Power factor for rectifier/capacitor input

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

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bob prohaska
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** 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

Reply to
Phil Allison

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

Reply to
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
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John Larkin

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r

** 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 ch arging current peaks and improves the PF a bit - at he expense of reduced p eak voltage. OK with regulated SMPSs but not otherwise.

The proper fix is to use *active* PF correction - a high frequency switchin g technique that keeps the current draw proportional to the instantaneous v oltage 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 s ame AC circuit.

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

.... Phil

Reply to
Phil Allison

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

Are you speaking of domestic or industrial?

Thanks for reading, and your guidance!

bob prohaska

Reply to
bob prohaska

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formatting link

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

.... Phil

Reply to
Phil Allison

Thank you, that's a good explanation.

bob prohaska

Reply to
bob prohaska

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