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AN2761 Datasheet(PDF) 13 Page - STMicroelectronics

Part # AN2761
Description  Solution for designing a transition mode
PDF  36 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN2761 Datasheet(HTML) 13 Page - STMicroelectronics

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AN2761
Designing a TM PFC
Doc ID 14690 Rev 2
13/36
Ton and Toff are respectively the ON-time and the OFF-time of the power MOSFET, ILpk is
the maximum peak inductor current in a line cycle, and
θ is the instantaneous line phase in
the interval [0,
Π]. Note that the ON-time is constant over a line cycle.
As previously stated, ILpk is twice the line-frequency peak current (16), 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:
The switching frequency is the minimum at the top of the sinusoid (
θ = Π /2 rad =>
sin
θ =1), maximum at the zero-crossings of the line voltage (θ = 0 rad or Π rad=> sin θ =0),
where Toff =0 µs.
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:
After calculating the values of the inductor at low mains and at high mains L(VACmax),
L(VACmin) (35), the minimum value has to be taken into account. It becomes the maximum
inductance value for the PFC dimensioning.
For this application a 0.7 mH boost inductance has been selected.
(34)
(35)
(36)
(37)
(
)
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
73
.
0
V
400
W
02
.
86
kHz
35
2
)
Vac
85
2
V
400
(
Vac
85
)
VAC
(
L
2
min
=
⋅
⋅
⋅
⋅
−
⋅
=
()
mH
83
.
0
V
400
W
02
.
86
kHz
35
2
)
Vac
265
2
V
400
(
Vac
265
)
VAC
(
L
2
max
=
⋅
⋅
⋅
⋅
−
⋅
=



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