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AN2782 датащи(PDF) 14 Page - STMicroelectronics

номер детали AN2782
подробное описание детали  Solution for designing a 400 W fixed-off-time controlled
PDF  39 Pages
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производитель  STMICROELECTRONICS [STMicroelectronics]
домашняя страница  http://www.st.com
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AN2782 датащи(HTML) 14 Page - STMicroelectronics

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Designing a fixed off-time PFC
AN2782
14/39
Doc ID 14763 Rev 2
If the PFC stage has to guarantee a specified holdup time, the selection criterion of the
capacitance changes. Co has to deliver the output power for a certain time (tHold) with a
specified maximum dropout voltage (Vout min) that is the minimum output voltage value
(which takes load regulation and output ripple into account). Vout min is the minimum output
operating voltage before the 'power fail' detection and consequent stopping by the
downstream system supplied by the PFC.
A 20% tolerance on the electrolytic capacitors has to be taken into account for the right
dimensioning.
Following the relationship (38), for this application a capacitor Co = 330 µF (450 V) has been
selected in order to maintain a holdup capability for 20 ms. The actual output voltage ripple
with this capacitor is also calculated. In detail:
As expected, the ripple variation on the output is:
4.3.4
Boost inductor
In the continuous mode approach, the acceptable current ripple factor, Kr, can be
considered between 10% to 35%. For this design, the maximum specified current ripple
factor is 35%.
To calculate the required inductance L of the boost inductor, use the following formula with a
4.2 µs OFF-time set at 90Vac (see the following ZCD pin dimensioning for finding the correct
value):
After calculating the values of the inductor at low mains and at high mains L(VACmax),
L(VACmin) (41) depending also on the OFF-time, the minimum value has to be taken into
account. It became the maximum inductance value for the PFC dimensioning.
(38)
(39)
(40)
() 2
min
out
2
out
out
Hold
out
O
V
V
V
t
P
2
C
−
Δ
−
⋅
⋅
=
() (
)
F
3
.
242
V
300
V
10
V
400
ms
20
W
400
2
C
2
2
O
μ
=
−
−
⋅
⋅
=
(
)
out
2
min
out
2
out
out
O
Hold
P
2
V
V
V
C
t
⋅
⎥⎦
⎤
⎢⎣
⎡
−
Δ
−
⋅
=
(
) (
)
[
]
ms
22
W
400
2
V
300
V
10
V
400
F
330
t
2
2
Hold
=
⋅
−
−
⋅
μ
=
O
l
out
out
C
f
2
I
V
⋅
⋅
π
⋅
=
Δ
V
2
.
10
F
330
Hz
47
2
A
0
.
1
V
out
=
μ
⋅
⋅
π
⋅
=
Δ
(41)
)
VAC
(
T
IL
V
)
k
1
(
)
VAC
(
L
OFF
pk
out
min
Δ
⋅
−
=
H
520
s
2
.
4
A
11
.
2
V
400
)
32
.
0
1
(
)
VAC
(
L
min
μ
=
μ
⋅
−
=



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