Why is boost convert mostly used for PFC?

[faikelvan]

Hi,

I was wondering why boost converter is mostly used for PFC applications? I have a couple of ideas myself, and I wanted to confirm these with expert opinions. I have just compared buck and boost, these two being the most simple form of SMPS. I don't know if any other topology is used in PFC applications. I think the reasons may be:

1- Driving MOSFET in boost converter is easier than high side switching for buck converter.
2- Since input voltage is a sine wave or a rectified sine wave, there are zero crossings. Trying to step down these areas in a buck converter might be problematic, hence lowering the input current THD might be more challenging.

What are your opinions?

 

[mtwieg]

#2 is the main reason. Also the boost has continuous input current which relaxes the demand put on input filters to meet emc specs.

 

[Orson Cart]

A buck converter needs to have a Tx if it wants to process power off the rectified mains down to low volts, the boost works from 0-> mains peak, a plain buck can only work when Vin > Vout.... and usually Vout is >385VDC, so a buck will never work here....