Yes!
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.
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!
Being usually of a boost- flyback- topology it can generate the required
output from input voltages lower than the output so that it can draw current
from the input during a greater part of the input alternance time. Hence,
the averaged peak current is lowered (longer conduction time),and averaging
the switching wave form causes the input wave form to approximate a
sinusoidal law, thereby reducing twice-line-frequency and higher harmonics.
I hope this helps:
Alex 4Z5KS
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Manfred
Mornhinweg
Sent: Saturday, September 28, 2013 8:33 PM
To: amps@contesting.com
Subject: Re: [Amps] Direct rectification of AC mains to drive the amp,
Bill,
> Power factor correction is an area I am a bit hazy on. Could someone
> explain why the power factor would be different between a
> direct-rectified PS and a transformer-isolated one?
I have mentioned it before, but I can explain it again: Yes, both charge the
capacitor with pulses, so the difference between the two is not very large.
But when there is a transformer, this transformer places both resistance and
inductance in series with the circuit, moderating the intensity of the
pulses, stretching them out, and even getting them a little closer to the
correct phase.
Note that the transformer does this through its imperfections: Winding
resistance, and leakage inductance. The better a transformer is, the worse
the power factor gets! A power supply using a perfect transformer would have
the same power factor as a direct, transformerless one. In reverse, a
transformerless power supply's power factor can be improved by
deliberatingly adding series inductance and resistance.
Manfred
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