FAN6863R Fairchild Semiconductor, FAN6863R Datasheet - Page 12

• Manufacturer Part Number: FAN6863R
• Description: A highly integrated PWM controller, FAN6863 provides several features to enhance the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2011 Fairchild Semiconductor Corporation
FAN6863 • Rev. 1.0.1
Protections
Self-protective functions include V
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,
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
auxiliary winding. When V
protection is reset and the operating current reduces to
startup current, which causes V
resumes normal operation when V
(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).
Latch-Mode Protection
Once this protection is triggered, switching is terminated
and the MOSFET remains off. The latch is reset only
when V
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).
DD
to fall because no more power is delivered from
SCP,
DD
Figure 28. Auto Restart Operation
is discharged below 4V by unplugging the
and
SSCP
DD
are
falls to V
DD
auto-restart
DD
to rise. FAN6863
DD
DD-OFF
reaches V
Over-Voltage
(7V), the
mode
DD-ON
12
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.
V
V
by over voltage on the VDD pin. The OVP is triggered
when V
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
below
immediately after t
DD
DD
Over-Voltage Protection (OVP)
over-voltage protection prevents IC damage caused
DD
0.7V,
reaches 22.5V. A debounce time (typically
Figure 29. OLP Operation
it
DOTP
DOTP2
triggers
debounce time. If this pin drops
debounce time.
the
latch-off
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protection