> > 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.

And finally ... :-)

One common method of driving LEDs from smaller smps boost converters
is to "ring" an inductor into the LED, allowing the LD to act as
rectiifer with no series diode and no smoothing capacitor.The LED is
driven by pulsed DC at the smps frequency - really essentially AC
pulses. The current will tend to be in very narrow high magnitude
peaks.

Based on what has been said in this thread, this is an extremely poor
practice. The LED will be pulsed by unknown by high  magnitude
short duration pulses. If the LED is driven to about design brightness
then it will be vastly overdriven current wise.

Circuits like this will shorten LED life, possibly very substantially,
and you'd hope that application note circuits would not show this
system. Alas, many do.

Perhaps a year ago wrote to Zetex about a typical and bad example of
this in one of their app notes. I received no response.
A bit sad given Zetex's competence in product performance.  Mayhaps
their designers and app note writers don't talk to each other.




    Russell



             Russell
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.