Overdriving LED's at reduced duty cycles - truth or fiction? - Page 1
Forum section: electronics / projects

author: Corporate666
author_rank: Unknown
timestamp: September 26, 2013, 05:29:40 am
content: So I've multiplexed plenty of LED's with great success (insert Borat pic).But I have always noticed that the datasheet will give a listing for max pulsed current and have a note giving duty cycle and pulse duration below. Case in point - I am looking at an RGB Cree LED now which can take 50mA/35mA/35mA respectively for R, G and B. And it lists 200mA/100mA/100mA for max pulsed current, but says that is max 0.1ms pulse width, 1/10th duty cycle.I have always heard that multiplexing LED's with high pulse currents is fine, so long as the average current remains below the specified threshold. However based on that, with a 1/10th duty cycle, one would expect to be able to pump 500mA/350mA/350mA into this LED. I realize the LED is much less efficient at 200mA than at 50mA, but it seems the brigheness is massively compromised by multiplexing only at the manufacturers specified rate.So, what's the deal - are the manufacturer numbers needlessly conservative? Is average current what matters? Is it the reduced efficiency at high drive currents (i.e. heat) that causes the manufacturer listed cap?

author: Whuffo
author_rank: Regular Contributor
timestamp: September 26, 2013, 05:47:48 am
content: The manufacturer's specs are conservative; they need to be because the parts are guaranteed to operate at those specs over the full range of environmental conditions.At your desired current levels, you might melt the bonding wire or overheat the chip. Or maybe not; the only way to be sure is to give it a try. If you're designing a commercial product, stick to the specs. But for personal use, push them and see what happens. The worst that could happen is you trash some LEDs.

author: Corporate666
author_rank: Unknown
timestamp: September 26, 2013, 06:14:41 am
content: Quote from: Whuffo on September 26, 2013, 05:47:48 amThanks for the info. But what I am wondering is why the discrepancy between the idea that average current is what matters, and the manufacturer specs? Is it the inefficiency of LED's at higher drive currents that means more of the power dumped in is going to wind up as heat? Or is there some other physical process going on that causes the LED to not withstand greater currents?I also find it interesting that they don't list a different number based on how many dies are illuminated. This is a PLCC-6 package so each die has separate anodes and cathodes, so it makes sense in that regard, but there is also a PLCC-4 common anode version of this LED with the same specs. I would have thought that with common conductors, one could get away with more current if only using one of the dies, but they don't spec that.

author: David_AVD
author_rank: Super Contributor
timestamp: September 26, 2013, 06:14:58 am
content: I've wondered the same thing. There must be some upper limit to the current where even a narrow pulse will kill the LED.

author: johansen
author_rank: Super Contributor
timestamp: September 26, 2013, 06:36:01 am
content: take a look at the forward voltage drop vs current graph and calculate the rms watts, rather than multiplying average current and assuming the forward voltage drop stays the same.

author: Corporate666
author_rank: Unknown
timestamp: September 26, 2013, 06:43:25 am
content: Quote from: johansen on September 26, 2013, 06:36:01 amUnfortunately the LED's I've seen have very non-linear V-I curves but on the datasheet they never go as high as the max pulsed current, so it's anyone's guess what the voltage is. But you're right, the voltage is not linear, so 60mA is not triple 20mA in terms of the power. But not having seen any LED's with a VI curve that hits the max pulsed current, are you saying that the max pulse current spec does tend to be 10x the power at a 10% duty cycle compared to the continuous rating? Because those curves get very steep on the high end, so very small voltage differences result in big changes in current. On this Cree LED, just extrapolating a WAG (wild ass guess), I would say there is only 0.1 or 0.2 volts increase from 50mA drive to where 200mA drive current would be.I've noticed that when driving LED's at the max pulse width/duration/current, the LED appears significantly dimmer than at the continuous rating. Of course the LED is much less efficient at higher drive levels, but AFAIK, most of that is down to heat and the effect it has on the die/light output, which should be much less of a problem at very short durations. So I think the average power through the LED at the pulse specs is significantly less than at the continuous rating. I don't know why, though. I've heard other engineers say the average is all that matters, which makes sense (especially with short pulse widths).

