FAN6862_10 Datasheet, PDF (11/16 Pages) Fairchild Semiconductor – Highly Integrated Green-Mode PWM Controller

Protections
Self-protective functions include VDD Over-Voltage
Protection (OVP), Open-Loop / Overload Protection
(OLP), Over-Current Protection (OCP), Short-Circuit
Protection, and Over-Temperature Protection (OTP).
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 (8.5V), the protection is reset and the
operating current reduces to startup current, which
causes VDD to rise. FAN6862 resumes normal operation
when VDD reaches VDD-ON (16V). In this manner, the
auto-restart can alternately enable and disable the
switching of the MOSFET until the fault condition is
eliminated (see Figure 26).
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.
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 27, no current flows through the opto-coupler
transistor, which pulls up the feedback voltage to 5.2V.
When the feedback voltage is above 4.6V longer than
56ms, OLP is triggered. This protection is also triggered
when the SMPS output drops below the nominal value
longer than 56ms due to the overload condition.
Figure 27. OLP Operation
Figure 26. Auto Restart Operation
Over-Current Protection (OCP)
FAN6862 has over-current protection thresholds. It 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 (56ms).
VDD Over-Voltage Protection (OVP)
VDD over-voltage protection prevents IC damage caused
by over voltage on the VDD pin. The OVP is triggered
when VDD reaches 25V. A debounce time (typically
30µs) prevents false triggering by switching noise.
Over-Temperature Protection (OTP)
The OTP circuit is composed of current source and
voltage comparators. Typically, an NTC thermistor is
connected between the RT and GND pins. Once the
voltage of this pin drops below a threshold of 1.0V,
PWM output is disabled after tDOTP debounce time. If
this pin drops below 0.7V, it triggers the latch-off
protection immediately after tDOTP2 debounce time.
© 2009 Fairchild Semiconductor Corporation
FAN6862 • Rev. 1.0.2
11
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