SLUSC60B December 2017 – October 2019 UCC28064A
Due to the parasitic resonance between the drain-source capacitance of the switching MOSFET and the boost inductor, conventional transition-mode PFC circuits may not be able to absorb power from the input line when the input voltage is near zero. This limitation increases total harmonic distortion as a result of ac-line current waveform distortion in the form of flat spots. To help reduce line-current distortion, the UCC28064A increases switching MOSFET on-time when the input voltage is near 0 V to improve the power absorption capability and compensate for this effect.
Figure 1 in the Typical Characteristics section shows the increase in on-time with respect to VINAC voltage. Excessive filtering of the VINAC signal will nullify this function. In cases where small inductances are used (< 250 µH) the increased MOSFET on time can be excessive, increasing distortion instead of decreasing. If this is the case the external circuit shown in Figure 17 can help limit this effect.