Power factor for rectifier/capacitor input

[bob prohaska]

Power supplies that take AC line power, rectify it to DC and then
use a DC-DC converter to generate whatever output is desired seem
fairly common. Those that I've dissected don't seem to have any
inductors between the caps and the AC line.

What's the power factor in such a setup? It's clearly not zero,
because some power gets to the load, but all the AC line sees
is a pure reactance, unless the rectifier does something close
to magic.

Thanks for reading,

bob prohaska

 

[Phil Allison]

bob prohaska wrote:

---------------------------

Power supplies that take AC line power, rectify it to DC and then
use a DC-DC converter to generate whatever output is desired seem
fairly common. Those that I've dissected don't seem to have any
inductors between the caps and the AC line.


What's the power factor in such a setup? It's clearly not zero,
because some power gets to the load, but all the AC line sees
is a pure reactance, unless the rectifier does something close
to magic.

** The rectifier does just that, but it ain't magic - by preventing continuous ant current flow, normal capacitive reactance is *eliminated*.

In most cases, current flow is reduced to a few mS each half cycle of the AC supply. The PF is typically about 0.6 and it is impossible to say if it is leading or lagging.

All you can safely say is that peak current and peak voltage closely coincide.

So it's resistive, but intermittent.




..... Phil

 

[bob prohaska]

Phil Allison <pallison49@gmail.com> wrote:

** The rectifier does just that, but it ain't magic - by preventing continuous ant current flow, normal capacitive reactance is *eliminated*.

In most cases, current flow is reduced to a few mS each half cycle of the AC supply. The PF is typically about 0.6 and it is impossible to say if it is leading or lagging.

All you can safely say is that peak current and peak voltage closely coincide.

So it's resistive, but intermittent.

That clears up some of the fog. For a small load like a cell phone charger
or LED fixture a 60% power factor is probably ok. Are inductors added for
larger (kW-sized) loads?

Thanks for replying,

bob prohaska

 

[John Larkin]

On Fri, 28 Jul 2017 03:45:00 +0000 (UTC), bob prohaska
<bp@www.zefox.net> wrote:

Power supplies that take AC line power, rectify it to DC and then
use a DC-DC converter to generate whatever output is desired seem
fairly common. Those that I've dissected don't seem to have any
inductors between the caps and the AC line.

What's the power factor in such a setup? It's clearly not zero,
because some power gets to the load, but all the AC line sees
is a pure reactance, unless the rectifier does something close
to magic.

Thanks for reading,

bob prohaska

The AC line sees a big narrow current spike close in time to the
voltage peak. There is real power there.

One authority says that power factor (defined as cos of the current
phase angle) is undefined for a non-sinusoidal waveform. More
reasonable people say that PF is (true watts)/(RMS volts * RMS amps),
which can be pretty low for a capacitor-input power supply.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Phil Allison]

bob prohaska wrote:

-----------------------

Phil Allison

** The rectifier does just that, but it ain't magic -
by preventing continuous ant current flow, normal
capacitive reactance is *eliminated*.

In most cases, current flow is reduced to a few mS each
half cycle of the AC supply. The PF is typically about
0.6 and it is impossible to say if it is leading or lagging.

All you can safely say is that peak current and peak
voltage closely coincide.

So it's resistive, but intermittent.


That clears up some of the fog. For a small load like a cell phone charger
or LED fixture a 60% power factor is probably ok. Are inductors added for
larger (kW-sized) loads?


** There is no capacitive phase angle to correct, peak current and voltage are in phase like I already stated.

However, adding a series L increases the AC supply impedance at frequencies above 50 or 60 Hz. This has the effect of reducing the amplitude of cap charging current peaks and improves the PF a bit - at he expense of reduced peak voltage. OK with regulated SMPSs but not otherwise.

The proper fix is to use *active* PF correction - a high frequency switching technique that keeps the current draw proportional to the instantaneous voltage of the supply.

This is becoming common in industrial & commercial situations where poor PF and or high inrush currents limit the number of devices that can use the same AC circuit.

Most of the larger LED lighting fixtures are active PF corrected these days..



..... Phil

 

[bob prohaska]

Phil Allison <pallison49@gmail.com> wrote:

The proper fix is to use *active* PF correction - a high frequency switching technique that keeps the current draw proportional to the instantaneous voltage of the supply.

Ok, this is what I was wondering about. Is there a link handy? I've never
encountered the idea before.

Most of the larger LED lighting fixtures are active PF corrected these days.

Are you speaking of domestic or industrial?

Thanks for reading, and your guidance!

bob prohaska

 

[Phil Allison]

bob prohaska wrote:

------------------------

P

The proper fix is to use *active* PF correction - a high frequency switching technique that keeps the current draw proportional to the instantaneous voltage of the supply.

Ok, this is what I was wondering about. Is there a link handy? I've never
encountered the idea before.

http://sound.whsites.net/lamps/pfc.html

Most of the larger LED lighting fixtures are active PF corrected these days.

Are you speaking of domestic or industrial?

** I see you are a snip and then forget kinda guy.




..... Phil

Thanks for reading, and your guidance!

bob prohaska

 

[bob prohaska]

Phil Allison <pallison49@gmail.com> wrote:

http://sound.whsites.net/lamps/pfc.html

Thank you, that's a good explanation.

bob prohaska