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AN3112 Fiches technique(PDF) 22 Page - STMicroelectronics

No de pièce AN3112
Description  Solution for designing a fixed off-time controlled PFC pre-regulator
PDF  36 Pages
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Fabricant  STMICROELECTRONICS [STMicroelectronics]
Site Internet  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN3112 Fiches technique(HTML) 22 Page - STMicroelectronics

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Designing a fixed-off-time PFC
AN3112
22/36
Doc ID 16820 Rev 3
The two bode plot charts are relevant to the PFC operating at the main voltage set point of
265 Vac and full load. In this condition the crossover frequency is fc = 4 Hz, the phase
margin is 50° and the third harmonic distortion is below 3%.
Pin 4 (CS): The #4 pin is the inverting input of the current sense comparator. Through this
pin, the L6564 senses the instantaneous inductor current, converted in a proportional
voltage by an external sensing 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 set again by the ZCD signal. The pin is
equipped with 150 ns (typ.) leading-edge blanking for improved noise immunity.
The sense resistor value (Rs) can be calculated as follows. For the 400 W PFC it is:
Equation 44
Where:
●
ILpk: it is the maximum peak current in the inductor, calculated as described in 4.2
●
Vcsmin = 1.0 V, it is the minimum voltage admitted on the L6564 current sense (on 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 may be calculated considering the
maximum voltage Vcsmax allowed on the L6564 (on datasheet):
Equation 45
The calculated ILpkx is the value at which the boost inductor must not be in saturation and it
is used for calculating the inductor number of turns and air-gap length.
Figure 12.
Open loop transfer function-bode
plot
Figure 13.
Phase
-200
-100
0
100
0. 1
1
10
100
1000
f [Hz]
-200
-150
-100
0.1
1
10
100
1000
f [Hz]
max
pk
min
s
IL
Vcs
R
<
Ω
=
<
124
.
0
A
01
.
8
V
0
.
1
R
s
s
max
pksat
R
Vcs
IL
=
A
67
.
9
12
.
0
V
16
.
1
IL
pksat
=
Ω
=



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