Question on PFC front ends for SMPS

[Ralph Barone]

Do PFC circuits inherently result in unity power factor or can they be
"fudged" to produce a leading or lagging power factor?

 

[Tim Williams]

You can take the line in through an inductor and chop it with a bridge (so
it looks like you're driving the line with the bridge's output), and
operate with PWM in a current loop so that the line current is always
proportional to the line voltage, or some phase shift thereof. This is
scaled by the DC error voltage on the output rail, so the voltage remains
regulated while 120Hz ripple (or 360Hz if you do this with a three phase
chopper for industrial use) appears on the filter cap.

You're basically running a VFD in reverse. Two of these, back to back,
would do a great job running motors; unsurprisingly, this is often done.

Tim