AN3142 Datasheet, PDF (31/44 Pages) STMicroelectronics – Solution for designing a 400 W fixed-off-time controlled

AN3142
Designing a fixed-off-time PFC
Equation 68
τ = ^{TOFF (VACmin )}
K₂
τ = ^3.76µs = 336.35ns
11.17
Now, by selecting a capacitor C in the hundred picofarad range or a few nanofarads, for
example a C = 220 pF, it is possible to determine the required equivalent resistance value:
Equation 69
R_eq
=
τ
C
R_eq
=
336.35ns
220pF
= 1.53kΩ
From Equation 62, R and R0 are found:
Equation 70
R = ^Req R = ^1.53kΩ = 15.79kΩ
1− K₁
1− 0.903
Equation 71
R₀
=
R_eq
K₁
R₀
=
1.53kΩ
0.903
=
1.5kΩ
A commercial value R = 15 kΩ and an R0 = 1.5 kΩ has been chosen.
Figure 19 and Figure 20 show the trend of the off-time and the switching frequency vs the
input mains voltage. The PFC inner current loop is working in the range 80 kHz-150 kHz.
Due to the tolerance of the capacitor selected C and the two discharging resistors, it is
important to take into account a variation on the switching frequency in a real board of about
± 10%.
Figure 19. Off-time vs. input mains voltage Figure 20. Switching frequency vs. input
mains voltage
Finally limiting resistor Rs should be selected according to the inequalities in Equation 6:
Doc ID 17005 Rev 3
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