Re: Current-driving a powerful IR-illuminator array

On 14 Apr 2006 04:52:59 -0700, "BW" <bjorn@xxxxxxxxxxxx> wrote:

qrk wrote:
I've done some tests with the HSDL-4400 recently. At a 1 kHz rep rate,
on time = 2 us, current = 9 Amps, I ran the LED for 1.8 million pulses
without problem. The same LED went through a battery of tests from <1A
to 9A without degredation. Probably around 5 million flashes total
until I cooked the poor thing above 10 A and a high duty cycle. I
found that you don't want to run these LEDs at much more than 2 Amps
as the efficiency gets pretty bad. 1 Amp seems like a nice compromise
for a 2 us pulse at 0.2% duty cycle. For your parameters you need to
do some tests to see where the intensity starts dropping off due to
heating. Find a PIN photodiode to use as a receiver and start running
some tests.

Interesting. So what you are saying is that if I hook up a sufficiently
linear (or calibrated) photodetector, I could measure how the LED
behaves when scanning over duty-cycles and/or currents, and where it
starts going non-linear is probably where I don't want to be because it
goes non-linear as an effect of heat buildup at the junction (that's
what kills it right) ?

/Bjorn

Your main concern isn't really non-linearity, but overheating of the
die. But yes, by monitoring the LED voltage, current, and light output
you will come to some sort of conclusion of an appropriate drive
level. Running the LED at currents over 6A you can see the light
output sag with time after about 1 us. Since your using longer
on-times, you need to operate the diode at a saner current level.

---
Mark
.

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