What is Active PFC?

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krotchy

Golden Member
Mar 29, 2006
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I built a DCM Flyback transformer to turn a 120VAC input into a 12V output in power electronics class. Idealy I should have done a 1:1 or 1:2 transformer to create the emulated resistor, and steped it down with a buck to the necessary voltage level. as it stood, my professor is a dipshit and mislead us in the design of it so we used a very poorly made 1:4 DCM Flyback Transformer, and it was quite lossy (Lots of snubber explosions).

We eventually closed the loop, so that the input voltage and load could change and it would still stay at 12V (using a fairly slow 10 Hz compensator to filter out the 120 Hz Harmonics + a 10mF Cap on the output for low ripple). However I read on wikipedia that Active PFC does not contain inductors and capacitors? This is very confusing and I cannot figure out for the life of me what it is.

A properly designed 2 stage DCM Flyback(1:1 AC->DC) -> Buck(DC Stepdown) with a well designed compensator should create 12, 5 or 3.3 V with very low loss and an emulated resistor accross the input if I am correct. This would effectively allow for "dirty power" from the wall to be compensated for and leave you with a 12,5,3.3 etc V output no matter what your input is. Also it has low harmonic distortion due to the DCM Flyback's emulated resistor quality. This seems like the definition of Active PFC, as it compensates input and load changes for a steady output, but from what I can tell is simply passive PFC?

So basically what is Active PFC if it doesnt use caps/inductors?
 
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bobsmith1492

Diamond Member
Feb 21, 2004
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What you're describing sounds to me like active PFC - "emulated resistor." The goal of an AC load with PFC is to keep your input current and voltage in phase with each other, making it appear to be a resistive load, so if your system appears to be a purely resistive load, it would have a PF=1 and no need for passive PFC. Beyond that, I don't know how to implement active PFC. :p

EDIT: Here's my first intro to active PFC....
http://www.vicr.com/documents/application_notes/an1_active-pfc.pdf

 
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Mark R

Diamond Member
Oct 9, 1999
8,513
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Originally posted by: krotchy

So basically what is Active PFC if it doesnt use caps/inductors?
Click to expand...

It's a conventional boost regulator, powered directly off the bridge rectified line input, and outputing into the reservoir capacitors, with a 2nd feedback loop that serves to match input current to input voltage. A compensation network is used to balance the competing demands of the 2 feedback loops. (PFC feedback operates short term, with voltage regulation over the longer term).

Just like a standard boost regulator, an active PFC boost can be operated in DCM or CCM, as required by the application.

High-power designs usually use a high PFC boost to produce a high voltage DC bus (approx 400 V), from which a conventional DC:DC converter can run (at this power, usually forward, half bridge or full bridge topology).

It is also possible, if minimum part count is desired, to replace the boost inductor with a flyback inductor, so reducing your PSU design to a single stage. This is most suitable for low-power supplies, which are suitable for the flyback topology.

A significant advantage of active PFC, is the fact that it is a regulator. In the 2 stage scheme described above, the 2nd DC:DC converter gets a partially-regulated voltage to work with, so inductor core size/turns ratio/duty cycle can be optimised for high efficiency. In some cases, adding using such a 2 stage design, can be signficantly more efficient than a single stage line-powered DC:DC converter.

Simple introduction Application note
 
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