FSFM300 Fairchild Semiconductor, FSFM300 Datasheet - Page 12

• Manufacturer Part Number: FSFM300
• Description: The FSFM260N/261N/300N is an integrated Pulse Width Modulator (PWM) and SenseFET specifically designed for high-performance offline Switch Mode Power Supplies (SMPS) with minimal external components
• Manufacturer: Fairchild Semiconductor

FSFM260N / FSFM261N / FSFM300N • Rev. 1.0.1
© 2009 Fairchild Semiconductor Corporation
3.2 Over-Voltage Protection (OVP): If the secondary
side feedback circuit malfunctions or a solder defect
causes an open in the feedback path, the current
through the opto-coupler transistor becomes almost
zero. Then, V
overload situation, forcing the preset maximum current
to be supplied to the SMPS until the overload protection
is activated. Because more energy than required 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
FSFM260/261/300
monitoring the output voltage. If V
OVP circuit is activated, terminating the switching
operation. To avoid undesired activation of OVP during
normal operation, V
3.3 Thermal Shutdown (TSD): The SenseFET and the
control IC are built in one package. This allows for the
control IC to detect the heat generation from the
SenseFET.
approximately 140°C, the thermal shutdown is activated.
3.4 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 LEB time. Even
though the FPS has overload protection, it is not enough
to protect the FPS in those abnormal cases, since
severe current stress is imposed on the SenseFET until
OLP triggers. This IC has an internal AOCP circuit
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 latch, resulting in the shutdown of the SMPS.
CC
Figure 21. Abnormal Over-Current Protection
is proportional to the output voltage and the
2.5R
AOCP protection
FB
R
When
climbs up in a similar manner to the
CC
uses
OSC
should be designed below 19V.
the
LEB
V
S
R
CC
Q
Q
temperature
instead
CC
exceeds 19V, an
driver
Gate
V
FSFM260 Rev: 00
OCP
of
exceeds
directly
1
GND
12
4. Soft-Start: The FSFM260/261/300 has an internal
soft- start circuit that increases PWM comparator
inverting input voltage, together with the SenseFET
current, slowly after it starts up. The typical soft-start time
is 15ms. The pulse width to the power switching device
is progressively increased to establish the correct
working conditions for transformers, inductors, and
capacitors. The voltage on the output capacitors is
progressively increased to smoothly establish the
required output voltage. It also helps prevent transformer
saturation and reduce the stress on the secondary diode
during startup.
5. Burst Operation: To minimize power dissipation in
standby mode, the FSFM260/261/300 enters burst mode
operation. As the load decreases, the feedback voltage
decreases. As shown in Figure 22, the device
automatically enters burst mode when the feedback
voltage drops below V
switching stops and the output voltages start to drop at a
rate dependent on standby current load. This causes the
feedback voltage to rise. Once it passes V
switching resumes. The feedback voltage then falls and
the process repeats. Burst mode operation alternately
enables and disables switching of the power SenseFET,
thereby reducing switching loss in standby mode.
FSFM260 Rev: 00
Vds
V
0.35V
Vo
0.5V
Ids
Vo
FB
set
Figure 22. Waveforms of Burst Operation
T1
Switching
disabled
BURL
T2 T3
(350mV). At this point,
Switching
disabled
BURH
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T4
(500mV)
time