Im teaching myself about boost pfc and while I've seen several datasheet/app note diagrams showing how the gain modulator/error feedback system is schematically, I've not yet seen a more intuitive explanation that perhaps has more diagrams showing how the feedback loop actually works to convert DC current demand at the output into a phase locked sinusoidal current at the input. I get that the input current waveforum must be:
-sinusoidal (easy to understand, otherwise its not PFC)
-in phase with the input voltage (easy to understand, otherwise its not PFC)
-proportional to the current being demanded by the output (easy to understand as a GOAL, but not so sure how its actually accomplished)
I'm imagining that the input current waveform basically just changes amplitude proportional to how much current is being demanded at the output. Therefore there is only ONE variable, and thats amplitude of the input current. Its always a sine wave, and its always phase locked to the input voltage. Is that right??
But how is that done? And is it on a cycle-by-cycle basis or can changes to it occur mid-cycle? How fast does it respond? Etc..
I'm guessing alot of those details are loop compensation, so I can shelve those questions for the moment.
What I need more clarification on is how the loop functions period. I seem to see a sine wave reference being generated by the input voltage. Then that times an output voltage signal creates a modulating signal to drive the switching fets PWM. But it still seems muddy. Maybe I just need to stare at those error amp block diagrams until it makes sense.
Hoping someone has seen a really good intuitive explanation of it.
The best I've yet seen is probably Unitrode 134, heres an excerpt
-sinusoidal (easy to understand, otherwise its not PFC)
-in phase with the input voltage (easy to understand, otherwise its not PFC)
-proportional to the current being demanded by the output (easy to understand as a GOAL, but not so sure how its actually accomplished)
I'm imagining that the input current waveform basically just changes amplitude proportional to how much current is being demanded at the output. Therefore there is only ONE variable, and thats amplitude of the input current. Its always a sine wave, and its always phase locked to the input voltage. Is that right??
But how is that done? And is it on a cycle-by-cycle basis or can changes to it occur mid-cycle? How fast does it respond? Etc..
I'm guessing alot of those details are loop compensation, so I can shelve those questions for the moment.
What I need more clarification on is how the loop functions period. I seem to see a sine wave reference being generated by the input voltage. Then that times an output voltage signal creates a modulating signal to drive the switching fets PWM. But it still seems muddy. Maybe I just need to stare at those error amp block diagrams until it makes sense.
Hoping someone has seen a really good intuitive explanation of it.
The best I've yet seen is probably Unitrode 134, heres an excerpt
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