AFPC Questions

[ACharnley]

I've a question that's been bugging me for a while on AFPC. The majority of designs I've come across using a boost either after the bridge or as a dual bridge-less. That means a high voltage DC is created of at least the input voltage so 110-220v. Bucking efficiency is generally worse the further the input voltage moves away from the output voltage so this high voltage would be less than ideal for many consumer electronics.

Alternatively there could be a transformer before all of this, so the 110-220v it taken down to 6-12v, which is APFC boost corrected and a buck to the target voltage works on that.

I asked this question of an ex-Texas employee who knows his stuff but lacks a bit in the explanation area. He said transformers are never used for this due to size/cost, but he left it there and it's bugged me ever since. Perhaps someone can enlighten!

 

[ACharnley]

Another question, I may as well get them all out.

All the designs I've looked at include the usual smoothing/storage capacitor after the bridge and an inductor as part of the PFC. This suggests to me that main's AC power supply is already reactance corrected when it comes into the house, otherwise there'd need to be some inline capacitors to retard the phase either in the mains box or as part of every circuit (an inductor can't retard phase inductance). Is this true?