FSGM300N View Datasheet(PDF) - Fairchild Semiconductor
Part Name
Description
View to exact match
FSGM300N Datasheet PDF : 16 Pages
| |||
4.3. Output-Short Protection (OSP): If the output is
shorted, steep current with extremely high di/dt can
flow through the SenseFET during the minimum turn-
on time. Such a steep current brings high-voltage
stress on the drain of the SenseFET when turned off.
To protect the device from this abnormal condition,
OSP is included. It is comprised of detecting VFB and
SenseFET turn-on time. When the VFB is higher than
1.6V and the SenseFET turn-on time is lower than
1.0μs, the FSGM300N recognizes this condition as an
abnormal error and shuts down PWM switching until
VCC reaches VSTART again. An abnormal condition
output short is shown in Figure 22.
5. Soft Burst-Mode Operation: To minimize power
dissipation in standby mode, the FSGM300N enters
burst-mode operation. As the load decreases, the
feedback voltage decreases. As shown in Figure 23, the
device automatically enters burst mode when the
feedback voltage drops below VBURL (500mV). At this
point, 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
VBURH (700mV), switching resumes. The feedback
voltage then falls and the process repeats. Burst-mode
operation alternately enables and disables switching of
the SenseFET, thereby reducing switching loss in
standby mode.
Figure 22. Output-Short Protection
4.4 Over-Voltage Protection (OVP): If the
secondary-side feedback circuit malfunctions or a
solder defect causes an opening in the feedback path,
the current through the opto-coupler transistor
becomes almost zero. Then VFB climbs up in a similar
manner to the overload situation, forcing the preset
maximum current to be supplied to the SMPS until the
overload protection is triggered. Because more energy
than required is provided to the output, the output
voltage may exceed the rated voltage before the
overload protection is triggered, resulting in the
breakdown of the devices in the secondary side. To
prevent this situation, an OVP circuit is employed. In
general, the VCC is proportional to the output voltage
and the FSGM300N uses VCC instead of directly
monitoring the output voltage. If VCC exceeds 24.0V,
an OVP circuit is triggered, resulting in the termination
of the switching operation. To avoid undesired
activation of OVP during normal operation, VCC should
be designed to be below 24.0V.
4.5 Thermal Shutdown (TSD): The SenseFET and
the control IC on a die in one package makes it easier
for the control IC to detect the over temperature of the
SenseFET. If the temperature exceeds ~135°C, the
thermal shutdown is triggered and stops operation.
The FSGM300N operates in auto-restart mode until
the temperature decreases to around 95°C, when
normal operation resumes.
Figure 23. Burst-Mode Operation
6. Random Frequency Fluctuation (RFF): Fluctuating
switching frequency of an SMPS can reduce EMI by
spreading the energy over a wide frequency range. The
amount of EMI reduction is directly related to the
switching frequency variation, which is limited internally.
The switching frequency is determined randomly by
external feedback voltage and internal free-running
oscillator at every switching instant. This Random
Frequency Fluctuation scatters the EMI noise around
typical switching frequency (67kHz) effectively and can
reduce the cost of the input filter included to meet the
EMI requirements (e.g. EN55022).
© 2009 Fairchild Semiconductor Corporation
FSGM300N • Rev. 1.0.3
Figure 24. Random Frequency Fluctuation
www.fairchildsemi.com
11
Share Link: 