FAN6861 Fairchild Semiconductor, FAN6861 Datasheet - Page 11

• Manufacturer Part Number: FAN6861
• Description: A highly integrated PWM controller, FAN6861 provides several features to enhance the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN6861 • Rev. 1.0.1
Protections
Self-protective functions include V
Protection
(OLP),
Temperature Protection (OTP). Among them, OLP,
OCP, and SCP are auto-restart mode protections; while
OVP and OTP are latch-mode protections.
Auto-Restart Mode Protection: Once a fault condition
is detected, switching is terminated and the MOSFET
remains off. This causes V
power is delivered from auxiliary winding. When V
falls to V
operating current reduces to startup current, which
causes V
when V
auto-restart can alternately enable and disable the
switching of the MOSFET until the fault condition is
eliminated (see Figure 21).
Latch-Mode Protection: Once this protection is
triggered, switching is terminated and the MOSFET
remains off. The latch is reset only when V
discharged below 4V by unplugging AC power line.
Two-Level Over-Current Protection (OCP)
FAN6861 has two levels of over-current protection
thresholds. One is for pulse-by-pulse current limit,
which turns off MOSFET for the remainder of the
switching cycle when the sensing voltage of MOSFET
drain current reaches the threshold. The other threshold
is for the over-current protection, which shuts down the
MOSFET gate when the sensing voltage of MOSFET
drain current is above the threshold longer than the
shutdown delay time (780ms).
This
applications with peak load characteristics, such as
printers and scanners.
These applications have motor load and inherently
impose over-load condition on the power supply during
17.5V
9.5V
3mA
8uA
Vds
V
DD
Operating Current
two-level
DD
DD
DD-OFF
Figure 21. Auto Restart Operation
Over-Current
Power
reaches V
On
to rise. FAN6861 resumes normal operation
(OVP),
Operation
(9.5V), the protection is reset and the
Normal
OCP
Protection Triggers
DD-ON
Open-Loop/Overload
protection
Protection
Situation
(17.5V). In this manner, the
DD
Fault
to fall because no more
is
DD
(OCP),
Fault Removed
Operation
Normal
designed
Over-Voltage
Protection
Over-
DD
for
DD
is
11
acceleration mode. Therefore, the protection circuit
should be triggered after a specified time to determine
whether it is a transient situation or an abnormal
situation.
Open-Loop / Over-Load Protection (OLP)
When the upper branch of the voltage divider for the
shunt regulator (KA431 shown) is broken, as shown in
Figure 23, there is no current flowing through the opto-
coupler transistor, which pulls up the feedback voltage
to 5.2V.
When the feedback voltage is above 4.6V longer than
780ms, OLP is triggered. This protection is also
triggered when the SMPS output drops below the
nominal value longer than 780ms due to the overload
condition.
V
FB
Figure 22. Two-Level OCP Operation
Figure 23. OLP Operation
OLP Shutdown Delay Time
V
OLP Triggers
FB-OLP
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5.2V
(4.6V)