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26th May 2007, 11:50 PM  #1 
diyAudio Member
Join Date: Mar 2006
Location: croatia

PFC question
LETS presume we theoreticly have two PSU whit the SAME output power: one has PFC another has not
SHALL power consumption clock in my home measure MORE consumption using NO PFC power supply? And another question: WHAT MAKES SMPS capacitive or inductive load instead "resistive" load : it has only rectifier at the imput ? Can someone explan that 
27th May 2007, 04:15 AM  #2 
diyAudio Member
Join Date: Mar 2007

The (active) PFC would act as a preregulator so the power supply with PFC should regulate better, everything else equal. And since the main converter operates with a more carefully controlled input, it can run at its optimum input voltage more of the time. Not sure if it's enough to offset the losses in the PFC itself, but most of the high efficiency power supplies include it.
I remember about a power supply with PFC drawing less than half the current of an otherwise identical power supply without PFC while powering the same load. 
27th May 2007, 05:10 AM  #3 
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Join Date: Dec 2001
Location: SIUE, Illinois, USA

this all comes down to tigonometry.
the average value of any sine wave is zero, unless the frequency is 0hz. the product of two sine waves will create terms at the sum and difference of the frequencies. thus if the voltage contains only a 60hz component, only the 60hz current will contribute to power. (as the product of the two waves will create a 120hz and a 0hz component of power) using this same line of reasoning, you can also find the phase shift of this fundamental component of current from the fourier transform of the current waveform. the actual SMPS is sometimes modeled as a constant power load (eg, drawing more current if voltage is reduced). The diode + cap combo just has bad power factor. this is clear when you try to calculate the RMS value of the current waveform. there is a high degree of harmonic content which contributes to the RMS current, even though only the 60hz component is used.
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27th May 2007, 05:45 AM  #4  
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Re: PFC question
Quote:
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27th May 2007, 10:33 AM  #5  
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Join Date: Mar 2006
Location: croatia

Re: Re: PFC question
Quote:
So it means power consumtion clock mesures ONLY EFFECTIVE POWER? and no difference if neglect wire looses: are you 100% sure? 

27th May 2007, 10:40 AM  #6  
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Join Date: Mar 2006
Location: croatia

Quote:
Thanks, but do maybe someone has detailed mathematical explanation how diode+cap can act as inductive or capacitive load ? I mean what does PFC concrete in the SMPS way, or classic diode+capacitor recitfier imput Are greatz + capacitor inductive or capacitive load and does PFC capacitive or inductive: I saw that SMPS with passive PFC have inductive PFC so it means SMPS itself is capacitive load WOULD greatz without capacitor be capacitive load ? 

27th May 2007, 02:51 PM  #7 
diyAudio Member

Yes, your "power consumtion clock" measures effective power although they are usually protected to avoid running backwards.
Diode bridge + capacitor is a nonlinear load and is neither inductive nor capacitive. The fundamental is almost in phase with the voltage, but there are plenty of out of phase harmonics. Anyway, the problem here is the higher rms and peak current required given the same power level. Passive PFC is intended to tame the harmonics and draw more power at the fundamental frequency. Active PFC simulates a resistive load.
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27th May 2007, 02:58 PM  #8 
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Join Date: Dec 2001
Location: SIUE, Illinois, USA

again, power factor is defined as the ratio of actual power to apparent power.
no actual power is transmitted, except at 60hz (50hz where applicable). Yet all of the harmonics will count towards RMS current. thus RMS voltage * RMS current will be higher than actual power. At the same time, we can look only at the voltage and current waveforms into the circuit. the voltage is assumed to be a pure sine wave, but the current is a series of pulses (as no current flows until the diode is forward biased). The fourier transform can be applied to this wave to show what phase shift exists for the fundamental 60hz component. lets look at one half of a sine wave cycle to see if we can make sense out of it: * Starting from a voltage input of 0V, the capacitor is discharging and the diode is reversed biased. * the voltage increases until it it equal to the voltage that the capacitor has decreased to . at this time, the diode will begin conducting * the source will force the capacitor to charge, and a current equal to C*dv/dt will flow (in addition to some current to the actual load) * finally the peak is reached, the cap is fully charged. * now the voltage decreases at a rate faster then the cap discharges. the diode will stay reversed biased for the remainder of the half cycle. The key is that the highcurrent peak occurse on the leading edge of the sine wave. unsure what "greatz" is. The active PFC is really more concerned about removing the distortion from the current then with shifting the phase of the input. it is not a simple inductor approximation. adding seriese inductace to the diode+capacitor network will filter the harmonic currents to a higher degree, and will improve power factor.
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27th May 2007, 03:06 PM  #9 
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Join Date: Jun 2004
Location: Warsaw

On the one hand it is as simple as a principle of conservation of energy, on the other some power consumption clocks can be fooled with a capacitive load, especially the older ones, which do not measure a true effective power.

28th May 2007, 12:21 PM  #10  
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Join Date: Mar 2006
Location: croatia

Quote:
It makes sense now How can inductance in series reduce those current peaks ? maybe it works like ballast and reduces current.. wont those peaks make HV pulses on the inductance in series ? Can SMPS work on the imput that has not capacitor in parallel just diode rectifier? 

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