Need help for 5a pulsed dc led driver

[protocol217]

I want to power this led with 4.5a - 5a pulsed dc:

SFH 4715S IR OSLON Black 3.4 W 1a max and pulsed dc 5a max
http://www.osram-os.com/osram_os/en...500mw/emitters-with-850nm/sfh-4715s/index.jsp
datasheet: http://www.osram-os.com/Graphics/XPic8/00100755_0.pdf/SFH 4715S Lead (Pb) Free Product - RoHS Compliant.pdf

i need it for my nightvishion gen1, i alredy have an illuminator with same led at 1a, now i want to buy PWM led driver or build a driver to give out 4.5-5a pulsed dc to drive this led, any suggestions, ideas?

 

[ronsimpson]

What do you hope to get by driving at 5A?
I think at 5A the part will be on for only 1percent of the time.

 

[protocol217]

i hope to be much brighter, i read somewhere that this led dived with 5a pulses will give out much more light then with constant 1a.
i don't know the pulse frequency needed for driving this led.

any ideas on what driver should i buy or build ?

 

[ronsimpson]

i hope to be much brighter, i read somewhere that this led dived with 5a pulses will give out much more light then with constant 1a.

Yes it is brighter when the current is on but no bright when the current is off.
So: 5x brighter when on but the part must be off 90% of the time so the power is only 50% of what it would be if you left it on all the time at 1A.
On the graph the bottom line is 1A for any amount of time.
The next line is for 50% on. So you could do 1.5A max fro 1mS then 0A for 1mS on/off/on/off
The next line is for 20% on/80% off. 1mS on 4mS off 2.25A max.
Next line shows 10% on 90% off. Lets pick 100uS on and 900uS off and 5A.

Bottom line: higher peak but lower average light. I think you want highest average light. So stay with DC.
From Data sheet:

 

[audioguru]

The datasheets for most high power LEDs shows that the brightness compresses when the current is higher so that doubling the current produces maybe 1.8 times more light, not 2 times the light. So instead of producing more light it also produces more heat.
Since you are pulsing the current then the average current is low then the average amount of light is low. Pulsing is used for dimming, not for brightening an LED.

You cannot increase the current high enough to produce anywhere near the same light output from a pulsed light compared to a continuous light because the LED will overheat during the pulses, so you must follow the rules printed on the datasheet for pulses.
Your LED has a maximum continuous current rating of 1A for a duration of only 100us when it has no heatsink and it can be pulsed at 1A for 10ms when it has a pefect heatsink. Its maximum surge current is 5A for 500us when this high current is not repeated.

 

[protocol217]

Ok, thanks for explaining, i understand now.

By the way a have one more question:
i have a several of these led-s and i have made several ir flashlights from them, i made one with 1.3 A driver (measured with ampere-meter) with a single lithium cell 3.7V, it has a good heatsink with thermal paste in cree mini flashlight housing , and tested it for around one hour continuously.
the housing does get hot ( not that much hot, i can hold it ), when i use it i never use it more than 5 min in one use. Its brighter then the 1A ones, but for how long will it last when overdrive with 1.3 Amps ?

 

[audioguru]

Some LEDs will fail when their current is at the maximum rating and others will survive. How long it will last is your guess.
I never run an electronic part near its maximum rating and they all last forever.

 

[ronsimpson]

LEDs age for several reasons. Heat is top on the list. Some light sources measure the temp of the LED. On a cold night they can run brighter.
The real reason is that they run 1.3x more current until the temp gets up. (20-30 seconds) This way they can advertise more light.
If you really want to push the edge then you need to measure the LED die temp. On a hot day you might be at 0.9x.

 

[Colin]

I lot of points and fact have been missed in the answers above.
Basically a LED does not produce more light by being pulsed, however pulsing a LED with a high current for a very short period of time will produce a higher brightness than delivering a very small current is due to PERSISTENCE OF VISION and your eye maintains the same brightness when the LED is not illuminated.

 

[ronsimpson]

I lot of points and fact have been missed in the answers above........
PERSISTENCE OF VISION and your eye maintains the same brightness when the LED is not illuminated.

You missed the point to: Night Vision (IR) No "your eye".

 

[protocol217]

yes, i read about that, and the frequency of pulses need to be higher then human eye fps, so the eye see it like it is always on and brighter.

PERSISTENCE OF VISION

Did you want to say that?

 

[ronsimpson]

yes, i read about that, and the frequency of pulses need to be higher then human eye fps, so the eye see it like it is always on and brighter.

So if this is true then all high end flashlights would pulse the LED.
I have pulsed white LED bulbs to make them appear dimmer. Some white LEDs turn colored when the current is low. So I pulsed them at full current but 25% duty cycle to keep the color right.

 

[audioguru]

Our vision's sensitivity to light is slow. It takes a pulse width of at least 30ms for us to see the pulses at the same brightness as continuous current with the same amount of current. Reducing the width of the pulses reduces the brightness that we see.
Persistence of vision is the after image that we perceive for a short time period after a light is turned off.