SMPS filter cap choise

[ElectroWarrior]

Hey guys, it my first post on this forum!
I am making a very simple buck converter to charge phones and such - the point behind the project is to make it as cheap as possible.
There are some data on the project:
- Converts 9VDC into 5VDC
- Output current 500mA
- Switching frequency 100kHz
- RMS ripple current estimated to be 30mA
- Max voltage ripple: 40mA
- Max cap ESR 0.5Ohm

To achieve the desired ripple i need a cap at minimum 33uF. I have been reading about filtercaps, and there is a lot of different advises out there. As I understand i only got aluminium electrolytic caps and tantalium to choose from.. I would prefer to use a aluminium due to the price, however i dont know how they respond at 100kHz frequency, and the further harmonics. Any advise or ideas?

[ddavidebor]

They've a datasheet you can read

[nctnico]

I'd recommend using SMT MLCC ceramic caps. 10uf is very affordable and results in a circuit which will last for a long time. A problem may be that the ESR of an MLCC ceramic cap is too low for the SMPS chip. Check the datasheet for information on which caps are recommend. I'd look into a different chip if it needs tantalum or electrolytic caps.

[ElectroWarrior]

Thank you very much for the replies!

They've a datasheet you can read

Not many datasheets on cheap electrolytic caps has much frequency information - or maybe i dont know how to read them right:)
Regarding having multiple MLCC in parallel is a very good idea - Thank you! I looked into it, and it is actually quite cheap.
I have never heard that buck controllers are filter cap ESR dependent.. could you possible elaborate?

[Psi]

I'd recommend using SMT MLCC ceramic caps.  A problem may be that the ESR of an MLCC ceramic cap is too low for the SMPS chip.

He can always add an external 0.1R resistor to the cap

[Niklas]

Check the datasheet of the Buck converter to see if there is some information about input and output capacitors. Some converters like more ESR on the output and some like less ESR. As Psi wrote, you can also add a series resistor to a low ESR capacitor to increase the amount of ESR that the Buck converter sees.

If you go with ceramic capacitors, use X7R or something similar dielectrica. Y5V is not a good option.

[ElectroWarrior]

awesome! thank you.. I checked the datasheet, and there is no information about the ESR, however its also a very simple switcher.
ill test it with and without a resistor.
I choose a X5R, should be quite stable over 0-50 degree temperature range need:)

Thanks for great advise!