> I'm currently reading up on LED lamps, and this is one of the Cree
> articles I looked at yesterday, dealing with failure mechanisms

> http://www.cree.com/products/pdf/XLamp-Pulsed-Current.pdf

That's an extremely good document to have available, not because it
says anything (that I noticed) stunningly new or different that can't
be understood from available data sheets and common sense BUT because
it puts a reputable manufacturers stamp of approval on the common
sense conclusions that any number of people seem to want to ignore or
criticise.

A very significant point is that modern white phosphor LEDs specify
the ratio of Ipeak to Ioptg max as not much above 1. In some cases
Imax and Iptgmax are the same, ratios of only about 1.1:1 are common
enough and as high as 2;1 would be rare. Add thermal data and a little
more and you come up with the sort of results that have been given in
this thread and in the paper Joe referenced.

Disturbing is the fact that the overall mix of factors is that Cree
actually DERATE the mean power able to be put into a pulsed LED.
eg they say

Based on the 1 KHz pulse testing we have reviewed in this application
note, Cree suggests the following guidelines for
pulsed current operations:

1. For duty cycles between 51-100%, do not exceed 100% of the maximum
rated current;
2. For duty cycles between 10-50%, do not exceed more than 200% of the
maximum rated current;
3. For duty cyles less than 10%, do not exceed more than 300% of the
maximum rated current

It seems to me that there is an element of arbitrariness in there and
also that the results are slightly conservative in some areas, but
only slightly.

A rule of thumb which falls on the optimistic side of what Cree says
is "Total power in should never be more than Pmax_DC and maximum
current ever is <=3D 3 x Imaxoptg.

That's going to probably reduce total lifetimes at the upper end but
probably not vasyly.

NB LED life time is decreased *independently* by either of increased
current or increased temperature.



         Russell





>
> from this page
>
> http://www.cree.com/products/xlamp7090_xre.asp
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