FAN6861 Datasheet, PDF (11/15 Pages) Fairchild Semiconductor – Low-Cost, Highly Integrated, Green-Mode PWM Controller for Peak Power Management

Protections
Self-protective functions include VDD Over-Voltage
Protection (OVP), Open-Loop/Overload Protection
(OLP), Over-Current Protection (OCP), Over-
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 VDD to fall because no more
power is delivered from auxiliary winding. When VDD
falls to VDD-OFF (9.5V), the protection is reset and the
operating current reduces to startup current, which
causes VDD to rise. FAN6861 resumes normal operation
when VDD reaches VDD-ON (17.5V). In this manner, the
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 VDD is
discharged below 4V by unplugging AC power line.
Vds
Power
On
Protection Triggers
Fault Removed
VDD
17.5V
9.5V
Operating Current
3mA
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.
Figure 22. Two-Level OCP Operation
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.
8uA
Normal
Operation
Fault
Situation
Normal
Operation
Figure 21. Auto Restart Operation
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
VFB
MOSFET gate when the sensing voltage of MOSFET
drain current is above the threshold longer than the
shutdown delay time (780ms).
This two-level OCP protection is designed for
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
© 2009 Fairchild Semiconductor Corporation
FAN6861 • Rev. 1.0.1
11
5.2V
VFB-OLP (4.6V)
OLP Shutdown Delay Time
OLP Triggers
Figure 23. OLP Operation
www.fairchildsemi.com