[SOLVED] Significance of Pi Filter in SMPS Power supply.

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ankitvirdi4

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Hello all,




In the above schematic can somebody explain why C1 C2 R2 and L1 are used? I understand they are in a Pi filter configuration but why is a Pi Filter used there? Can a LC filter or Just an electrolytic capacitor be used?

If yes, what all considerations should be made while designing the said filter?
 

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It's likely to just provide some extra filtering against the very high frequency EMI conducted from the flyback transformer. How it's designed depends on what frequencies you're trying to attenuate; you would likely select the bead such that its real impedance is high (much lower than C9) at such frequencies.
 
It is an input filter to reduce EMC to meet compliance requirements. Normally, you will design the filter after measuring the attenuation required at a certain frequency to meet a specific emission's standard.

So after doing a pre compliance measurement with an LISN box, you will know which frequencies need attenuation to meet the standard.
https://www.youtube.com/watch?v=RgQ4ebegjuc
 
E-design said:
It is an input filter to reduce EMC to meet compliance requirements. Normally, you will design the filter after measuring the attenuation required at a certain frequency to meet a specific emission's standard.
Click to expand...

I have no compliance requirements and I want to built a low cost power supply so can I remove this part form the design? Or filtering is important after the bridge rectifier?
 

You can drop the inductor, but you will still need the mains filter capacitance. You could use a single 10uF capacitor with the correct rated voltage.
 
You must decide how much ripple you want on the input. With the circuit shown and taking a worst-case efficiency of 70% you will have best about 11 V of ripple using a 10 uF capacitor. This could be a bit more depending on the ESR of the capacitor you use.
 

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