AN4149 Datasheet, PDF (36/43 Pages) STMicroelectronics – Designing a CCM PFC pre-regulator based on the L4984D

FOT PFC boost control loop
AN4149
Equation 104
( ) H _{2 fL} = H 4 ⋅ π ⋅ f _{LINE _ min} = 0.011
From equation 90 and equation 104 the actual 3^rd harmonic distortion can be found:
Equation 105
D 3 = ^{H 2 fL ⋅ ΔVout} ⋅ ¹⁰⁰ = 1.253
V_COMP − V_ref 2
The comparison between Figure 20 and Figure 21 shows the practical difference between
two different configurations of compensation network. In the first case, where the series
resistor R2 is such to move the "phase boost" far from the crossover frequency, the phase
margin is very low (around 22 °) then the system is underdamped, experiencing large
overshoots and ringings. In the second case where the phase margin is around 56 ° the
response of the system to the load variation is fast and without ringing, thus with limited
deviations.
Figure 20. 115 V_ac step load (50 % to 100 Figure 21. 115 V_ac step load (50 % to 100
%): improper compensation
%): good compensation
36/43
DocID023523 Rev 2