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Why a multiplier in a PFC control IC?

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eem2am

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Hello,

I am wondering about doing an offline flyback pfc....eg...

http://www.st.com/stonline/products/literature/an/5956.pdf

-that uses the following PFC control IC...

http://www.st.com/stonline/books/pdf/docs/5109.pdf

-The thing is, can't i just use a standard current mode PWM control IC such as...

UC3842:-
http://focus.ti.com/lit/ds/symlink/uc3842.pdf

I appreciate that the L6561 has a multiplier in it and the UC3842 does not.

but, if my application is for a constant output load then isn't the multiplier a waste of time?
 

FvM

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A PFC controller has to achieve an input voltage proportional (sinusoidal) input current. There a different methods, a multiplier with an input voltage derived reference waveform is a straightforward way, but there are other options, consult the manufacturer datasheets and application literature. It may work with a standard current mode controller, I didn't think about it yet
 

eem2am

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thankyou,

yes because i know that a flyback, if put after a FWB with no bus capacitor, it will naturally draw more current as the bus voltage raises, and the result woul dbe pfc....especially if the feedback control loop network was low..........so i am wondering why they need the multiplier?
 

FvM

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Yes, some simple PFC controllers are using a similar technique and no multiplier.
 

snafflekid

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To get high PFC, the current draw from the power source needs to be synchronized to the sinusoidal voltage waveform. If you multiply the sinusoidal voltage input waveform by the current feedback of the switching converter, the current feedback signal becomes proportional to the voltage waveform, the loop controls the switching to create an average sinusoidal current draw.

Or you can have another system consisting of a front converter that draws more current proportional to the input voltage, creates a dc voltage and then that gets converted to the output voltage.
 

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