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AN3112 bảng dữ liệu(PDF) 26 Page - STMicroelectronics

tên linh kiện AN3112
Giải thích chi tiết về linh kiện  Solution for designing a fixed off-time controlled PFC pre-regulator
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Designing a fixed-off-time PFC
AN3112
26/36
Doc ID 16820 Rev 3
Because the MULT divider also determines the mains input voltage at which the PFC starts
and stops (brownout function), these values are calculated using the actual divider ratio:
Equation 52
And also the stop voltage:
Equation 53
Start and stop PFC mains voltage are compatible with the input mains voltage range
(1).
In order to obtain the required startup and shutdown voltage, a reiteration may be required,
by selecting MULT resistors and checking the actual PFC start and stop mains voltage.
Pin 7 (ZCD): This is the input of the zero current detector circuit. In FOT mode, it is
connected to the line-modulated fixed-off-time circuit seen in
Figure 6. Taking into account
the information in
Section 3: Implementing the line-modulated fixed-off-time, the starting
point for the design of that circuit is the pair of the desired values for TOFF on the top of the
line voltage sinusoid at minimum (TOFF @VACmin) and maximum line (TOFF @VACmax)
obtained by setting the switching frequency on the peak of the sinusoid at low mains and
considering the minimum on-time of the L6564:
Equation 54
Equation 55
Where Fswmin is the switching frequency on top of the sinusoid of the input voltage at
VACmin = 90 Vac (
Figure 16) and 220 ns is a corrector factor in order to consider the delay
between the ZCD and GD signal.
Considering the ratio between
Equation 55, Equation 54, we have:
Equation 56
In the formula,
Equation 55 and Equation 54, the delay between the ZCD signal and the
gate drive signal is taken into account in order to increase the accuracy of the mathematical
model.
multL
multL
multH
START
R
R
R
2
V
88
.
0
V
+
⋅
==
V
8
.
84
k
51
k
51
M
9
.
6
2
V
88
.
0
V
START
=
Ω
Ω
+
Ω
⋅
=
multL
multL
multH
STOP
R
R
R
2
V
80
.
0
V
+
⋅
==
V
1
.
77
k
51
k
51
M
9
.
6
2
V
80
.
0
V
STOP
=
Ω
Ω
+
Ω
⋅
=
min
sw
min
min
OFF
f
k
)
VAC
(
T
=
s
76
.
3
ns
220
kHz
80
32
.
0
)
VAC
(
T
min
OFF
µ
=
−
=
max
max
min
ON
max
OFF
k
1
k
T
)
VAC
(
T
−
⋅
=
s
1
.
6
ns
220
94
.
0
1
94
.
0
ns
450
)
VAC
(
T
max
OFF
µ
=
−
−
⋅
=
)
VAC
(
T
)
VAC
(
T
min
OFF
max
OFF
x =
ρ
63
.
1
s
76
.
3
s
1
.
6
x
=
µ
µ
=
ρ



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