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#11
11-07-2010, 01:03 PM
 notoriusjt2 Senior Member Join Date: Feb 2010 Posts: 209

recalculating with D=0.316 gives me...

but that answer is incorrect. am I not using the right formula here?
#12
11-07-2010, 01:17 PM
 mik3 Senior Member Join Date: Feb 2008 Location: Cyprus, but now in UK (GMT+0) Posts: 4,845 Blog Entries: 9

What is this formula?

http://en.wikipedia.org/wiki/Buck_converter
#13
11-07-2010, 09:21 PM
 notoriusjt2 Senior Member Join Date: Feb 2010 Posts: 209

based on this graph off of wikipedia, it appears that on time is only from 0 to DT.

so would duty cycle actually be DT/T

which would be equal to 0.316?
#14
11-07-2010, 09:39 PM
 mik3 Senior Member Join Date: Feb 2008 Location: Cyprus, but now in UK (GMT+0) Posts: 4,845 Blog Entries: 9

The duty cycle is duty cycle, it doesn't change. You set the duty cycle and the output voltage depends on the duty cycle. Now, the relationship of the output voltage to the duty cycle depends whether the converter operates in CCM or DCM.
#15
11-07-2010, 10:26 PM
 t_n_k Senior Member Join Date: Mar 2009 Posts: 4,649

Quote:
 Originally Posted by notoriusjt2 recalculating with D=0.316 gives me... but that answer is incorrect. am I not using the right formula here?
Kindly permit me an observation or two ...

You've been tinkering with these problems for some time now and still seem to be struggling.

As Mik3 & others point out, the particular converter mode of operation and topology dictate the approach required. I'm sure you've grasped that fact.

You asked me some posts back whether I remember the formulas for any given situation. In brief - no I don't. Well that's not exactly correct - I recall the relationship between switching duty and output voltage in a continuous mode buck converter. Rather than remembering a formula which may not be relevant to a particular case, I try to understand what's happening physically and then analyse the circuit.

The present example is a case in point. I came momentarily came 'unstuck' because I didn't initially think it through carefully.

You may derive some benefit from putting the text book aside and systematically analyzing the various cases for yourself - or make sure you understand why a certain formula is relevant for a particular case. Then you may be able to apply the formulas with confidence.
#16
11-07-2010, 11:04 PM
 notoriusjt2 Senior Member Join Date: Feb 2010 Posts: 209

Quote:
 Originally Posted by t_n_k Kindly permit me an observation or two ... You've been tinkering with these problems for some time now and still seem to be struggling. As Mik3 & others point out, the particular converter mode of operation and topology dictate the approach required. I'm sure you've grasped that fact. You asked me some posts back whether I remember the formulas for any given situation. In brief - no I don't. Well that's not exactly correct - I recall the relationship between switching duty and output voltage in a continuous mode buck converter. Rather than remembering a formula which may not be relevant to a particular case, I try to understand what's happening physically and then analyse the circuit. The present example is a case in point. I came momentarily came 'unstuck' because I didn't initially think it through carefully. You may derive some benefit from putting the text book aside and systematically analyzing the various cases for yourself - or make sure you understand why a certain formula is relevant for a particular case. Then you may be able to apply the formulas with confidence.
thats a very good idea, i will take that into consideration

however, the graph that i posted from wikipedia and the graph that is in my textbook shows two different on/off concepts. if i would have seen the wikipedia version from the beginning i probably would have gotten this concept much earlier

 Tags buck, converter, cycle, duty

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