author: Whuffo
author_rank: Regular Contributor
timestamp: September 26, 2013, 07:00:55 am
content: It's usually a heat dissipation issue; the epoxy package the LED is in tends to be a very poor conductor of heat. Figuring the amount of heat that the package radiates versus the amount of heat generated requires data that the manufacturer isn't likely to share. The heat weakens the die mounting and bonding wire; get it hot and a little overcurrent will pop it.

author: johansen
author_rank: Super Contributor
timestamp: September 26, 2013, 07:28:57 am
content: Quote from: Corporate666 on September 26, 2013, 06:43:25 amyou've got an oscope right?measure it yourself and do some tests with a light meter or solar panel in a closed box.if you can visibly see the difference between say 10% pwm and constant power for the same average light output then that is very serious.. the eye is logarithmic you know.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: September 26, 2013, 03:32:29 pm
content: When computing DC-equivalent "averages", isn't rms the norm? I get 63ma as the rms current for the pulse, which is a lot closer to 50ma.

author: johansen
author_rank: Super Contributor
timestamp: September 26, 2013, 05:18:20 pm
content: Quotethat sounds too good to be true, but the LED industry has grown by two orders of magnitude since i last paid any attention to it.

author: Corporate666
author_rank: Unknown
timestamp: September 27, 2013, 02:03:38 am
content: Quote from: johansen on September 26, 2013, 07:28:57 amSure, I can empirically test it, but that doesn't change the specs... so I wouldn't feel comfortable running outside the stated specs unless I really understood exactly why the pulse current is derated so much.I'm not sure what you mean about seeing the difference between 10% PWM and constant power... my experience is that an LED run at 1/10th duty cycle at the manufacturer stated maximum pulsed current is dramatically dimmer than one run at the max continuous current. Have you observed otherwise? If so, perhaps it is something to do with the newer LED's, as you say, because I have pretty much only worked with the higher brightness stuff over the last few years. And I've never seen an LED that wasn't noticeably dimmer (by quite a lot, actually) when run at the max pulse current limit vs. the max continuous current.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: September 27, 2013, 02:32:10 am
content: I have wondered the same thing. I guess it has to be a heat issue and maybe similar to a i2t rating on a fuse.Also I have read that you can get more effective light out of a pulsed LED for the same power, I don't really believe this and when I asked for further info, possibly on this forum, no details were forthcoming.In my limited experience I like to parallel a cap across the LEDs to smooth the current flow. It doesn't cause me any oscillation issues. My reasons for doing this are to reduce EMI and maybe get more life out of the LEDs on the basis they will be less stressed.

author: PedroDaGr8
author_rank: Super Contributor
timestamp: September 27, 2013, 04:19:52 am
content: An excellent application note from Cree on the subject:http://www.cree.com/~/media/Files/Cree/LED%20Components%20and%20Modules/XLamp/XLamp/Application/Notes/XLampPulsedCurrentThere are a few interesting failure modes discussed. This is besides the usual burning out of the bond wires. One I found very intersting was:QuoteThey also discuss the effects of "current crowding" on the substrate and how current gradients can cause failure as well.

author: Druzyek
author_rank: Contributor
timestamp: September 27, 2013, 04:40:00 am
content: QuoteMy fear about this has always been that things will work fine when I am testing but then fail after a month or two due to the small amount of extra stress on the LEDs day after day. Would you really trust something like this to work long term if it doesn't overheat after a few days in your lab?

author: Whuffo
author_rank: Regular Contributor
timestamp: September 27, 2013, 09:27:46 am
content: Quote from: Druzyek on September 27, 2013, 04:40:00 amI'd give it 30 days of constant operation - if it survived that long, I'd have a high level of confidence that it'd last for many years.

author: Corporate666
author_rank: Unknown
timestamp: September 27, 2013, 07:21:59 pm
content: Quote from: HackedFridgeMagnet on September 27, 2013, 02:32:10 amI've heard the same thing many times over the years - in my experience and opinion it is an old wives tale, similar to the one that "LED's don't generate any heat". I get that second one a lot - we make LED lighting products and often get people who want to buy the "guts" to adapt to their own use. They often stubbornly refuse to accept that they cannot ditch our housing and thermal design to use in their own housing - because they are convinced that "LED"s don't generate any heat".