdown delay time is the time required to
charge C
Figure 18.
Figure 19.
FB
DELAY
from 2.4V to 5V with 2.7µA current source.
FB
) starts to charge C
increases until it reaches 5V, when the
Auto-Restart Protection Waveforms
Overload Protection (OLP)
FB
). If V
FB
FB
exceeds 2.4V, the
slowly up. In this
O
) decreases
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 20. 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 the control IC are integrated,
making it easier to detect the temperature of the
SenseFET. When the junction temperature exceeds
approximately 135°C, thermal shutdown is activated and
the FPS is restarted after decreasing temperature
reaches 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 17).
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
designed to be below 24.5V.
Figure 20.
FB
climbs up in a similar manner to the overload
Abnormal Over-Current Protection
CC
CC
CC
is proportional to the
CC
should be properly
instead of directly
exceeds 24.5V,
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