FEDEVEL
Platform forum

Imad_RF , 10-07-2024, 10:26 AM

Hi, I'm designing an audio SMPS that has an auxiliary supply rails of plus and minus 15V, the idea is simple, the output of the transformer is connected to a full bridge rectifier built with *ES1D * ultra fast recovery rectifiers, afterwards the voltage is going to a post-regulator LDO, specifically the LM317 & LM337 Adjustable voltage regulators.

I'm wondering about what type of capacitors I should use for Cadj.
MLCC? Tantalum? lower or higher ESR is prefered? Also, does adding a ferrite bead somewhere help reducing the ripple even further? on the Input/output or in the feedback path?

Thanks!

QDrives , 10-07-2024, 07:52 PM

Class II ceramic capacitors (X5R, X7R, Y5V, etc) are piezo electric. So I would not use them. Class I (NP0/C0G) are no problem, but only have low capacitances.
If you want to reduce noise, make both the input and output capacitors a lot bigger than shown in the picture.
Additional filtering, if used, should be on the input.

Imad_RF , 10-07-2024, 08:25 PM

Thank you for your answer, so you suggest using tantalum capacitor for Cadj for example?

Imad_RF , 10-07-2024, 08:25 PM

And what about ESR? do we prefer to have it high or low for this specific capacitor?

QDrives , 10-07-2024, 08:51 PM

I would go for (relative) low ESR. Either tantalum or aluminium polymer (hybrid).