FA5500P/N, FA5501P/N
17
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winding. A current flows out of the clamp circuit and the
ZCD pin voltage is clamped to 0.6V (typ.). While
MOSFET is off, positive voltage is generated in the
auxiliary winding. A current flows into the clamp circuit
and the ZCD pin voltage is clamped to 5.7V (typ.).
Clamp
5
sub
R5
ZCD
ZCD.comp
0.6V
Q1 : ON
Clamp circuit
current
Fig.10
Clamp circuit of ZCD pin(1)
Clamp
5
sub
R5
ZCD
ZCD.comp
5.7V
Q1 : OFF
Clamp circuit
current
Fig.11
Clamp circuit of ZCD pin(2)
(7)Compensation circuit for light load
If the output of multiplier, which determines the
threshold of current comparator, does not have offset
voltage, the input current to the converter is
approximately zero under condition that the PFC
converter operates in no load. But an actual multiplier
may have offset voltage. If the offset voltage is positive,
the input current , which corresponds to the offset
voltage, flows into the converter even when the PFC
converter operates in no load. In this case, the PFC
output voltage rises abnormally because of too much
input current.
To avoid these, this IC has an automatic offset
correction circuit (AOC) for light load. The output
voltage of error amplifier is approximately 2V or higher
in normal operation. When the output voltage of error
amplifier drops below 2V, AOC circuit operates.
If the output of multiplier has a positive offset, the
output voltage of error amplifier falls below 2V in the
case that the PFC converter operates in no load or light
load. Then, the offset voltage is corrected in the current
comparator by AOC circuit. Because of this operation,
even in the no load or light load condition, the PFC
output voltage does not rise abnormally, but is always
kept constant. The amount of correction changes
linearly according to the output of error amplifier so that
the operation can be made stable.
Input current
full load
no load
AC
input
current
offset currnt
0
t
1/2 of line frequency
without compensation
result of compensattion
0
with compensation
AC
input
current
full load
no load
1/2 of line frequency
t
Output voltage
0
PFC
output
voltage
PFC output power
without compensation
with compensation
Fig.12
Outline of operation when multiplier has a
positive offset
(8)Restart timer
In steady state, set signals from the ZCD circuit turns
MOSFET on at each switching cycles. But a trigger
signal is needed when starting up or stabilizing
operation in the light load condition. This IC includes a
restart timer. If off period of output pulse continues
200s or more, it automatically generates a trigger
signal.