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Alain Stas
Alain Stas
11/15/2018 11:18:31 AM
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Newbie
Re: Ceramic Cap Models
Hello Doug

In fact, during the simulation, the user can measure the current passing into and the voltage across the capacitors.

Also the tolerances tolCmin and tolCmax are independently tunable.These are respectively the minimal and maximal tolerances on the capacitor. So if at 0 Hrs you have a tolerance of +/-10% , you can indicate tolCmin= -10 and tolCmax=+10 (if you don't, it's already implemented into the modelsanyway) . But for a drift between -5% and -10 % during the lifetime, then it will be worth to perform the simulation while writing tolCmin =-20 and tolCmax = +5 or any other limits given by the manufacturer.

By the way the models presented in this article will be uploaded on the Vishay website soon but they are availble on simple mail request at edesign.ntc@vishay.com.

I like your last sentence: the devil is always in the details.......Thanks for your reaction.

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Doug.Leeper
Doug.Leeper
11/14/2018 6:06:16 PM
User Rank
Newbie
Ceramic Cap Models
It would be nice to see the common DC Bias (drop in capacitance when DC is on the capacitor), value drop in the model.

Also lacking from most models is how the caps reduce in value as they age.

The difficult thing, is as they have pushed to high C*V values in a given size, they have gone to 5um ceramic layers, and the DC Bias you see in a particular series, like X7R, can vary *quite* a bit between suppliers. Some at 50% of the DC rating drop by 80% of their value, others only 20%.  Each has different "doping" of the ceramic with various elements.

Conversely, Reliabilty would be good to see in a model.  If you dig down to an individual datasheet for a specific part, instead of just the family series sheet, you will see that instead of the standard 200% to 300% applied voltage for the Reliability- Durability or also called Long Term High Operating Temp. So, basically- like with an X5R you put 200% to 300% of it's working voltage on it and run it at the characteristics maximum temp, so X5R you would use 85 degrees C, for 1000 hours, and you have to pass a number of parameters.

However, all the vendors have had to drop the overvoltage value, many are down to 100% of the working voltage for this test.  And there are some that are intended for Mobile applications, that not only do this, but then have a chart, and depending on your voltage and temp, you will only get a 3 year life out of the part before 1% will fail.

Anyhow it would be nice to see some of this in the models.

One needs to really dig down in the datasheets for the parts these days, lots of cutting corners to worship at the Altar of BOM.  The devil is always in the details....

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