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1st May 2013, 18:35 #1
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capacitor calculation for ripple for smps i/p and o/p
Dear all..
greetings..!!
i have been suffered with capacitor calculations.. as i m getting confused by reading too mch books..
i want a correct direction to select the capacitor to minimize the ripples... also the tolerances considerations for i/p and o/p...
waiting to hearing you sooon..!!

1st May 2013, 18:35

2nd May 2013, 01:00 #2
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Re: capacitor calculation for ripple for smps i/p and o/p
Rough calculations for output smoothing cap:
The capacitor charge drops a certain amount during an idle gap.
The amount of drop depends on the time constant, TC = R * C.
One time constant is the time it takes to change by 63% (by definition).
Suppose your load is 20 ohms. Operating frequency is 6 kHz.
You set a limit of 15% ripple. (A high figure but it shows up well on a simulated oscilloscope.)
Idle time can be as long as 1/6,000 sec. So the capacitor is permitted to fall 15% in 167 uSec.
This translates to 63% in 700 uSec. (To make things easy we use linear calculations.)
Therefore the time constant is .0007 second.
So solving TC = R * C, gives a cap value of 35 uF.
The 10% duty cycle creates an idle time which is close to the maximum, and the ripple is approximately the expected amount for our cap value.

2nd May 2013, 03:58 #3
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Re: capacitor calculation for ripple for smps i/p and o/p
Dear..
not satiesfied...:(
as one of my team mates keeps 940uF/450V CAP after bridge... n i am going to load that capacitor with 750 watts..
how that guy keep above cap after the bridge..??
he waz telling that...
ripple=vpp/loading power *(1/100(freq)*940uF)... SOMETHING LIKE THAT..
But he showed me exactly 17 v ripple by calculations and also practically wen we connect scope..
plz help me out hw..??

2nd May 2013, 03:58

2nd May 2013, 09:48 #4
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Re: capacitor calculation for ripple for smps i/p and o/p
450V CAP
Effective load 270 ohms.
1/100 is the idle gap time (time between current bursts of rectified 50 Hz mains AC).
A 940 uF cap, and 270 ohms, makes a time constant of .25 sec. Charge level would fall 63% in that time.
Therefore in .01 seconds it falls 2.5%. ( 63 * .01 / .25 ) (Although the discharge curve is not straight, we assume it is, to make things easy.)
2.5% of 450 makes 11.3 V of ripple.
This can only be a rough estimate. It's not a surprise that it's lower than your colleague's figure.

Consulting another source, here is a simulation:
It agrees better with your colleague's figure.
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2nd May 2013, 09:48 #5
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Re: capacitor calculation for ripple for smps i/p and o/p
450V CAP
Effective load 270 ohms.
1/100 is the idle gap time (time between current bursts of rectified 50 Hz mains AC).
A 940 uF cap, and 270 ohms, makes a time constant of .25 sec. Charge level would fall 63% in that time.
Therefore in .01 seconds it falls 2.5%. ( 63 * .01 / .25 ) (Although the discharge curve is not straight, we assume it is, to make things easy.)
2.5% of 450 makes 11.3 V of ripple.
This can only be a rough estimate. It's not a surprise that it's lower than your colleague's figure.

Consulting another source, here is a simulation:
It agrees better with your colleague's figure.
1 members found this post helpful.

2nd May 2013, 09:48 #6
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Re: capacitor calculation for ripple for smps i/p and o/p
450V CAP
Effective load 270 ohms.
1/100 is the idle gap time (time between current bursts of rectified 50 Hz mains AC).
A 940 uF cap, and 270 ohms, makes a time constant of .25 sec. Charge level would fall 63% in that time.
Therefore in .01 seconds it falls 2.5%. ( 63 * .01 / .25 ) (Although the discharge curve is not straight, we assume it is, to make things easy.)
2.5% of 450 makes 11.3 V of ripple.
This can only be a rough estimate. It's not a surprise that it's lower than your colleague's figure.

Consulting another source, here is a simulation:
It agrees better with your colleague's figure.
1 members found this post helpful.

2nd May 2013, 09:48

3rd May 2013, 07:10 #7
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Re: capacitor calculation for ripple for smps i/p and o/p
thank u sir..!!
   Updated   
another one doubt is that,
in dc to dc converter how i can calculate.??
by taking switch frequency of converter..??
consideration criteria for keeping ripple after bridge..??
1% is for the final o/p of converter...

3rd May 2013, 11:27 #8
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Re: capacitor calculation for ripple for smps i/p and o/p
Yes, the frequency is important. At high frequencies you do not need such a large value capacitor.
consideration criteria for keeping ripple after bridge..??
1% is for the final o/p of converter...
Or using time constant calculations...
Your ripple spec is 1%, inside a .01 second timespan.
This translates to a 63% drop in .63 sec. Therefore time constant is 0.63.
Suppose load is 270 ohms. Then the smoothing capacitor is 2,333 uF. ( .63 / 270 )
You can get more info from the list of similar threads at the bottom of this page.
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3rd May 2013, 21:15 #9
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Re: capacitor calculation for ripple for smps i/p and o/p
Dear sir,
i agreed for the above explanation..
i want to calculate the o/p capacitor of my half bridge topology...
switch freq iz 45 khz..
converter worked at 380 v dc...
the o/p is loaded 1.7 amp @400v dc...
the o/p windings of 200v of 2 windings series together to produce 400v output...
please help me out..
i gone similar threads bt my fullfillments not completed..
like c=i*(1/f)/ripple... it coudnt helping me to find optimum solution..

4th May 2013, 08:25 #10
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Re: capacitor calculation for ripple for smps i/p and o/p
Method #1.
There is always the method of installing a low value capacitor...
and watching to see if the ripple is too much...
and if it is then try a higher value.
Etc.

Method #2.
Or, a simulator is also useful for this purpose. 'Let the computer do the math.'
8.4uF. (Duty cycle 10% assumed).

Method #3.
1% ripple on 400V is 4V. Then the formula:
C = 1.7 * (1/45000) / 4
= 9.4 uF

Method #4.
The load calculates to 235 ohms. (400 / 17).
We have an idle period up to 1/45,000 sec. This is 22.2 uSec.
Ripple allowed is 1% during that time. Translates to 63% in .0014 sec. The time constant is .0014.
Divide by 235 ohms, gives us 6 uF for the output capacitor.
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