Driving LED with overcurrent ( what is the usual ratio ? )

[andre_luis]

Hi friends,



I´m working with IR image capture.
And I must develop a light source to drive current throught the LED array.
In order to increase lighting, most strobe controllers systems operate with pulsed wave trains.

According datasheet, we can handle small energy packs ( Dutycicle=1% / Overcurrent = 20x ).
However, I don´t know what is the common ratio used.

Could somebody give to me some guidance ?





PS.: I also don´t know the impact on long term life cicle

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[umesh49]

I am not sure about this LED array, but for a common LED, you should mentain two things;
Peak current should be underlimit and average current should be less than the derated average current at worst case operating temperature.

 

[jiripolivka]

While it sounds clear than you can keep the average current constant and making shorter pulses (lower duty cycle) would allow you to increase the LED current, please be careful. There are many LED designs; some are made to survive such high current under short pulse duration, others would die soon.

The advice is: if possible, find a LED as close similar to those in your illuminator matrix, and kill it by increasing the DC current while monitoring it by an ammeter. Then use half of such current in pulse regime.

Notice than "common" LEDs are inside contacted with a quite thin gold wire or whisker- this is the fuse that would melt or burn by overcurrent. As this wire is molded in plastic, it cannot cool by convection . Other LED designs use a ribbon instead of a wire; such ribbon might survive more current.

 

[andre_luis]

Hi,

I´m very grateful to the advice regarding risk to life cicle decreasing.
However, the main question stated is to determine the best choice to the overcurrent ratio.

I worked many times ago with a lighsource driven by 100us pulses and 1A peak current, but dutycycle was variable.
That values agrees to the chart bellow ( Note : The LED nominal rated current is 100mA ).

Is that values appropriated ?

**broken link removed**

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[jiripolivka]

The "overrating" curves are fine, but I think they were defined for a certain LED design.
I would not dare to use the >10x overcurrent for any LED. Using 100 us pulses is also interesting.. it may be that for certain LED design, the contact wire would not heat and melt.
Recently there are dozens or hundreds new LED designs; the companies try to increase their brightness also by pulse duty cycle.
LED life is at stake; any overdrive which increases active region temperature would limit the lifetime.

Semiconductor-element reliability and MTBF strongly depends on junction or active region temperature. Also, LEDs connected in series are expected to die not all at the same time.

I would stay with my earlier advice. If you can afford losing your LED illuminator, overdrive it but expect that it dies early.

 

[andre_luis]

Hi jiripolivka


I thank for your advice concerning life cicle decrease.
However, the pulsed driving approach is usual ( almost mandatory ) at the target application, and is not my conception.

The only issue to me is know what the overcurrent ratio commonly practiced.


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