UCC28070: Active power factor correction

Part Number: UCC28070

Hello,

I am using UCC28070 interleaved PFC controller for 1.2kW battery charger design. I have few queries regarding this IC. These are mentioned below.

1. Does duty cycle in boost PFC varies according to input sine voltage? 

2. What are the advantages and disadvantages if we keep duty cycle constant?

3.  What are the advantages and disadvantages if we keep duty cycle variable?

4. Why loop bandwidth of PFC is always kept low such as at 20Hz?

Regards,

Dhananjay