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AN2782 Datasheet(PDF) 24 Page - STMicroelectronics

Part # AN2782
Description  Solution for designing a 400 W fixed-off-time controlled
PDF  39 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN2782 Datasheet(HTML) 24 Page - STMicroelectronics

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Designing a fixed off-time PFC
AN2782
24/39
Doc ID 14763 Rev 2
In the formula (65), (66) 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.
From the theory of the line-modulation fixed off-time, TOFF is increasing with the line voltage
so that at maximum line voltage, the condition TON>TONmin = 450 ns is always true for the
L6562A [4]. This is important in order to avoid line distortion [7].
Figure 15.
Switching frequency function on the peak of the sinusoid input voltage
waveform and the corresponding off-time value
Now considering the two discharging resistors R and R0 of the circuit of Figure 6, the ratio
K1 is defined:
where 0 < K1 < 1.Through the definition of the parameter K2 it is underlined the expected
time constant
τ =(R//R
0)C necessary to achieve the desired TOFF at 90 Vac.
Finding a way to obtain K1 and K2 means increasing the values of R and R0 and the
discharging time constant of the capacitor C.
The following part describes the mathematical way to obtain the two parameters K1 and K2.
Combining (65), (66), (68) and (69) with the expression of the off-time (5) the following
expressions are obtained:
(67)
(68)
(69)
)
VAC
(
T
)
VAC
(
T
min
OFF
max
OFF
x =
ρ
63
.
1
s
2
.
4
s
8
.
6
x
=
μ
μ
=
ρ
0
1
2
3
4
5
6
012
34
5
usec
90Vac
265Vac
0
p
fswmin
fswmax
Input Line Voltage
Switching
Frequency
Function
Vin [Vac]
Angle
0
1
2
3
4
5
6
0
2
468
usec
TOFF@VACmin
TOFF@VACmax
0
1
2
3
4
5
6
012
34
5
usec
90Vac
265Vac
0
p
fswmin
fswmax
Input Line Voltage
Switching
Frequency
Function
Vin [Vac]
Angle
0
1
2
3
4
5
6
0
2
468
usec
TOFF@VACmin
TOFF@VACmax
90Vac
265Vac
0
p
fswmin
fswmax
Input Line Voltage
Switching
Frequency
Function
Vin [Vac]
Angle
0
1
2
3
4
5
6
0
2
468
usec
TOFF@VACmin
TOFF@VACmax
R
R
R
K
0
1
+
=
τ
=
)
VAC
(
T
K
min
OFF
2



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