Capicitors for the buck convertor

[naseerak]

Hi
I want to design a buck converter from 42v to 28v I have calculated the input and output capacitors but fear that i would not be able to find low ESR caps in my local stores so I want to parallel many capacitors in the order of 4 or 6 and also want to increase the capacitance for low ripple, I want a suggestion here.

 

[PA4TIM]

Depends on how much current it should be able to supply. More parallel adds selfinduction and do not forget the resistance of the extra traces but it is possible. I do it sometimes but first measure ESR, impedance and selfresonance. Most times one or two caps and a LC filter works fine

 

[naseerak]

Thanks for the input but can you tell me any rule of thumb for this additional capicitances.

 

[PA4TIM]

You need the capacitance you calculated and that depends on current and frequency. So does the ESR. At 100 kHz skin effect and dielectric loss will enlarge ESR more as at 15 kHz but ESR is most times higher at 15 kHz, and there are things like self resonance, self inductance.
Just order the caps advised in the datasheet or experiment with what you have. I assume it is hobby use so if it goes wrong, start over ;-)
But be carefull if you are talking about high currents. You have not told how much current.
Strange value from 48 to 24V, something with (car) batteries ?

 

[naseerak]

No these are solar panels feeding the batteries.

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The current is around 30A

 

[PA4TIM]

I would take no risk with 30A and buy good low ESR caps, do a real good pcb desing and most important an inductor suitable for this current and your frequency.
And look in the datasheet of the caps of it can hansle the di/dV you need . Do you have experience with smaller or simmilar big power smps design and building ?

 

[naseerak]

Unfortunately the caps available here have no datasheets with them.

 

[PA4TIM]

That is most times not a good sign. Most times Chinese crap. Can you not buy from digikey, Farnell, mouser or some other serious supplier ? If not, just measure their behaviour at the working frequency. Do you have a VNA or impedance analyser or a very good LCR meter ?
If not, buy a decent converter or just try your homebuild one and hope for the best, monitor ripple current with your scope and temperature of caps and wear some protection while testing. If good caps for converters are hard to get where you live than it supprizes me they do sell suitable inductors.

but like I asked before, do you have some experience in building converters ?

 

[naseerak]

Yes, I have some experience in building some offline smps but for such high power buck this is my first time, however I will try to nullify the effect of ESR by paralleling caps and will enhance my pcb tracks with soldering a copper wire over the copper tracks which are supposed to carry the most of the current.