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AN2782 Datasheet(PDF) 23 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) 23 Page - STMicroelectronics

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AN2782
Designing a fixed off-time PFC
Doc ID 14763 Rev 2
23/39
From (60) the maximum required divider ratio is calculated as:
Supposing a 300 µA current flowing into the multiplier divider the lower resistor value can be
calculated:
A commercial value of 10 k
Ω for the lower resistor is selected. The upper resistor value can
now be calculated:
In this application example RmultH = 1240 MΩ and a RmultL = 10 kΩ have been selected.
Please note that for RmultH a resistor with a suitable voltage rating (> 400 V) is needed, or
more resistors in series must be used.
The voltage on the multiplier pin with the selected component values re-calculated is 1.01 V
at minimum line voltage and is 2.99 V at maximum line voltage. The multiplier works
correctly within its linear region.
●
Pin 5 (ZCD) is the input to the zero current detector circuit. It is connected to the line-
modulated, fixed off-time circuit seen in the previous Figure 6 on page 6. Referring to
Section 3: The circuit implementing the line-modulated fixed off-time with the new
L6562A
, the starting point for the design of the zero-current detector (ZCD) circuit is the
pair of the desired values for TOFF on the top of the line voltage sinusoid at minimum
(TOFF at VACmin) and maximum line (TOFF at VACmax) obtained by setting the switching
frequency on the peak of the sinusoid at low mains and considering the minimum on-
time of the L6562A:
where fswmin is the switching frequency on the top of the sinusoid of the input voltage at
VACmin = 90 Vac (Figure 15).
Let
ρx as the ratio between (66), (65):
(62)
(63)
(64)
(65)
(66)
3
max
max
MULT
p
10
08
.
8
Vac
265
2
V
02
.
3
VAC
2
V
k
−
⋅
=
⋅
=
⋅
=
Ω
=
μ
=
μ
=
k
03
.
10
A
300
V
02
.
3
A
300
V
R
max
MULT
multL
Ω
=
Ω
⋅
⋅
−
=
−
=
−
−
M
238
.
1
k
10
10
08
.
8
10
08
.
8
1
R
k
k
1
R
3
3
multL
p
p
multH
min
sw
min
min
OFF
f
k
)
VAC
(
T
=
s
2
.
4
ns
220
kHz
72
32
.
0
)
VAC
(
T
min
OFF
μ
=
−
=
max
max
min
ON
max
OFF
k
1
k
T
)
VAC
(
T
−
⋅
=
s
8
.
6
ns
220
94
.
0
1
94
.
0
ns
450
)
VAC
(
T
max
OFF
μ
=
−
−
⋅
=



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