FAN4800CU Fairchild Semiconductor, FAN4800CU Datasheet - Page 16

• Manufacturer Part Number: FAN4800CU
• Description: The highly integrated FAN4800A/C and FAN4801/02/02L are specially designed for power supplies that consist of boost PFC and PWM
• Manufacturer: Fairchild Semiconductor

© 2011 Fairchild Semiconductor Corporation
FAN4800AU/CU • Rev. 1.0.0
AC Line Drops Out
FAN4800AU/CU is designed such that the operation of
PFC part is not perturbed by AC line dropout. Once line
voltage disappears, the error amplifier can be saturated,
resulting in abnormal current waveforms when the line
voltage is recovered if proper preventive measures are
not employed.
With a limited gain modulator operation, FAN4800AU
/CU guarantees stable PFC operation even when AC
line is recovered from dropout, as shown in Figure 36.
Line Sag Protection
When the line sags below its normal operational range,
the PFC part keeps operating until the brownout
protection is triggered, which has 1s debounce time.
Due to the low line voltage, the gain modulator for
current loop is saturated and input current of PFC is
limited, resulting in a drop of the PFC output voltage at
heavy-load condition. Since the PWM part has a V
comparator that shuts down PWM operation when the
FBPFC voltage drops below 1.3V, the downstream
DC/DC converter can stop operation while the PFC
output voltage drops during line sag. Once the
downstream converter stops operation, even the limited
PFC input current can charge up the PFC output since
the PFC part has no load current. Because this can
cause repeated startup and shutdown of downstream
converter during line sag, FAN4800AU/CU has line sag
protection.
There are two conditions that trigger line sag protection,
as shown in Figure 37 and Figure 38. The first condition
is when V
than t
condition is when V
and VF
Figure 38. Once line sag protection is triggered, the
PWM and the PFC stop operation until V
above 1.9V.
SAG
BPFC
(33ms), as shown in Figure 37. The second
RMS
is lower than V
Figure 36. AC Cycle Drop
is lower than V
RMS
is lower than V
IN-OFF
RMS-SAG
(1.3V), as shown in
(0.85V) for longer
RMS-SAG
RMS
increases
(0.85V)
IN
OK
16
Figure 38. The Second condition of Sag Protection
Figure 37. The First Condition of Sag Protection
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