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PFC for 15W offline LED bulb

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treez

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Hello,
We are doing a 15W flyback with extremely minimal post mains rectifier capacitance. Also, we have made the feedback loop very slow.

The mains current looks well power factor corrected until we add the LC line filter......with this LC filter added, the line current no longer looks as well power factor corrected.

Is their a rule with LC line filters for SMPS's that have high power factor, does the total phase shift through the filter have to be zero degrees, ...and so do we need several LC stages?
 

mtwieg

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I don't see how a LC filter being added could degrade the PF, so long as its output impedance is low. You're monitoring AC voltage at the output of the LC filter, correct? Can you post a schematic?
 
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treez

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here is the schem without the LC filter..

its 15W and vout = 40V.
 

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  • pfc bcm _LT3799.pdf
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mtwieg

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Sorry for the late reply.

So is this behavior observed in the simulation, the actual circuit, or both?

Here are a couple things to check:
1. Does the shape of the pre/post rectifier voltage change much?
2. Does the error amplifier output (input to the mixer) have any increased signal content above ~10Hz?
 

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