Author Topic: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor  (Read 16177 times)

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Offline ketil b

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Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« on: October 27, 2013, 09:25:12 am »
Hi all

Just found this via reddit and thought it might interest sume of you

http://www.maximintegrated.com/app-notes/index.mvp/id/5527
 

Offline DutchGert

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #1 on: October 27, 2013, 09:57:04 am »
Something even seasoned engineers often not realise :).

A very quick and dirty solution could be to always use 50V rated ceramics in your lets say <10V circuits. A 50V ceramic at 5 or 3.3V is very close to its rated capacitance. Saves u a lot of datasheetreading and u dont have to specify every ceramic in detail in the BOM.

 Ofcourse goodluck finding your 0402 variants ;)
« Last Edit: October 27, 2013, 10:00:01 am by DutchGert »
 

Offline Kjelt

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #2 on: October 27, 2013, 10:09:09 am »
Yes this is quite a problem if you are not aware of this, that is why I suggested it as a subject for a fundamental friday:
Maybe designing and choosing the right components (which have their quirks), for instance like a standard ceramic X7R vs NPO SMD capacitor, capacitance derating (Voltage and Temperature Coefficient, last year this got even the pros at our company in trouble.
 

Offline tszaboo

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #3 on: October 27, 2013, 11:23:24 am »
Once I was measuring 96% loss:
http://nandblog.com/avoid-y5v-capacitors/
Former username: NANDBlog
 

Offline mzzj

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #4 on: October 27, 2013, 12:25:22 pm »
Once I was measuring 96% loss:
http://nandblog.com/avoid-y5v-capacitors/
Plenty of people already know to avoid Y5V capacitors but don't really realize how bad X5R and X7R can also be, ie. -90% of the rated capacity for small high-capacity X5R or X7R caps.  :scared:
 

Offline nukie

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #5 on: October 27, 2013, 01:08:14 pm »
Newbie trap. Theres more to study about capacitors.
 

Offline jmaja

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #6 on: October 27, 2013, 01:20:45 pm »
A very quick and dirty solution could be to always use 50V rated ceramics in your lets say <10V circuits. A 50V ceramic at 5 or 3.3V is very close to its rated capacitance.

It's not really about voltage rating.  In some cases the lower voltage rating one can have higher capasitance at 5 V than a higher one. Voltage spec is about capacitance durability not about staying close enough to it's rated capacitance.

Best choise is to look at the specs (not typically in the datasheet) and choose big enough size. 22+ uF at 5 V you really need to pick 1210. Requiring 50 V makes you choose a bigger package, but then you can select the lower voltage version of that package as well.

I always look the DC biases for critical caps from these:
http://www.tdk.co.jp/ccv/step1.asp
http://www.kemet.com:8080/webspice/
http://ds.murata.co.jp/software/simsurfing/en-us/index.html (this one is the easiest for comparisons)

One X5R 0805 22 uF 6.3 V is ~6 uF at 5V and the 25 V version is 10 uF
One X5R 1210 22 uF 6.3 V is ~17 uF at 5 V and 25 V version 20 uF

So 1210 6.3V is a much better choise than 0805 25V for 5V DC bias.
 

Offline SeanB

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #7 on: October 27, 2013, 01:23:34 pm »
And if you are dealing with high voltages then dielectric absorption can really be nasty.
 

Offline DutchGert

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #8 on: October 27, 2013, 02:15:47 pm »
A very quick and dirty solution could be to always use 50V rated ceramics in your lets say <10V circuits. A 50V ceramic at 5 or 3.3V is very close to its rated capacitance.

It's not really about voltage rating.  In some cases the lower voltage rating one can have higher capasitance at 5 V than a higher one. Voltage spec is about capacitance durability not about staying close enough to it's rated capacitance.

Best choise is to look at the specs (not typically in the datasheet) and choose big enough size. 22+ uF at 5 V you really need to pick 1210. Requiring 50 V makes you choose a bigger package, but then you can select the lower voltage version of that package as well.

I always look the DC biases for critical caps from these:
http://www.tdk.co.jp/ccv/step1.asp
http://www.kemet.com:8080/webspice/
http://ds.murata.co.jp/software/simsurfing/en-us/index.html (this one is the easiest for comparisons)

One X5R 0805 22 uF 6.3 V is ~6 uF at 5V and the 25 V version is 10 uF
One X5R 1210 22 uF 6.3 V is ~17 uF at 5 V and 25 V version 20 uF

So 1210 6.3V is a much better choise than 0805 25V for 5V DC bias.

