What will happen if the output voltage becomes higher than the input voltage? How can I take measures against the state?

We are going to adopt PFC circuit for the first time. If REDC has equivalent devices, let us know the appropriate site link.

What is 'Power Factor'? Why the power factor must be improved?

What is Zero volt switching (ZVS)? Why is the loss reduced by ZVS?

What are Class X capacitors and Class Y capacitors? How to select Class X capacitors and Class Y capacitors?