FSB117H Fairchild Semiconductor, FSB117H Datasheet - Page 14

• Manufacturer Part Number: FSB117H
• Manufacturer: Fairchild Semiconductor

© 2011 Fairchild Semiconductor Corporation
FSB117H / FSB127H / FSB147H • Rev. 1.0.4
Protections
The FSB-series provides protection function, that
include Overload / Open-Loop Protection (OLP), Over-
Voltage Protection (OVP), and Over-Temperature
Protection (OTP). All the protections are implemented
as Auto-Restart Mode. Once the fault condition is
detected, switching is terminated and the SenseFET
remains off. This causes V
6V, the protection is reset and HV startup circuit
charges V
Open-Loop / Overload Protection (OLP)
Because of the pulse-by-pulse current-limit capability,
the maximum peak current through the SenseFET is
limited and maximum input power is limited. If the
output consumes more than the limited maximum
power, the output voltage (V
voltage. Then the current through the opto-coupler LED
and the transistor become virtually zero and FB voltage
is pulled HIGH as shown in Figure 27. If feedback
voltage is above 4.6V for longer than 56ms, OLP is
triggered. This protection is also triggered when the
feedback loop is open due to a soldering defect.
.
V
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
virtually zero. Then feedback voltage 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 triggers. Because more
energy than required is provided to the output, the
output voltage may exceed the rated voltage before the
overload protection triggers, resulting in the breakdown
of the devices in the secondary side. To prevent this
situation, an OVP circuit is employed. Since V
DD
Over-Voltage Protection (OVP)
Figure 26. Burst-Mode Operation
DD
up to 12V voltage, allowing re-startup.
Figure 27. OLP Operation
DD
to fall. When V
O
) drops below the set
DD
DD
voltage
falls to
14
is proportional to the output voltage by the transformer
coupling, the over voltage of output is indirectly detected
using V
voltage reaches 28V. Debounce time (typically 150µs) is
applied to prevent false triggering by switching noise.
Over-Temperature Protection (OTP)
The SenseFET and the control IC are integrated in one
package. This makes it easy for the control IC to detect
the abnormal over temperature of the SenseFET. If the
temperature exceeds approximately 140°C, the OTP is
triggered and the MOSFET remains off. When the
junction
temperature, the FSB-series resumes normal operation.
Two-Level UVLO
Since all the protections of the FSB-series are auto-
restart, the power supply repeats shutdown and re-
startup until the fault condition is removed. FSB-series
has two-level UVLO, which is enabled when protection is
triggered, to delay the re-startup by slowing down the
discharge of V
power of the power supply during the fault condition,
minimizing the voltage/current stress of the switching
devices. Figure 28 shows the normal UVLO operation
and two-step UVLO operation. When V
without triggering the protection, PWM stops switching
and V
Meanwhile, when the protection is triggered, FSB-series
has a different V
is triggered, the IC stops switching and V
V
small and V
naturally discharged down to 6V, the protection is reset
and V
V
DD
DD
reaches 12V, the IC resumes switching operation.
drops to 9V, the operating current becomes very
DD
DD
DD
is charged up by the HV startup circuit. Once
temperature
is charged up by the HV startup circuit.
voltage. The OVP is triggered when V
Figure 28. Two-Level UVLO
DD
DD
DD
is slowly discharged. When V
. This effectively reduces the input
discharge profile. Once the protection
drops
by
25°C
DD
DD
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drops. When
drops to 6V
from
DD
OTP
DD
is