AN1059 Datasheet, PDF (15/20 Pages) STMicroelectronics – DESIGN EQUATIONS OF HIGH-POWER-FACTOR

AN1059 APPLICATION NOTE
Figure 15. Feedback network and connection to the error amplifier.
R7 ^R8
COMP
IF
C2
L6561
INV VCC
R4
R1
R6
VE
IC
R5
VK
TL431
C1 R3
R2
Vout
≈1 µF
Therefore:
R₄
<
Vout
−1−
2.5
2.5
⋅
CTR_min
⋅
R₅,
where 1V is the typical drop across optocoupler’s photodiode. Keep R4 close to the maximum for a low
gain. R1 and R2 are selected to get the desired output voltage:
R₂
=
2.5
I_R2
;
R₁
=
Vout − 2.5
2.5
⋅
R₂
where 2.5 is the internal reference of the TL431 and IR2 the current flowing through R2.
To have a low gain at twice line frequency, the zero of H(s) will be placed below 100Hz and R3 will be 4-
5 times less than R1. This yields the value of C1.
The value of R6 will be such that the twice mains frequency ripple superimposed on the static VE cannot
trip the dynamic overvoltage protection of the L6561 (40 µA entering pin COMP). Approximately:
R6
>
R5
+
R5
R4
⋅
CTR_max ⋅ ∆V_o
40 ⋅ 10^−6
R7 will be selected so as to allow the output of the error amplifier to swing all the dynamics. Finally, R8
and C2 will be adjusted so that the crossover frequency of the open-loop gain is a good compromise be-
tween a high enough PF and an acceptable transient response, ensuring also sufficient phase margin.
The optional capacitor (in the µF range) connected in parallel to R1 acts as a soft-start circuit that pre-
vents overvoltages of the output at start-up, especially at light load. The two diodes decouple the capaci-
tor during steady-state operation so that it does not interfere with the loop gain and provide a discharge
path when the converter is turned off.
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