# Buck Converter inductor sizing question

Discussion in 'Homework Help' started by notoriusjt2, Oct 18, 2010.

1. ### notoriusjt2 Thread Starter Member

Feb 4, 2010
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my work

through trial and error I chose 22.5microH to work with

D=duty ratio is the fraction of the switching period that the switch is closed

I think this statement may be throwing me off
"Determine the size of the inductor such that the minimum inductor current is 25% of the average inductor current."

so does that mean that if the average inductor current is 1.25A then the minimum inductor current has to be 25% of that? or .3125A

I have tried all these values of inductors and none of them result in that answer

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
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You have the mean current of 1.25A.

The minimum ripple value is to be 25% of that or 0.3125A.

The difference between 1.25A and 0.3125A is 0.9375A.

The peak-to-peak ripple current is then 2*0.9375 = 1.875A

ΔIl=1.875={[Vs-Vo]/L}*DT

DT=0.6666/40000=16.67 us

L= [Vs-Vo]*DT/ΔIl=[30-20]*16.67/1.875 uH = 88.9 uH

3. ### notoriusjt2 Thread Starter Member

Feb 4, 2010
209
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so$\Delta$Il is the peak to peak ripple current??

in my textbook it defines it as the "change in current"

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Yes - my wording is intended to imply the same thing.

BTW - Why did you delete the other post - it might have been of interest to others?

Last edited: Oct 18, 2010
5. ### notoriusjt2 Thread Starter Member

Feb 4, 2010
209
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I didn't delete it... It just disappeared. I still need to work out that problem so I guess I'll repost it. Could a mod have deleted it?

6. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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That's strange. There was nothing I recollect in the content that would make it a target for deletion - even something as trivial as duplication of an existing thread. Might be worth informing the administrator.

Apr 5, 2008
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Hello,

I made a remark on the missing posts to the other forum leaders.

Bertus

8. ### hgmjr Moderator

Jan 28, 2005
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Sorry notoriusjt2,

Mia culpa.

I think I deleted the thread since it appeared to be an accidental double post.

Can you repost the question?

hgmjr

Feb 4, 2010
209
0
no worries