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FA5500AP/AN, FA5501AP/AN
19
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11. Design advice
(1)Designing a PFC converter
The following description is a sample of designing of
a PFC converter with FA5500A/FA5501A using a circuit
shown in Fig. 13. However, this is just a sample of
calculating. If you want to use the components or
circuits calculated in this process, be sure to test and
determine in actual circuit. In addition, be sure to
consider and check the characteristics, the tolerance
and the rating of each component including this IC.
(1-1)
Specification of PFC converter
To begin designing, the following specification of PFC
converter is determined.
Input voltage range (Vrms):
Vac(min.) to Vac(max.)
Output voltage (V):
Vo (> 2 ×Vac (max.) )
Maximum Output power(W):
Po
Output voltage (Vo) should be set higher than the
peak value of input voltage (= 2 ×Vac (max.)) because
the PFC converter is a boost type topology.
(1-2)
Designing inductance of L1
The switching frequencies are determined with
input-output conditions and the value of inductor
because PFC converter operates in critical conduction
mode (see Supplement). Therefore, the value of
inductor L1 (Lp) can be determined with input-output
conditions and the minimum operating frequency.
When efficiency of PFC is
η and the minimum
operating frequency is fsw (min.), Lp is calculated by
following equation.
Lp=
Vo
Po
fsw
2
)
Vac
2
(Vo
Vac
.)
(min
.)
(min
2
.)
(min
×
η
×
×
It
is
recommended
to
set
fsw(min)
between
20kHz-100kHz.
Assume
that
the
efficiency
η is
approximately 90% in calculating.
Supplement:
Inductance and switching frequencies
On and off period of each switching cycle can be
calculated with the following equation.
η
×
=
2
Vac
Po
Lp
2
Ton
ω
×
η
×
=
1
t
n
si
Vac
2
Vo
Vac
Po
Lp
2
Toff
2
where,
ω=2×π×fac
fac: AC line frequency (Hz)
In theory, according to the equation above, if
input-output conditions are constant, Ton is also
constant.
On
the
other
hand,
Toff
changes
corresponding to each instantaneous voltage of AC line,
maximum at
ωt =90°, minimum at ωt =0°.
Then,
switching frequencies can be calculated with the
following
equation
according
to
the
relationships
described above:
(
)
Vo
Po
Lp
2
t
n
si
Vac
2
Vo
Vac
fsw
2
×
η
×
ω
×
×
=
The
switching
frequencies
always
change
corresponding to each instantaneous voltage of AC line.
Vac
C1
L1 Np
Ns
R3
R6
R7
D2
R5
R8
FA5500A/01A
FB
COMP OUT
VCC
MUL
GND
IS
ZCD
C4
R4
C6
C3
C5
Rs
Q1
D1
C2
R1
R2
Po
Vo
Fig.13
Typical application circuit