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Question on PFC front ends for SMPS

Started by Ralph Barone January 18, 2012
Do PFC circuits inherently result in unity power factor or can they be
"fudged" to produce a leading or lagging power factor?
On Wed, 18 Jan 2012 06:18:26 GMT, Ralph Barone
<address_is@invalid.invalid> wrote:

>Do PFC circuits inherently result in unity power factor or can they be >"fudged" to produce a leading or lagging power factor?
The common flyback PWM can't output current to the AC line, so can't accurately fake a capacitance or inductance. Their positive peak current could be shifted in time, which would technically be a power factor shift. Other circuits could, given adequate reservoir capacitance, really sim an L or C over the whole cycle. John
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

-- 
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in 
message news:qmndh7pjbcs1d17r5c9drceu0arsla3k3t@4ax.com...
> On Wed, 18 Jan 2012 06:18:26 GMT, Ralph Barone > <address_is@invalid.invalid> wrote: > >>Do PFC circuits inherently result in unity power factor or can they be >>"fudged" to produce a leading or lagging power factor? > > The common flyback PWM can't output current to the AC line, so can't > accurately fake a capacitance or inductance. Their positive peak > current could be shifted in time, which would technically be a power > factor shift. > > Other circuits could, given adequate reservoir capacitance, really sim > an L or C over the whole cycle. > > John > > >
John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
> On Wed, 18 Jan 2012 06:18:26 GMT, Ralph Barone > <address_is@invalid.invalid> wrote: > >> Do PFC circuits inherently result in unity power factor or can they be >> "fudged" to produce a leading or lagging power factor? > > The common flyback PWM can't output current to the AC line, so can't > accurately fake a capacitance or inductance. Their positive peak > current could be shifted in time, which would technically be a power > factor shift. > > Other circuits could, given adequate reservoir capacitance, really sim > an L or C over the whole cycle. > > John
Darn. I guess that approach won't work then. Thanks for the info.