I know its not about voltage rating :). But a lot of times u don't want to specify all of your C's in detail and let procurement decide what is going to be on your board. Also, you want to use as much of the same to keep your BOM small and your production process fast.
Thats why I said its a quick and dirty way of working aroudn this problem. If you simply pick a high rated version of lets say 50V for lyour 5/3.3V circuits you may not have the best compromise between rating, size and real capacitance but it saves u a lot of time designing and gives procurement a lot more freedom. The fact that u use a higher. Voltage rating automatically ensures the bigger physical size thats it about.
 

Offline ElectroIrradiator

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #9 on: October 27, 2013, 02:56:13 pm »
Something even seasoned engineers often not realise :).

A very quick and dirty solution could be to always use 50V rated ceramics in your lets say <10V circuits. A 50V ceramic at 5 or 3.3V is very close to its rated capacitance. Saves u a lot of datasheetreading and u dont have to specify every ceramic in detail in the BOM.

The data in the linked article contradicts this. The only factors involved seem to be the physical size of the capacitor, the manufacturer and the dielectric used. The curves for caps with the same capacitance and physical size yet different maximum operating voltage overlaps perfectly.

So using a 50V cap on a low voltage rail accomplishes nothing.

 

Offline Rufus

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #10 on: October 27, 2013, 03:19:57 pm »
The curves for caps with the same capacitance and physical size yet different maximum operating voltage overlaps perfectly.

1210 does not specify physical size. The 3rd dimension is required and it can vary over about a 5:1 range.
 

Offline DutchGert

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #11 on: October 27, 2013, 03:21:18 pm »
Something even seasoned engineers often not realise :).

A very quick and dirty solution could be to always use 50V rated ceramics in your lets say <10V circuits. A 50V ceramic at 5 or 3.3V is very close to its rated capacitance. Saves u a lot of datasheetreading and u dont have to specify every ceramic in detail in the BOM.

The data in the linked article contradicts this. The only factors involved seem to be the physical size of the capacitor, the manufacturer and the dielectric used. The curves for caps with the same capacitance and physical size yet different maximum operating voltage overlaps perfectly.

So using a 50V cap on a low voltage rail accomplishes nothing.



There are no 50V ones in the graph.
What I mean is that if u take a 50V one u automatically end up with a physical bigger cap that probably has about the same real capacitance at 3.3 or 5V as what u want it to have.
Ofcourse u can find better ones if u look at the data but often u just want to be able to say to procurement "just place any A-brand 10uF 50V 1210 u can buy and it will be fine".

 

Offline ElectroIrradiator

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #12 on: October 27, 2013, 03:33:36 pm »
1210 does not specify physical size. The 3rd dimension is required and it can vary over about a 5:1 range.

Footprint then.

Still trying to find enough datasheets with the relevant info on them to check for this.
 

Offline free_electron

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #13 on: October 27, 2013, 03:53:22 pm »
Here is another one: a freshly soldered ceramic cap decays at a rate of 1% the first hour, 1% the next day, 1% the next month and a 1% the next 2 years (it's a 1:25 ratio... 24 hours a day , 24 days a month , 24 months : 2 years).

Heating the dielectric above 148 degree centrigrade resets the decay process !

So: you can't make frequency stable oscillators with these suckers ! Worse. If you got the frequency spot on , don't reheat the cap : it'll reset !
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Offline jmaja

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #14 on: October 27, 2013, 04:24:59 pm »
What I mean is that if u take a 50V one u automatically end up with a physical bigger cap that probably has about the same real capacitance at 3.3 or 5V as what u want it to have.
Ofcourse u can find better ones if u look at the data but often u just want to be able to say to procurement "just place any A-brand 10uF 50V 1210 u can buy and it will be fine".

Is there some standard for that or does it just happen to be so? You can find many 35 V caps, which give poor performance at 5V DC (e.g. 0805 10 uF 35V is only about 4 uF at 5 V), but 50V caps seem to be clearly better (=bigger footprint).

