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AN3027 Datasheet(PDF) 13 Page - STMicroelectronics

No. de pieza AN3027
Descripción Electrónicos  How to design a transition-mode PFC pre-regulator
PDF  41 Pages
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Fabricante Electrónico  STMICROELECTRONICS [STMicroelectronics]
Página de inicio  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN3027 Datasheet(HTML) 13 Page - STMicroelectronics

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AN3027
Designing a TM PFC
Doc ID 16134 Rev 4
13/41
As previously stated, ILpk is twice the line-frequency peak current Equation 4, which is
related to the input power and the input mains voltage. Substituting this relationship in the
expressions of ton and toff, after some algebra it is possible to find the instantaneous
switching frequency along a line cycle:
Equation 21
The switching frequency is minimum at the top of the sinusoid (
θ = Π /2 ==> sin θ =1),
maximum at the zero crossings of the line voltage (
θ = 1 or Π ==> sin θ =0), where toff =0.
The absolute minimum frequency fswmin can occur at either the maximum VACmax or the
minimum mains voltage VACmin, thus the inductor value is defined by the formula:
Equation 22
After calculating the values of the inductor at low mains and at high mains L(VACmax),
L(VACmin) Equation 23, the lowest value must be used. It becomes the maximum
inductance value for the PFC dimensioning.
Equation 23
For this application a 0.52 mH boost inductance has been selected.
Figure 4.
Switching frequency, fixing the line voltage
The figure above shows the switching frequency versus the
θ angle calculated with Equation
22, a 0.52 mH boost inductance and fixing the line voltage at minimum and maximum
(
)
out
out
2
in
off
on
sw
V
)
sin(
VAC
2
V
VAC
P
L
2
1
T
T
1
)
,
VAC
(
f
θ
⋅
⋅
−
⋅
⋅
⋅
⋅
=
+
=
θ
out
in
min
sw
out
2
V
P
f
2
)
VAC
2
V
(
VAC
)
VAC
(
L
⋅
⋅
⋅
⋅
−
⋅
=
()
mH
642
.
0
V
400
W
38
.
106
kHz
40
2
)
Vac
90
2
V
400
(
Vac
90
)
VAC
(
L
2
min
=
⋅
⋅
⋅
⋅
−
⋅
=
()
mH
515
.
0
V
400
W
38
.
106
kHz
40
2
)
Vac
265
2
V
400
(
Vac
265
)
VAC
(
L
2
max
=
⋅
⋅
⋅
⋅
−
⋅
=



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