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> Rms Value Of This Current Waveform?, Discontinuous current
treez
Posted: August 18, 2012 04:17 pm
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Hello,

Do you know of mathematical expressions that give the RMS value of this Discontinuous current waveform?

http://i49.tinypic.com/152gryo.jpg

(i.e. supposing I know the peak value, the rise time, fall time and dead time)
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Abdullah M.A.
Posted: August 19, 2012 01:58 pm
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-> 1st Tri.

i(t) = [ t / (4.71 - 4.7)] * 2

-> 2nd Tri.

i(t) = [(4.5 - 4.4 )/(4.74 - 4.71)] * 2

Now, Find the RMS. wink.gif

Abdullah


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Sch3mat1c
Posted: August 19, 2012 06:11 pm
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Define duty cycle D as the ratio between the time when current is nonzero versus the full period (eyeballing, about 75%). RMS is 1/sqrt(3*D) times the peak current. Note that average is only Ipk/(2*D), so the crest factor gets huge for light loads.

Tim


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treez
Posted: August 22, 2012 07:21 pm
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D = 0.8

The RMS is actually 1.03A.

The formula above......I.E...... 1/[sqrt(3D)] gives 0.645A
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Sch3mat1c
Posted: August 22, 2012 08:45 pm
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I specifically stated "times peak". You applied it wrong. I get about 1.3A RMS. Was 1.03A calculated in LTSpice? Then why ask for a formula you don't know how to use?

Tim


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treez
Posted: August 24, 2012 06:16 am
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1.03A was indeed from ltspice

But its also the sum of the sqares of the individual rms's of the waveform split into two trains of triangles (both right angled).

so it seems correct.........i know i should be doing the integration to check it, though i find i need a single excel spread calc...as its quicker and clearer for those jobs that you need to do quickly
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