then it must be high frequency, many people has the ability to see LED blinks if frequency is not high enough.
 
Funny N.
New Bedford, MA
http://www.AuElectronics.selfip.com

----- Original Message ----
From: Bill van Dijk <bvdijk@igs.net>
To: Microcontroller discussion list - Public. <piclist@MIT.EDU>
Sent: Wednesday, January 23, 2008 6:51:23 PM
Subject: RE: [EE]:: White LED lifetime issues.


Russel,

Interesting. I'm curious, you propose at the end that:

> Needless to say, to have any chance of achieving rated
> lifetimes LEDs need to be driven in a constant current mode

A common thing to do is a PWM drive. This is often hailed as a way to
 extend
life with very little loss of light. In some cases the even suggest you
 can
over drive them slightly and not cause harm. Views?

Bill van Dijk






> -----Original Message-----
> From: piclist-bounces@MIT.EDU [mailto:piclist-bounces@MIT.EDU] On
 Behalf
> Of Apptech
> Sent: Wednesday, January 23, 2008 5:22 AM
> To: PIC List
> Subject: [EE]:: White LED lifetime issues.
> 
> Papers pertinent to white LED lifetimes.
> 
> It's worth noting that the "100,000 hour LED lifetime" often
> quoted for LED products is usually grossly untrue.
> 
> Another useful point to note is that lifetime will be
> decreased at constant current as junction temperature rises
> OR at constant junction temperature as current rises. So
> both a low current and low Tj are desirable for best
> lifetimes.
> It's also worth noting that encapsulation material can make
> a vast difference to lifetimes. All epoxies are far from
> equal in such applications and silicones (no great surprise)
> tend to be superior due both to their superior UV handling
> (due to Si-O-Si bonds being higher energy than epoxy C-C),
> higher temperature ratings and gentler action on bond wires
> etc due to their softer and more flexible nature.
> 
> Needless to say, to have any chance of achieving rated
> lifetimes LEDs need to be driven in a constant current mode
> at rated current or less. The common mass market practice of
> hard paralleling multiple LEDs is an invitaiton to vastly
> unbalanced performance and very early failures.
> 
> 
>         Russell
> 
> _______________________
> 
> Superb slideshow on white LED lifetime issues
> UKNC Jan 2007.
> 
>         http://www.uknc.org/papers/Jan07/Humphreys.pdf
> 
> Excellent lumileds paper on "Understanding Power LED
> lifetime analysis".
> The dread Waloddi Weibull takes a bow.
> 
>         http://www.lumileds.com/pdfs/WP12.pdf
> 
> Good general LED lifetime and related intro
> Low technical level.
> 
>         http://www.ledsales.com.au/cart.php?page=led_faqs
> 
> Philips ra ra but in passing useful news article on lifetime
> prediction tools
> 
> 
>
 http://www.eetimes.com/press_releases/bizwire/showPressRelease.jhtml?artic
> leID=603418&CompanyId=2
> 
> TOC for a LED manufacturing technologies report.
> Full version will cost you $US4990 or more.
> TOC alone has some value.
> 
> 
> http://www.electronics.ca/reports/technology/led_manufacturing.html
> 
>  Already date but still useful 2004 intro to "How white LEDs
> work".
> 
>         http://www.compoundsemi.com/documents/articles/news/4706.html
> 
> 
> 
> 
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