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

[ankitvirdi4]

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?

 

[mtwieg]

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.

 

[E-design]

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

 

[ankitvirdi4]

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.

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?

 

[E-design]

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.

 

[E-design]

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.