Re: High brightness white LEDs damaged by custom switcher

Al a écrit :
In article <azN_h.527$LR5.87@xxxxxxxxxxxxxxxxxxxxxxxxxx>,
<hapticz@xxxxxxxxxxxxx> wrote:

is device to device parameter variations being accounted for?

most of these LED's are not anywhere near being matched pairs/triples or
other, unless you get the manufacturer to custom mount them from the same
die and then do a parametric test on each and then sort them and mark them,
(Very costly)

even clusters are assembled from random devices!

MIL specs may offer you some leeway, but even controlled groups will be bulk
tested only for basic params.

if spiked current is failing some devices, they may have beeen borderline to
begin with, as most will just heat up and dim before catastrophic failure
occurs. usually over time.

your blown bonds and similar symptoms tells me that you had EXTREME current.

rise time between units may force some to go into failure befor the adjacent
units absorb the available current also.

as with all design, the simplest is the best. trying to parlay your
education of PICs and neatsy circuits willl yield expensive and hard to
manufacture systems.

take some lessons from the chinese,tiawanese and others, cheap simple and
basic.

stay within some easy set standard limitations and you may have good
results.

Amen!

Many years ago I did some analysis one a power switching circuit which used two transistors in the primary of a center tapped transformer. The transistors would switch alternately to produce a square wave on the secondary which was then rectified to produce a higher voltage.

Modules which failed had one blown transistor on them. As it was blown, it was not possible to read the parameters. Analysis of stock transistors showed that their characteristics, although within spec, varied. When the characteristics of the transistors were matched and they were used in matched sets, the problem disappeared.

The problem disappeared until a couple of years later, when a new manufacturing guru wondered why they were going through the expense of checking all the transistors. He abolised the practice, and voila, problem reappeared.


Bad design to start with.
Bad answer to go on: for the cost of matching, you could easily pay for the components cost to fix the problem.


--
Thanks,
Fred.
.

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