Electronics > Beginners
Polarized caps in lieu of non-polarized?
eamoex:
Hi All,
I need to add a positive DC — positive DC converter to my circuit. Traco Power's TRS3-1250 datasheet shows a filtering scheme using non-polarized caps (fig. 1 below, Positive output cuircuit). My local supplier only has CMS versions of non-polarized caps for the specified values, and I can't do CMS. Can I use polarized, electrolytic caps here? If yes, where do I connect the negative side of those caps? Thanks for your help.
schmitt trigger:
The reason they show non-polarized caps, is to obtain such a low ESR value, the only caps that will consistently meet it are MLCC devices. Which are non polarized of course.
Having said this, you could also parallel several polarized tantalums and achieve the required capacitance and ESR. this would be perfectly fine,
Tantalums a few years ago gained a poor reputation due to the "blood tantalum" controversy. Capacitors which were certified to be "human right abuse-free" had their prices skyrocket. The unfortunate reason is that most of the World's tantalum supply comes from countries with notorious human rights abuses.
Siwastaja:
The fact they have not shown the polarization marks doesn't mean the caps can't be of polarized type - it's just a generic capacitor symbol.
It's not a question of being polarized vs. non-polarized, it's a much broader question to capacitor types and their pros and cons.
If you want to follow this appnote blindly, an aluminum polymer electrolytic capacitor might be the most sensible choice for fig1 C1, satisfying the low ESR condition. An MLCC bank would be massive, especially if you try to guarantee even close to the specified capacitance at the actual DC bias - you'd need over 1000uF total of MLCC there!
Their PI filter looks like it's simulated (worse), or prototyped (better) with some specific parts, but they fail to give the part numbers. As it stands, it's not very usable. You have a risk of creating ringing or amplifying noise at some frequencies, having no lossy elements in the filter.
Don't design in an tantalum unless you know what you are doing. Ignoring the political discussion, they have two massive engineering traps and people seem to consistently overlook at least one even when they think they know the stuff. A tantalum won't work in any of the positions shown anyway.
schmitt trigger:
I know one of them is that they can easily fail catastrophically;
What would be the other one?
GerryR:
In both figures 1 & 2, the "+" side goes toward pins 2 and 4, "-" side to pin 3. I think that was your original question.(?)
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