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PauloPT 14th November 2011 03:34 PM

Small 5mm 0.1uF caps - what's best?
 
Hi,

I have a board to fit with small 5mm pitch 0.1uF caps. These caps bypass decoupling electrolytic caps. Is there a better choice than Wima MKS2 ?

Thanks,
Paulo.

Elvee 14th November 2011 07:51 PM

Quote:

Originally Posted by PauloPT (Post 2781319)
Hi,

I have a board to fit with small 5mm pitch 0.1uF caps. These caps bypass decoupling electrolytic caps. Is there a better choice than Wima MKS2 ?

There is a better choice: thin air. As is shown in this thread, amongst others is that paralleling capacitors results most of the times in degradation.
If paralleling is actually needed (doubtful), it is a complex job, and requires multiple elements with scaled damping resistors.
If you're not prepared to go to such lengths, the simplest option is to bypass it (!)

simon7000 14th November 2011 07:56 PM

Quote:

Originally Posted by PauloPT (Post 2781319)
Hi,

I have a board to fit with small 5mm pitch 0.1uF caps. These caps bypass decoupling electrolytic caps. Is there a better choice than Wima MKS2 ?

Thanks,
Paulo.

good choice

nereis 15th November 2011 01:57 AM

I understand paralleling small film caps with large electrolytics is not the best idea, but with the common rail to rail bypass caps, I suppose those wouldn't be of any issue since there are usually no lytics across the rails?

marce 15th November 2011 11:59 AM

Just out of curiosity what are you going to bypass with these caps, as I would have thought by the time you take into account parasitic inductances they would be useless for decoupling high frequency noise.

Elvee 15th November 2011 02:33 PM

Quote:

Originally Posted by nereis (Post 2782047)
I understand paralleling small film caps with large electrolytics is not the best idea, but with the common rail to rail bypass caps, I suppose those wouldn't be of any issue since there are usually no lytics across the rails?

No but there are two in series ==> double the inductance.
If a low-impedance path (one or more capacitors) exists between two nodes, adding a capacitor will close the loop and create resonances.

PauloPT 15th November 2011 05:10 PM

Quote:

As is shown in this thread, amongst others is that paralleling capacitors results most of the times in degradation.
Thank you for sharing the link. Itīs a very interesting thread with scientific data to back it up! :)

But now I'm confused about this: In PSU I use to put two 100nF just after the rectifier (1 per rail) with the purpose of filtering noise and 2 more after the capacitor bank. I'm I correct to assume it's better to get rid of all the 4 caps?

rvrazvan 15th November 2011 05:22 PM

russian teflonS! :))

simon7000 15th November 2011 05:32 PM

Quote:

Originally Posted by PauloPT (Post 2782844)
Thank you for sharing the link. Itīs a very interesting thread with scientific data to back it up! :)

But now I'm confused about this: In PSU I use to put two 100nF just after the rectifier (1 per rail) with the purpose of filtering noise and 2 more after the capacitor bank. I'm I correct to assume it's better to get rid of all the 4 caps?

No. There is enough loss in the PC traces to spread out the potential resonances in all but badly designed cases.

AndrewT 15th November 2011 05:36 PM

I have seen recommendations to use the higher esr MKS rather than the lower esr of MKP.
I have also read that the lossy non C0G caps can better be used due to the damping of the lossy element.
Certainly use of resistor damping is fundamental to Hagerman's method.
Who's the digital/switching expert on the Forum? He did a long and detailed posting of test results for oscillation due to paralleled capacitances.

But absolutely not teflons alone.


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