AN2782 Datasheet, PDF (24/39 Pages) STMicroelectronics – Solution for designing a 400 W fixed-off-time controlled

Designing a fixed off-time PFC
AN2782
ρ_x
=
T_OFF (VAC_max )
T_OFF (VAC_min )
ρ_x
=
6.8 μs
4.2 μs
= 1.63
(67)
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, T_OFF is increasing with the line voltage
so that at maximum line voltage, the condition T_ON>T_ONmin = 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
TOFF@VACmax
6
5
4
3
2
1
0
0
2
4
6
8
usec
TOFF@VACmin
6
5
4
3
fswmin
2
1
0
0
1
2 _usec 3
4
5
fswmax
0
Switching
Frequency
Function
p
265Vac
90Vac
Input Line Voltage
Vin [Vac]
Angle
Now considering the two discharging resistors R and R₀ of the circuit of Figure 6, the ratio
K₁ is defined:
K₁
=
R
R₀ +
R
(68)
where 0 < K₁ < 1.Through the definition of the parameter K₂ it is underlined the expected
time constant τ =(R//R₀)C necessary to achieve the desired T_OFF at 90 Vac.
K₂
=
T_OFF (VAC_min )
τ
(69)
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:
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Doc ID 14763 Rev 2