FAN6862WLTY 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) and Over-Temperature Protection
(OTP). OLP, OCP and SCP 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 (7 V), the
protection is reset and the operating current reduces to
startup current, which causes V_DD to rise. FAN6862W
resumes normal operation when V_DD reaches V_DD-ON
(16 V). 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 5.4 V.
When feedback voltage is above 4.6 V for longer than
60 ms, OLP is triggered. This protection is also triggered
when the SMPS output drops below the nominal value
for longer than 60 ms due to the overload condition.
If the secondary output-short situation occurs when the
feedback voltage is above 4.6 V, protection time is 7 ms
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 4 V by unplugging the
AC power line.
Over-Current Protection (OCP)
FAN6862W over-current protection is a pulse-by-pulse
bias current limit threshold (V_LIMIT) that turns off the
MOSFET once the sensing voltage of MOSFET drain
current reaches the threshold. The V_LIMIT compensates
the power-limit variation over the universal input range
and adaptively keeps the power limit constant.
Figure 29. OLP Operation
V_DD Over-Voltage Protection (OVP)
V_DD over-voltage protection prevents IC damage caused
by over voltage on the V_DD pin. The OVP is triggered
when V_DD reaches 22.2 V. A debounce time (typically
50 µs) 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 1.0 V, PWM output
is disabled after t_DOTP debounce time. If this pin voltage
drops below 0.7 V, it triggers the latch-off protection
immediately after t_DOTP2 debounce time.
© 2012 Fairchild Semiconductor Corporation
FAN6862W • Rev. 1.0.0
12
www.fairchildsemi.com