FAN6863RTY Datasheet, PDF (12/16 Pages) Fairchild Semiconductor – Highly Integrated Green-Mode PWM Controller

Protections
Self-protective functions include V_DD Over-Voltage
Protection (OVP), Open-Loop / Overload Protection
(OLP), Over-Current Protection (OCP), Short-Circuit
Protection (SCP), SENSE pin Short-Circuit Protection
(SSCP), and Over-Temperature Protection (OTP). OLP,
OCP, SCP, and SSCP are auto-restart mode
protections; 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_DD to fall because no more power is delivered from
auxiliary winding. When V_DD falls to V_DD-OFF (7V), the
protection is reset and the operating current reduces to
startup current, which causes V_DD to rise. FAN6863
resumes normal operation when V_DD reaches V_DD-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 28).
Open-Loop / Overload Protection (OLP)
When the upper branch of the voltage divider for the
shunt regulator (KA431 shown in Figure 29) is broken,
no current flows through the photo-coupler transistor,
which pulls up the feedback voltage to 5.2V.
When feedback voltage is above 4.6V for longer than
60ms, OLP is triggered. This protection is also triggered
when the SMPS output drops below the nominal value
for longer than 60ms due to the overload condition.
If the secondary output-short situation occurs when the
feedback voltage is above 4.6V, protection time is 7ms
for shorter debounce time.
Figure 28. Auto Restart Operation
Latch-Mode Protection
Once this protection is triggered, switching is terminated
and the MOSFET remains off. The latch is reset only
when V_DD is discharged below 4V by unplugging the
AC power line.
Over-Current Protection (OCP)
FAN6863 has two 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 (60ms).
Figure 29. OLP Operation
V_DD Over-Voltage Protection (OVP)
V_DD over-voltage protection prevents IC damage caused
by over voltage on the VDD pin. The OVP is triggered
when V_DD reaches 22.5V. A debounce time (typically
50µs) 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. If the voltage
of this pin drops below a threshold of 1.0V, PWM output
is disabled after t_DOTP debounce time. If this pin drops
below 0.7V, it triggers the latch-off protection
immediately after t_DOTP2 debounce time.
© 2011 Fairchild Semiconductor Corporation
FAN6863 • Rev. 1.0.1
12
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