Author Topic: How to choose capacitor that have the same price?  (Read 337 times)

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Offline drakejest

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How to choose capacitor that have the same price?
« on: July 17, 2021, 01:31:16 pm »
Assuming a ceramic capacitor with the same Temp coefficient and the price is very close together, how would i choose capacitors? for example these two capacitors.

Samsung Cap vs YAGEO cap.

I am using them as general decoupling, Is lower ESR the way to go? Also why cant i find their ESR in the datasheet arent those important information?
« Last Edit: July 17, 2021, 01:47:21 pm by drakejest »
 

Online Siwastaja

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Re: How to choose capacitor that have the same price?
« Reply #1 on: July 17, 2021, 02:02:31 pm »
All normal MLCCs are very low ESR. Indeed it isn't usually listed, similarly ripple current rating is missing. This is maybe because ESR is considered often "so low" it doesn't matter; people work with rules of thumb like a certain sized part being 10mOhm, but often you just assume zero. But the fact is, there are some applications where it would be interesting to know; example would be a converter which transfers energy through the series capacitor, like a high-power SEPIC or Cuk.

Unless you need special features like soft termination / automotive grade, the #1 selection parameter would be the actual capacitance available under DC bias. Forget all incorrect and stupid rules of thumbs like X7R being much better DC-bias wise than Y5V. There are X7R parts that may lose 85% of their capacitance under nearly the rated voltage!

So if you really need, say, 1uF, now the part choice could be a 2.2uF part, or a 4.7uF part.

Some manufacturers do not publish the DC bias curves at all. Then all you can do is guess based on another component in similar volume package (footprint; but don't forget the height!) and similar ratings.

When actual capacitance and price matters, I like to create a spreadsheet where I calculate $ / actual uF, then pick the solution giving the best bang for buck. I may try different parts from different manufacturers here, but also different footprints, and different numbers of paralleled parts.
« Last Edit: July 17, 2021, 02:04:15 pm by Siwastaja »
 

Offline drakejest

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Re: How to choose capacitor that have the same price?
« Reply #2 on: July 17, 2021, 03:01:47 pm »
All normal MLCCs are very low ESR. Indeed it isn't usually listed, similarly ripple current rating is missing. This is maybe because ESR is considered often "so low" it doesn't matter; people work with rules of thumb like a certain sized part being 10mOhm, but often you just assume zero. But the fact is, there are some applications where it would be interesting to know; example would be a converter which transfers energy through the series capacitor, like a high-power SEPIC or Cuk.

Unless you need special features like soft termination / automotive grade, the #1 selection parameter would be the actual capacitance available under DC bias. Forget all incorrect and stupid rules of thumbs like X7R being much better DC-bias wise than Y5V. There are X7R parts that may lose 85% of their capacitance under nearly the rated voltage!

So if you really need, say, 1uF, now the part choice could be a 2.2uF part, or a 4.7uF part.

Some manufacturers do not publish the DC bias curves at all. Then all you can do is guess based on another component in similar volume package (footprint; but don't forget the height!) and similar ratings.

When actual capacitance and price matters, I like to create a spreadsheet where I calculate $ / actual uF, then pick the solution giving the best bang for buck. I may try different parts from different manufacturers here, but also different footprints, and different numbers of paralleled parts.

So basically in my situation get the cheaper one , even though i said price is very close because for general decoupling it does not matter which one i pick. Brand preference might come to play sadly i do not have enough experience to have one. If i really do want to know which one is better i need to have a few of them to test them out.
 

Offline TimFox

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Re: How to choose capacitor that have the same price?
« Reply #3 on: July 17, 2021, 03:31:12 pm »
Does the manufacturer of the cheaper part publish data on the voltage dependence of the capacitance?  This is not a trivial factor for the popular dielectrics in high-capacitance MLCCs.  The voltage rating on the part is merely a guarantee that the part will not smoke at that voltage, but the drop In capacitance can be substantial.  The dielectric material (e.g., X7R or Z5U) has its own characteristics in terms of voltage gradient (E-field) through the layer, but manufacturers have different construction details from each other and from rating to rating.
By the way, sometimes ceramic capacitors have a specified Q or D=1/Q factor, a function of frequency, from which you can calculate ESR.
« Last Edit: July 17, 2021, 03:33:14 pm by TimFox »
 

Online Siwastaja

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Re: How to choose capacitor that have the same price?
« Reply #4 on: July 18, 2021, 07:12:26 am »
In general-purpose DC power link decoupling, capacitance is usually irrelevant as long as you have enough, and typical suggested values already have ample capacitance margins. For example, a digital CMOS IC where 100nF is recommended per power pin may actually work exactly the same with anything in excess of 5-10nF. So if one 100nF MLCC is 30nF at the DC bias and another is 60nF, it doesn't matter. (Regarding regulator IC output caps, the requirements for loop stability are more important, and recommended capacitance value should be respected. If they recommend a "2.2uF MLCC", then they assume it has some amount of DC bias effect, likely that actual 1uF is enough, but you can still get in troubles if you pick the absolute worst offender 2.2uF part.)

What actually matters is parasitic inductance, and I have good news for you: because all MLCCs use the same physical construction pattern, the ESL is only dependent on the footprint size and your layout (how many vias, and how close, you use to get to the ground plane, etc.), so a 0402 is better than 1206, but the manufacturer does not matter.

In large sizes (1206 and up), reliability things like soft terminations start to matter.

Both Samsung and Yageo are known brands. Just buy whichever is available at the best price.

What's your intended application for the 330nF parts? I.e., by "general decoupling", do you mean these go in the Vdd pins of some bog standard CMOS chips like microcontrollers, logic gates, maybe opamps? If this is the case, the best way to really optimize the design is to go to a smaller package instead of comparing manufacturers. Though, don't get me wrong, 0805 is perfectly fine.
« Last Edit: July 18, 2021, 08:43:20 am by Siwastaja »
 


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