PFC boost converter.

Thread Starter

electr

Joined May 23, 2009
49
Hey,
I wanted to ask 2 questions please regarding this SMPS which contains power factor corrector.

1.
It is said in the article that the boost converter must meet with the condition that the current drawn from the line at any given instant must be proportional to the line voltage.
Why is that?

2.
It is also said that having the boost preconverter voltage higher than the input voltage forces the load to draw current in phase with the ac main line voltage that, in turn, rids harmonic emissions.
Could someone please thow light on this matter, how it brings the current and voltage to be in phase?

Thank you very much.

 

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Wendy

Joined Mar 24, 2008
21,840
Where did the illustration come from? Context is needed for the answer.
 

mik3

Joined Feb 4, 2008
4,846
It is said in the article that the boost converter must meet with the condition that the current drawn from the line at any given instant must be proportional to the line voltage.
Why is that?
It means that the line voltage and current are in phase and thus their ratio is a real number and not a complex number.
 

DrNick

Joined Dec 13, 2006
110
This circuit controls the current flowing in the inductor (and thus the current being drawn by the converter) by switching very quickly. Because the inductor current can not change immediately, one can switch the MOSFET very quickly computing any waveform within the ability of the inductor. SO, you can look at the voltage waveform, and create a current waveform that matches it.

It is desirable to have current in-phase (and have the same frequency...) because:

1. power = voltage X current.

if you have current being drawn as a cosine wave, and your voltage is a sine, there will be no power being transferred to the circuit. The power in this case will be circulating back and forth between the source and load. This give and take that happens from the source and load produce losses in the transmission lines = costs the power company money...

2. Power is only transferred if the frequency of the current matches the frequency of the voltage. If you are unfamiliar with fourier analysis, this topic may be a bit tricky. Therefore I will just say, you lose power by drawing current waveforms that are not of the exact same shape as the voltage waveform. When current and voltage are not of the same shape, losses are produced in the transmission line. In addition to losses, drawing strange current waveforms can cause distortions in the voltage waveform. This will start to degrade the power quality for other users of the supplied voltage....

In short, PFC is used make the voltage used and the current drawn have the same shape and phase to reduce disturbances and losses in the line providing the power. The current and voltage are matched by controlling the current in the inductor via pulse width modulating. Sorry if I am sounding repetitive...it's late hehe.
 
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