I have a application where my LED currents are switched as in a matrix. My current LED on time is 60 us and off time is 15240 us. The intensity is problem thus I pump a higher current in that 60 us (applied potential is arround 9V in that period of time ) and that gives the required intensity. LED doee not heat up since LED current duty cyle is extreamly small (rms current is also very small too). Is this method will reduce the LED life time ?. Could some one refer some articles on LED life time dependability with current.
LED die shouldn't mind the high peak currents, as long as thermals are within bounds. Where you will see some lifetime degradation is in the bonds between the die's bonding pads and the bond wires. The phenomenon you want to research is referred to as "electro-migration."
Wow. I don't like to go much beyond 8:1 to 10:1. You're doing 250+:1. I don't think you'll get away with it unless the peak current is not much more than 5-10x the allowable DC current (meaning the LEDs will be relatively dim).
Best regards, Spehro Pefhany
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My design works prefectly. IT gives almost like a continoues current because that current pulse is repeatedly applied. My concerened was with the life time.
Yes, yes, I know, 'prefectly'. So what does the LED data sheet say about the conditions you are subjecting it to? For example, here is a data sheet that specifies maximum PEAK current ("Limits of Safe Operation") at 25°C of 150mA (only 5 x the maximum DC current) for pulses of < 10 *microseconds*).
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It should be derated above 25°C, obviously.
Best regards, Spehro Pefhany
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"it\'s the network..." "The Journey is the reward"
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So the peak current is 150 mA at less than 10 us pulses for this LED type. Anyway my drivers provide arround 90 mA per LED. I think I am in the safe region.
Actually, the datasheet is strange. It gives a max DC current of 20mA, but a max pulse of 500mA at 1/20 duty. Which is a *25* mA average, and probably much more than that in terms of effective heating.
And is has a "continuous forward current" of "100mW"!
Very impressive part, Jameco must have access to some very high technology suppliers. Compare a similar Avago part:
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Applications ? Commercial outdoor signs ? Automotive interior lights ? Front panel indicators ? Front panel backlighting
Max DC current is 30mA (derate above 50°C) rather than 20mA (?) Peak current is only 100mA vs 500mA for the Jameco thing. Max Pd is 111mW vs. 100mW for the Jameco wonder.
Of course it could just be a sloppy translation and layout from a sloppy Chinese data sheet from a Guangzhou back-alley assembly outfit that hardly knows what they are doing, but that's just conjecture.
Best regards, Spehro Pefhany
--
"it\'s the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
The whole thing is strange - maybe "pub' writes his own datasheets for leds as the other data offered makes just about as much sense.
With a 1in 255 ratio and 90mA led current the average led current is only 353uA yet the stated led voltage for this current is 9V!! (unless it is meant that the average led current is 90mA whence the led 'on' current is almost 23Amps)
Clearly the fellow has no idea what he is doing else he would not have chosen such a ridiculous ratio to start with
My current driver can handle upto 100 mA Max (typically 90 mA) with the duty cycle I mentioned above. Data sheet is strange to me too ..... but I am in the safe region according to my duty cycle. I did not check my current pulses since my instruements giving some problems at this tiny duty cycle. But what I observe is that my LEDs are realy bright and I dont want to go beyond 9 V at all. My problem is with the life time. I working on measuring the current pulses.
My design is a LED matrix with 48 rows and 254 columns. I am using column scanning instead of row scanning. The way I designed it, it is much cheaper than using row scanning and also less over head for the uP.
Can some one tell me how do I theoretically calculate the average current in a such system ?
So you're not limiting the current, just hitting them with 9V? In that case, think you'd probably getting around 250mA peak current, which would be more like 5mA average, which would be fairly bright with a modern LED (you can compare with a DC current and see). The color may also shift substantially with such abuse.
Since you aim to have 12,000+ blue LEDs, it will be a bummer if/when you lose a whole bunch of them.
Multiply the theoretical *average* current per "on" LED times the number of remaining LEDs which should be on.
Best regards, Spehro Pefhany
--
"it\'s the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
, just another site sparse on technical information.
It appears the signage silicon market is dominated by start-ups. Macroblock seems to have a global presence and a product line that pretty much covers everything:
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, very impressive stuff, the big semis are just plagiarizers by comparison.
From his responses, it is apparent the OP's question is not worthy of consideration, let him take his confusion to the Basic ng...
I dont just apply 9V directly. It has a constant current driver. So it can not have 250 mA peak currents in anyway. max is 100 mA. Design has a safety circuit which cuts power to LEDs in case "on" time chages by
+/- 5 us. Thank for the information. My deisng is continouesly running for couple of month till now without seeing any intensity changes so far.
I would like to add that LED datasheets tend to be conservative in terms of short high current pulses.
At your own risk, you must do "sufficient reliability testing", but I give good odds of success with situations that the datasheet does not say is good.
Slight caution - the LED may have a design change not reflected in the datasheet and such design change could reduce ability to survive conditions that the datasheet never supported. Those who abuse LEDs are "on their own", although so far I have yet to blow an LED with hours of low-duty-cycle pulsing with peak current that according to the datasheet "should blow the LEDs sky-high" (my words).
I would like to add:
Look at the datasheet for both peak current and maximum pulse duration and also duty cycle. So far, I have yet to blow an LED with higher peak current as long as pulse duration is reduced in inverse-square relation to increase in peak current according to the datasheet, the average power dissipation does not exceed what the datasheet supports as allowable (even with higher voltage drop at higher current), and the RMS current does not exceed what is known safe. As in: For most LEDs rated 30 mA max continuous, 100 mA max peak at 10% duty cycle and for 10 milliseconds on-time (cycle period 100 milliseconds): RMS current tends to be allowed to be up to 32 mA or so according to the datasheet, though not documented when peak current gets past 100 mA.
I would extrapolate to pulse duration at current past 100 mA being 10 milliseconds times square of ratio of 100 mA to actual pulse current, and allowable duty cycle 10% times is square of ratio of 100 mA to actual pulse current.
So far, I have yet to bloe LEDs with such conditions with pulsing to an amp or so. However, I do warn that doing anything outside the datasheet's box is at your own risk!
One more thing - LEDs are nonlinear. Most white, blue, and non-yellowish-green ones are more efficient at lower peak currents! Same story at least with "low current red" - as in most red ones of a
30-plus-year-old chemistry but having nominal peak wavelength of 690, 697 or 700 nm (believe me, their bandwidth is on the high side for phosphorless LEDs). Most high brightness and high efficiency ones other than the above tend to have efficiency maximized when peak current is in or near the range of from half of maximum rated continuous current to double maximum rated continuous current.
One more thing with pulsing LEDs: Watch for higher voltage drop, and do not exceed maximum rated power dissipation (I find that is usually easy to obey with lower average current when peak current is high and a maximum RMS current is obeyed).
RMS is not 1.11 times average for most pulsed waveforms - for rectangular pulses, RMS is peak times square root of duty ratio. For example, 1 amp at duty ratio of .1% or 1/1,000 has RMS current of 31.6 mA.
If your led current is 100mA and the duty ratio is 1 in 255 the average led current is 392uA. Allowing nothing for the reduction in efficacy due to the high current, the observed 'brightness' of the led will be the same as if it was powered with a constant current of 392uA. Quite clearly this would not result in the 'bright' leds you report and one is led to conclude that your currents are much higher than you state.
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