Alex,
First of all, there is not so much of a difference between a transformer
power supply and a PFC: the both are based on an inductive input element- a
transformer primary here, and a series inductor there.
Well, not exactly... The input of a PFC is a bridge rectifier followed
by a capacitor, not an inductor. That capacitor is a small one, like
1uF, so that it doesn't draw any significant current pulses. Then
follows an inductor that goes ACROSS the line, in series with a MOSFET.
This is not used to provide any inductive correction, but instead as
part of a boost converter with tightly controlled current. The MOSFET
switches on and off at any rate between roughly 25 and 200kHz. So it
draws current pulses from the input at this rate, and the small filter
capacitor is there to smooth this out. The control circuit can control
in detail how much current is drawn, fast enough to respond to each
little detail of the incoming AC waveform, but slow enough to not impair
the basic high frequency switching. Let's talk of a response limit of
3kHz or so.
Second, the PFC does not allow the input current to assume whatever value it
wants, but it forces the successive peaks of the switching wave forms to
Follow a sinusoidal curve AT THE MAINS FREQUENCY!
In fact it's even better than that: The mains voltage us usually not a
clean sinewave, but a rather distorted one. Best power factor does not
result from drawing a clean sine wave current, but from drawing a
current having exactly the same waveform as the voltage does. Just like
a resistor would do. And a PFC does exactly that! It senses the actual
AC waveform, and controls the current to follow the same waveform -
regardless of whether it's a clean sine, or a severely deformed one!
Manfred
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