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AN2782 Datasheet(PDF) 21 Page - STMicroelectronics

No. de pieza AN2782
Descripción Electrónicos  Solution for designing a 400 W fixed-off-time controlled
PDF  39 Pages
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Fabricante Electrónico  STMICROELECTRONICS [STMicroelectronics]
Página de inicio  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN2782 Datasheet(HTML) 21 Page - STMicroelectronics

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AN2782
Designing a fixed off-time PFC
Doc ID 14763 Rev 2
21/39
voltage by an external sense resistor (Rs). As this signal crosses the threshold set by
the multiplier output, the PWM latch is reset and the power MOSFET is turned off. The
MOSFET stays in OFF-state until the PWM latch is reset by the ZCD signal. The pin is
equipped with 200 ns leading-edge blanking for improved noise immunity.
The sense resistor value (Rs) can be calculated as follows. For the 400 W PFC it is:
where:
●
ILpkmax is the maximum peak current in the inductor, calculated as described in (28)
●
Vcsmin = 1.0 V is the minimum voltage allowed on the L6562A current sense (in the
datasheet)
Because the internal current sense clamping sets the maximum current that can flow in the
inductor, the maximum peak of the inductor current is calculated considering the maximum
voltage Vcsmax allowed on the L6562A (in the datasheet):
The calculated ILpksat is the limit at which the boost inductor saturates and it is used for
calculating the inductor number of turns and air gap length.
The power dissipated in Rs is given by:
It does not exceed 1% of the rated output power (10), that is, 4 W.
According to the result, four parallel resistors of 0.47
Ω with 1 W of power rating have been
selected.
●
Pin 3 (MULT): The MULT pin is the second multiplier input. It is connected, through a
resistive divider, to the rectified mains to get a sinusoidal voltage reference. The
multiplier can be described by the relationship:
where:
●
VCS (multiplier output) is the reference for the current sense
●
k = 0.38 (typ) is the multiplier gain
●
VCOMP is the voltage on pin 2 (E/A output)
●
VMULT is the voltage on pin 3
(56)
(57)
(58)
(59)
max
pk
min
sx
IL
Vcs
R
<
Ω
=
<
124
.
0
A
07
.
8
V
0
.
1
R
sx
s
max
pksat
R
Vcs
IL
=
A
67
.
9
12
.
0
V
16
.
1
IL
pksat
=
Ω
=
2
rms
s
s
ISW
R
P
⋅
=
(
)
W
14
.
2
A
54
.
10
12
.
0
P
2
s
=
⋅
Ω
=
MULT
COMP
CS
V
)
V
5
.
2
V
(
k
V
⋅
−
⋅
=



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