Electrolyic vs ceramic

[Makaram]

Why are polarized capacitors used as bypass caps where ceramic arnt?

Im bucking 12v to 3v3 with a tl2575 and just about to design my external circuit but only have smd ceramic caps and was wondering if I could use them?

Thanks

 

[crutschow]

Electrolytics are used for power supply filtering because they are available in much larger capacity then ceramics. If the ceramic has sufficient capacitance for your application, then you can use it as it has better characteristics (lower ESR and inductance) than an electrolytic.

 

[Makaram]

! Best and quickest reply ever!
thank you

 

[jpanhalt]

Electrolytic (aluminum or tantalum) and ceramic capacitors are not always interchangeable, as the efficiency of filtering at high frequencies and tendency to cause oscillation of the regulator are influenced by the ESR of the capacitor.

Oscillation is covered in a nice application note from National Semi (TI):

**broken link removed** page 9

Output Capacitor Selection
Since the output capacitor is the user’s tool for compensating
a monolithic LDO regulator, it must be selected very
carefully. Most cases of oscillations in LDO applications are
caused by the ESR of the output capacitor being too high or
too low.

Filtering is discussed in several datasheets. You may want to consult the datasheet for the LM117 group.

John

Edit: That link is to the newer TI application note with more pages. This may take you to the National note:
**broken link removed**

 

[davenn]

Why are polarized capacitors used as bypass caps where ceramic arnt?

Im bucking 12v to 3v3 with a tl2575 and just about to design my external circuit but only have smd ceramic caps and was wondering if I could use them?

Thanks

Thats not really correct.

electrolytics with their higher values are used to bypass low frequencies

ceramics ARE USED as bypass capacitors in huge numbers and often along side electrolytics
the smaller values of ceramics are used to bypasss higher frequencies

you CANNOT use one in place of the other

Dave

 

[simonbramble]

For a dc/dc converter, you really need to use ceramics. the low ESR of the ceramic capacitor (10mOhms) compared with the high ESR of an electrolytic (several Ohms) will give you much lower ripple voltage on the output.

As with many applications, read the datasheet. Indeed ceramics have been known in the past to produce instability if used where the datasheet recommends electrolytic caps. You can always put a resistor in series with a ceramic to increase the ESR. this only tends to be a problem with devices older than about 15 years when all capacitors had a higher ESR than the ceramics used today.

I have also seen datasheets where the ESR has to be as low as possible so you cannot use electrolytics. It depends on how the chip designer has designed the internal compensation.

Hope this helps

 

[Boncuk]

Thats not really correct.

electrolytics with their higher values are used to bypass low frequencies

ceramics ARE USED as bypass capacitors in huge numbers and often along side electrolytics
the smaller values of ceramics are used to bypasss higher frequencies

you CANNOT use one in place of the other

Dave

Hi both of You,

You might want to throw a glance on that one. - "Multi layer ceramic cap" is the keyword. It replaces ceramic, metalized paper, electrolytic and tantalum caps.

And YES, each of these types of caps can be used in place of the other, since they are all the same.

Have fun reading the datasheet.

Regards

Boncuk

 

[simonbramble]

And YES, each of these types of caps can be used in place of the other, since they are all the same

Not necessarily - for all the reasons I mentioned above. A ceramic is as near to the ideal cap as you can get and generally present a better solution than electrolytics, but they are not always a drop in replacement.

 

[unclejed613]

ceramics are susceptible to piezoelectric effects. they are rarely used as bypass caps in certain applications because of this. two of the most prominent effects are self-resonance, where the cap acts in the same manner as a series resonant LC network, and the other effect is microphonic, where the ceramic cap produces a voltage when subjected to mechanical stresses.

https://www.electro-tech-online.com/custompdfs/2012/09/cdn_nb_brochure.pdf

https://www.electro-tech-online.com/custompdfs/2012/09/piezoelectric_effect.pdf

 

[davenn]

Hi both of You,
You might want to throw a glance on that one. - "Multi layer ceramic cap" is the keyword. It replaces ceramic, metalized paper, electrolytic and tantalum caps.
And YES, each of these types of caps can be used in place of the other, since they are all the same.
Have fun reading the datasheet.
Regards
Boncuk

hi ..
all very sweet, but relatively irrelevent to the discussion as none of those caps have the big values of electrolytic caps commonly used in smoothing of power supplies.
The largest value I could see was 100uF, a long long way from the many 1000's of uF often needed for smoothing raw DC
Only in circuits, where the capacitance value needed is small would those caps be ideal

On a side note ... its interesting to see one of those multilayer ceramics fail. A bit of gear that came into the workshop recently with the complaint of no go and a ticking sound could be heard. It was an accurate fault description. On opening the unit up, I could see a physically large SMD ceramic internally arcing with that glow radiating out from with in the cap.

Dave

 

[Boncuk]

On a side note ... its interesting to see one of those multilayer ceramics fail. A bit of gear that came into the workshop recently with the complaint of no go and a ticking sound could be heard. It was an accurate fault description. On opening the unit up, I could see a physically large SMD ceramic internally arcing with that glow radiating out from with in the cap.

Dave

You might increase the effect (increased volume) by hammering a nail through the cap which also increases the visual effect.

Boncuk

 

[davenn]

You might increase the effect (increased volume) by hammering a nail through the cap which also increases the visual effect.

Boncuk

hahaha

somehow I dont think Trimble would be overly impressed with that style of design modification

I suspect it would lead me to loosing my certification

cheers
Dave

 

[Makaram]

Uhmm thanks for the input guys but I am infinitely more unsure now! haha

 

[davenn]

Uhmm thanks for the input guys but I am infinitely more unsure now! haha

hi Makaram

it still, as as been said, boils down to your specific need .... in PSU's where you need hi value caps you really have no choice you will be using electrolytics for anything over ~ 100uF in value.
for other situations where 100 uF or smaller values are needed then tantalum or ceramics will be ideal and in general much more reliable

BTW just noticed your location .... Dunedin, NZ .... thats my home town... I have been in Sydney for the last 12 years.
most of my family are out in the town of Outram ... western end of the Taieri Plains

cheers
Dave

 

[Makaram]

Thanks for that dave, sums it up nicely! Maybe you can come home and mentor me?? haha

 

[audioguru]

Am I allowed to talk about the poor reliability of cheap Chinese capacitors?
A Chinese electrolytic, ceramic or poly capacitor will short soon. Or its connecting wires might fall off.
Instead use an American or European reliable capacitor that costs a little more.

 

[Makaram]

Hahaha the irony ^