capacitance vs frequency

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#1 21662117 11 May 2012 07:55

Zied Saad Zied Saad

Anonymous

Post #1
21662117 11 May 2012 07:55

hi
im trying to buil a boost converter.
i calculate my output capacitance as showen in this equation :
Cout=(Iomax *Dmax)/(Fs*deltaVo)
the matter that on data sheet of capacitance the capacitance are mesured at 100KHz
and my Fs=1Mhz
should i choose a capacitance 10 times greater than the C calculated from the equation above ?

thks !!!
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#2 21662118 11 May 2012 08:07

Mark Majewski Mark Majewski

Anonymous

Post #2
21662118 11 May 2012 08:07

If i see good this equasion You should get a 10 times lower
But in my personal opinion Cout stays the same. But You have to remember about ESR in pulse power suppliers.

B regards
Mark.
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#3 21662119 11 May 2012 08:09

Calinoaia Valentin Calinoaia Valentin

Anonymous

Post #3
21662119 11 May 2012 08:09

Electrolitic capacitors have a capacitance that decrease very much with increasing frequency. I don't think you must use a C 10 times greater than Cout, but you should use a bigger value, and at 1 Mhz Fs, be sure to use Low ESR capacitors.
#4 21662120 11 May 2012 08:22

Mark Majewski Mark Majewski

Anonymous

Post #4
21662120 11 May 2012 08:22

Caapacitance depends from frequency ? Are You sure...
http://en.wikipedia.org/wiki/Electrolytic_capacitor
ESR yes, Power of losses yes .... but capacitance ?

Here You have in polish:
http://www.google.pl/url?sa=t&rct=j&q=&esrc=s...jv9Z9e0m0d7DUn6dQ&sig2=hC4_N9v-WBBXzsJQ3kFiVA

B regards
Mark
#5 21662121 11 May 2012 08:32

Zied Saad Zied Saad

Anonymous

Post #5
21662121 11 May 2012 08:32

thak you !!!

what's the matter with ESR is increasing with frequency ?
#6 21662122 11 May 2012 08:34

Todd Hayden Todd Hayden

Anonymous

Post #6
21662122 11 May 2012 08:34

See pages 7 and 10: http://www.cde.com/catalogs/AEappGUIDE.pdf
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#7 21662123 11 May 2012 08:35

Mark Majewski Mark Majewski

Anonymous

Post #7
21662123 11 May 2012 08:35

Hello
tg delta = ESR/Xc
where
Xc = 1/(wC) (sorry w means omega)
C in Farads
w= (2 * pi * f)
P = U2 * ESR (dispatch power)
or
P = U2 * w * C * tg delta

B rgds
Mark
#8 21662124 11 May 2012 08:41

Mark Majewski Mark Majewski

Anonymous

Post #8
21662124 11 May 2012 08:41

Yes my mistake...
Or bad understanding of question .
Capacitance it is not depend of frequency.
But Capacitance of element as physical element depends. Because we see all equivalent schematic of capacitor i meand inductance etc. ).

Sorry.

But still i think that is no need to change Cout to 10 times bigger ...
My idea is to put couple capacitors in parallel then ESR will go down.
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#9 21662125 11 May 2012 08:53

Calinoaia Valentin Calinoaia Valentin

Anonymous

Post #9
21662125 11 May 2012 08:53

I'm sure that capacitance varies with frequency, because I measured a lot of electrolytic capcaitors with an RLC meter, and an 1000 uF capacitor has 1300 uF at 100 Hz, and only about 700 uF at 1 KHz.
And the formula Xc=1(wC) doesn't count that the dielectric material in the capacitor chagnes its properties with frequency.
#10 21662126 11 May 2012 09:17

Mark Majewski Mark Majewski

Anonymous

Post #10
21662126 11 May 2012 09:17

As i told at first ...
You should Use the same or maybe 2-3 times bigger capacitance ...
But i use couple capacitors in parallel configuration to low down ESR.
Always working good.

Here You have a nice link where are many graphs maybe it will help You to choose good capacitors:

http://www.electronicscomponentsworld.com/articleView~idArticle~72364_54018512315122009.html

Sorry by my bad conclusion. Hope I help You some :D

B rgds
Mark
#11 21662127 12 May 2012 03:50

Zied Saad Zied Saad

Anonymous

Post #11
21662127 12 May 2012 03:50

thank's
this is the datasheet ( http://www.kemet.com/kemet/web/homepage/kechome.nsf/file/T495 Series/$file/KEM_T2009_T495.pdf)
i think in my case it's 10 time greater ? or not ?
#12 21662128 12 May 2012 04:10

Mark Majewski Mark Majewski

Anonymous

Post #12
21662128 12 May 2012 04:10

Hi from this pdf you show.
On page 4 is shown a capacitance/freq characteristic....
So when You use a 1MHz freq capacitor has low down freq from almost 200uF to 15-20 uF... so
i recomend You to use a about 5 times greater .... but couple in parallel ....
I think 10 times it is not necesary but...
Also i recommend You to think if You can low down freq ... 100kHz and You do not have any trouble :-)

B regards
Mark
#13 21662129 12 May 2012 05:07

Zied Saad Zied Saad

Anonymous

Post #13
21662129 12 May 2012 05:07

thank you !!!
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