If you compare all the X5R/X7R 1210 10 uF caps Murata has available, the worst one is one 16V rated giving 8.5 uF at 5 V. All the rest (a few 10 V ones included) give more than 9.5 uF. The  one giving "only" 8.5 uF is a thin one. It is only 1.0 mm thick while all the others are 2.2 or 2.7 mm. Still at 10 V all the others give more than 8 uF and the thin one 6 uF. The 50V ones are 9.8 uF at 5V and 9.1uF at 10V.

50V ones are clearly more expensive, if that matters.
 

Offline ElectroIrradiator

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #15 on: October 27, 2013, 04:26:55 pm »
Oh this is gold. >:D

There is no rule that a higher VW cap of the same footprint and capacity is inherently better, as I just found a counter example: 10 uF 1206 size muRata GRM31CR71 series, available from 6.3 to 25VW. All of these are the same thickness, 1.6mm. The 10V edition is noticeable better between about 2.5(!) and 10V, compared to the 25V type. :D

For other types, like the 0.1uF 0603 size GRM188R71 family, 0.8mm thick, it doesn't matter one iota what VW you use between 0 and 6.3V. There is a worst case difference in capacity at 10V of about 5-7% between the 10V and the 50V specimen. The major differences are near the rated maximum voltages for each type, where you really need the elevated ratings. For instance the 50VW is down about 50% at 50V, while the 25VW is down 45% at 25V.

So tl;dr: You better double check in each and every case, if this parameter is important to you.
 

Offline DutchGert

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #16 on: October 27, 2013, 05:15:09 pm »
What I mean is that if u take a 50V one u automatically end up with a physical bigger cap that probably has about the same real capacitance at 3.3 or 5V as what u want it to have.
Ofcourse u can find better ones if u look at the data but often u just want to be able to say to procurement "just place any A-brand 10uF 50V 1210 u can buy and it will be fine".

Is there some standard for that or does it just happen to be so? You can find many 35 V caps, which give poor performance at 5V DC (e.g. 0805 10 uF 35V is only about 4 uF at 5 V), but 50V caps seem to be clearly better (=bigger footprint).

If you compare all the X5R/X7R 1210 10 uF caps Murata has available, the worst one is one 16V rated giving 8.5 uF at 5 V. All the rest (a few 10 V ones included) give more than 9.5 uF. The  one giving "only" 8.5 uF is a thin one. It is only 1.0 mm thick while all the others are 2.2 or 2.7 mm. Still at 10 V all the others give more than 8 uF and the thin one 6 uF. The 50V ones are 9.8 uF at 5V and 9.1uF at 10V.

50V ones are clearly more expensive, if that matters.

No, there is no standard, its just that in my experience a 50V usually is a step bigger in size so 'automattically' gives u better (satisfying) performance when using it at 3.3 or 5V.

Thats all, i wont argue that outting considerable time into each ceramic u use will give u better results. Ofcourse it does.
But it also increases your development costs (but could be lowering your BOM cost) and makes your BOM and procurement considerably more complex.
« Last Edit: October 27, 2013, 05:19:52 pm by DutchGert »
 

Offline DutchGert

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #17 on: October 27, 2013, 05:26:22 pm »
Oh this is gold. >:D

There is no rule that a higher VW cap of the same footprint and capacity is inherently better, as I just found a counter example: 10 uF 1206 size muRata GRM31CR71 series, available from 6.3 to 25VW. All of these are the same thickness, 1.6mm. The 10V edition is noticeable better between about 2.5(!) and 10V, compared to the 25V type. :D

For other types, like the 0.1uF 0603 size GRM188R71 family, 0.8mm thick, it doesn't matter one iota what VW you use between 0 and 6.3V. There is a worst case difference in capacity at 10V of about 5-7% between the 10V and the 50V specimen. The major differences are near the rated maximum voltages for each type, where you really need the elevated ratings. For instance the 50VW is down about 50% at 50V, while the 25VW is down 45% at 25V.

So tl;dr: You better double check in each and every case, if this parameter is important to you.

Nobody said that :). I think we can agree that physical size matters in this case. I am mentioning 50V in 5V and 3.3V circuits for a reason, not because the voltage rating is higher but because it fairly safe to say that it ensures the physical size needed to function properly at 5 or 3.3 volt because it will be close to the value u want it to be at that given voltage.
« Last Edit: October 27, 2013, 05:28:00 pm by DutchGert »
 

Offline free_electron

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #18 on: October 27, 2013, 06:24:07 pm »
Hang on guys. There is more.... Every cap maker has his own dielectric . And certain series behave differently than others. Never assume that a cap from tdk behaves the same as a murata...

