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AN3142 数据表(PDF) 31 Page - STMicroelectronics

部件名 AN3142
功能描述  Solution for designing a 400 W fixed-off-time controlled
PDF  44 Pages
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制造商  STMICROELECTRONICS [STMicroelectronics]
网页  http://www.st.com
标志 STMICROELECTRONICS - STMicroelectronics

AN3142 数据表(HTML) 31 Page - STMicroelectronics

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AN3142
Designing a fixed-off-time PFC
Doc ID 17005 Rev 3
31/44
Equation 68
Now, by selecting a capacitor C in the hundred picofarad range or a few nanofarads, for
example a C = 220 pF, it is possible to determine the required equivalent resistance value:
Equation 69
From
Equation 62, R and R0 are found:
Equation 70
Equation 71
A commercial value R = 15 k
Ω and an R0 = 1.5 kΩ has been chosen.
Figure 19 and Figure 20 show the trend of the off-time and the switching frequency vs the
input mains voltage. The PFC inner current loop is working in the range 80 kHz-150 kHz.
Due to the tolerance of the capacitor selected C and the two discharging resistors, it is
important to take into account a variation on the switching frequency in a real board of about
± 10%.
Finally limiting resistor Rs should be selected according to the inequalities in
Equation 6:
2
min
OFF
K
)
VAC
(
T
=
τ
ns
35
.
336
17
.
11
s
76
.
3
=
µ
=
τ
C
R
eq
τ
=
Ω
=
=
k
53
.
1
pF
220
ns
35
.
336
R
eq
1
eq
K
1
R
R
−
=
Ω
=
−
Ω
=
k
79
.
15
903
.
0
1
k
53
.
1
R
1
eq
0
K
R
R
=
Ω
=
Ω
=
k
5
.
1
903
.
0
k
53
.
1
R
0
Figure 19.
Off-time vs. input mains voltage
Figure 20.
Switching frequency vs. input
mains voltage



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