Ceramic caps drift under temperature, current, voltage, frequency , dc bias , size and dielectric material. Even the electrode material has an impact.

NPoO dont drift under temperature but they are susceptible to dc bias.
BaTio dielectric based also exhibit microphony efect. Send a pulsing signal throught them and they will 'sing' audibly ! Vibrate the board and you will get ripple voltage !

That is why those barium titanate dielectricc aps like x7r are a nono in filters or amplifiers that will be mounted in vinration rich environment !
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Offline ElectroIrradiator

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #19 on: October 27, 2013, 06:29:47 pm »
Hang on guys. There is more.... Every cap maker has his own dielectric . And certain series behave differently than others. Never assume that a cap from tdk behaves the same as a murata...

Ceramic caps drift under temperature, current, voltage, frequency , dc bias , size and dielectric material. Even the electrode material has an impact.

All of which means there are no rules of thumb, and that caps cannot be expected to substitute between brands. You always need to check the parameters, which are important to your application.
 

Offline free_electron

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #20 on: October 27, 2013, 07:07:25 pm »
Hang on guys. There is more.... Every cap maker has his own dielectric . And certain series behave differently than others. Never assume that a cap from tdk behaves the same as a murata...

Ceramic caps drift under temperature, current, voltage, frequency , dc bias , size and dielectric material. Even the electrode material has an impact.

All of which means there are no rules of thumb, and that caps cannot be expected to substitute between brands. You always need to check the parameters, which are important to your application.

bingo !

that is why cpaacitor mkers have software avaialble that will give you the plots for each and every cap they make. that osftware is free.
i have already posted the link to such software on the forum here.

Part of what i do professionally is tweak switching regulators . tuning the input and output caps is a tedious work. frequently we connect network analysers and sweep the impedance plots over frequency with bias applied to the caps. i call the cap makers and tell em : here is what i need : they run a couple of scenarios , send me samples , i build the board and verify it works fine with our switchers. once a good outcome is found you do not touch it. that is the bill of material going into production. there will be NO substitutes !
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Offline tszaboo

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #21 on: October 27, 2013, 08:20:18 pm »
That is why those barium titanate dielectricc aps like x7r are a nono in filters or amplifiers that will be mounted in vinration rich environment !
Well you can make "acceptable" filters with NP0 or COG capacitors. X5R and X7R is suitable for power supply, bypassing and such applications. I really expect a superior technology to be available in mass quantities in the near future, as there is the demand for it. In fact we already have it in the 18-20 bit SAR ADCs and zero drift PGAs. But it is time to give something for the discrete guys.
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Offline Kjelt

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #22 on: October 27, 2013, 09:38:14 pm »
I really wonder the following:
So where and when did we accept in this industry that capacitors are allowed to be graded as X uF and Y Volts when they only will reach that X uF's at 1/Y the Voltage, and only have 1/Xth the uF's at Y volts? I sense a deliberate form of false information.
If this is allowed what keeps electrolytic capacitor manufacturers from doing the same thing? This can become a sliding scale.
 

Offline SeanB

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #23 on: October 28, 2013, 05:00:23 am »
Electrolytics already have a +50 -85% rating so how could it be worse? At least they are not so voltage sensitive.
 

Offline DutchGert

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Re: Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
« Reply #24 on: October 28, 2013, 08:26:42 am »
Hang on guys. There is more.... Every cap maker has his own dielectric . And certain series behave differently than others. Never assume that a cap from tdk behaves the same as a murata...

Ceramic caps drift under temperature, current, voltage, frequency , dc bias , size and dielectric material. Even the electrode material has an impact.

NPoO dont drift under temperature but they are susceptible to dc bias.
BaTio dielectric based also exhibit microphony efect. Send a pulsing signal throught them and they will 'sing' audibly ! Vibrate the board and you will get ripple voltage !

That is why those barium titanate dielectricc aps like x7r are a nono in filters or amplifiers that will be mounted in vinration rich environment !

Nobody argues with that free_electron :). Just sharing a rule of thumb I myself find very useful.
 


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