Constant Current vs. PWM dimming Revealed

[Doug S]

[ QUOTE ]
McGizmo said:
However is there additional degradation to the life of the LED as a function of the current level beyond the thermal issues?


[/ QUOTE ]

Don, the Luxeon Application Brief AB25, Luxeon Reliability, seems to suggest so. It would be hard from this AB alone to say for sure whether CC or PWM would be preferred from a lumen maintenance standpoint but if the "all else equal" condition stipulates equal luminous output I would intuitively suspect that the CC method would be preferred.

 

[andrewwynn]

Well. this definitely bodes well for my LDO driver on my upcoming variable nano (which dims down to zero).. i think that it only takes like 80 micro amps to drive my circuit, so when dialed down to low double-digits i should be able to get double-digit hours of runtime with the varinano.

I have a demo online: http://rouse.com/nano click on 'varinano' on the top.

-awr

 

[NewBie]

Doug S said:
Nice work, Doug! I note that the emitter x-y chromacity coordinates fall outside the bin structure of the current Luxeon binning scheme. Do you know the bin of the emitter tested? BTW, On the Nichia datasheets for their high power white LEDs they provide some graphs related to those that you have done. They provide graphs of x-y chromacity as functions of temp and also of drive current.

I'm aware of the Nichia datasheet, haven't had a chance to look at it and compare. I don't know the bin, this BW01 is before hounding Future to give us the binning data on the emitters. It is a rather yellow emitter, fyi.

When you current dim, the wavelength of a blue LED shifts alot, see below:

 

[NewBie]

FYI, the data there is for the 380mA range.

 

[Kiessling]

This is one of the best threads I read for a while. As I cannot contribute in an adequate way here I simply say "Thanx" to those who actually did the work !
/ubbthreads/images/graemlins/thumbsup.gif /ubbthreads/images/graemlins/bowdown.gif
bernhard

 

[NewBie]

Thanks for the kind words Kiessling.

Here is some more data for everyone.

This is what the blue LED underneath the phosphor is actually doing, as far as wavelength shift, note how it stays put during PWM dimming:

Here is what the spectral output is actually doing, notice how it gets less pure when current dimming:

To help visualize things, here is a visual spectral plot of the blue LED. Notice how the spectral plot for each is not symetrical on each side of the wavelength peak, and more is on the green side. Next, note how the peak has moved, with the different current levels. Finally, check out the width of each plot, and notice how it broadens out at low current levels.

In contrast, with PWM dimming, the dominant and the peak wavelength do not change, and when you dim it the spectrum actually narrows, reverse of what current dimming does. And it doesn't shift in wavelength left or right. See below:

 

[McGizmo]

Well we can certainly see why Lumileds wants PWM dimming VS variable constant current systems! Facinating info Newbie!

In the name of efficacy, I say bring on the CC and let the color shift be hanged! /ubbthreads/images/graemlins/grinser2.gif Surely it is poetic that the shift is towards the warmer levels for the more intimate settings. /ubbthreads/images/graemlins/nana.gif

 

[NewBie]

McGizmo, you could do a combo of both and take advantages of each...

Anyhow, here is some more info in a different format:


This is the difference in the relative output when you PWM dim, shown in a spectral power plot:

Here is the difference of Current dimming in a spectral power plot:

Here below is the difference of the 0.05% power input levels for current dimming vs. PWM dimming. Take careful note of how with the current dimmed plot, how the blue peak shifted to a longer wavelength towards the red, and how the output of the phosphor (the big hump running from cyan through green to red), increased it's relative output (went higher) in relation to the blue peak at 100%.

Not every emitter has the same wavelength blue LED die in it (think of the luxeon lottery but in blue), and depending on where it's wavelength starts at, in relationship to the peak phophor conversion efficiency wavelength for the blue, emitters in the same batch and binning will do different things.

I tried to hold the slug temperature as constant as possible. But in a flashlight, it usually isn't possible to do provide such extreme cooling. Additional changes begin to occur once you start changing the temperature.


I hope this has been educational and enlightening for the general CPF crowd.

 

[Doug S]

Interesting data on the blue lux wavelength shift with current. If we take the 380mA as 100% and the 7.6mA as 2%, the shift from 2% to 100% is 5nm. Putting this into perspective, this is the width of a single color bin for the blue emitters. AB21 defines 6 bins for blue emitters, each 5nm wide. There is also a tempco associated with wavelength. The luxeon datasheets give this as 0.04nm/C for the blue emitters.

 

[NewBie]

Doug S said:
Interesting data on the blue lux wavelength shift with current. If we take the 380mA as 100% and the 7.6mA as 2%, the shift from 2% to 100% is 5nm. Putting this into perspective, this is the width of a single color bin for the blue emitters. AB21 defines 6 bins for blue emitters, each 5nm wide. There is also a tempco associated with wavelength. The luxeon datasheets give this as 0.04nm/C for the blue emitters.

Yes, compare the shift for 100% and 2% for current dimming.

Next, compare the shift for 100% and 2% for PWM dimming.

Think about what is occuring during this time with heat in the die.

Remember, I held the slug itself within a 2.3C range over the 380mA test range. There is a 15 C/W thermal resistance for the 1W emitter to the slug. 3.6V * 380mA = 1.368 Watt
1.368 Watt * 15 C/W = 20.52 C rise in the emitter die. Add in the slug temperature rise of 2.3C (monitored with a K-type thermalcouple embedded in the slug) and we get to a 22.82 C rise.

22.82 C * 0.04 nm/C = 0.9128 nm shift. We are seeing alot more than this when Current dimming...and there is no peak or dominant wavelength shift when PWM dimming (in the blue).

In fact, the PWM blue dominant wavelength and peak wavelength stay put, and match the emitter running 100% duty cycle (on continously at 1140mA), even clear down to a pulse width of 3.67 millionths of a second.

; )


BTW, red emitters when PWMing do change their wavelength.

 

[Canuke]

I always thought that PWM was less efficient, on the grounds that LED's are more efficient at lower currents... but it's cool to see the details.

Great post Newbie.

 

[Darell]

Freaking amazing stuff, Newb!

And now we know what Don's been doing under the blanket with his flashlights all these years...

 

[NewBie]

Canuke,

There was an un-answered assumption that a PWM'd LED would be more efficient, since it doesn't have power applied all the time, it would be cooler, and thus more efficient. Looking at it from the other side, you are pulsing it, but you are hitting it full tilt, whacking it with full power, during which time it would be less efficient. With the data, it shows that the efficiency of PWM is quite poor, and in one common scenario shown above, showing that current dimming is 230% more efficient. This equates into 2.3x the runtime. Additionally, you loose efficiency in the battery, due to I^2*R losses in the battery. Some day, I hope to put together two circuits that emit the same amount of light, one a dim current circuit, the other a dim PWM circuit, and hook them to a rechargeable cell, and do a runtime comparison.

Again, there should be a Holy Grail combination of both approaches, current dimming and PWM dimming, that gets a person limited color shift, without paying the high efficiency penalty of PWM dimming at low light output levels.

Thanks Darell.

Anyone that hops in bed with several thousand flashlights has to be a cpf'er.

 

[McGizmo]

Newbie,

Perhaps with the nanotubes you brought to our attention, the color shift will be a non issue. The future certainly may be bright and not too wasteful! /ubbthreads/images/graemlins/grinser2.gif

 

[NewBie]

That and silicon nanocrystals to replace phosphor...

Looks like they are attacking it from multiple aspects.


Anyhow, I was testing some red LEDs today, and found out that their efficiency is barely any different from PWM dimming compared to Current dimming. Their color also shifts dang near the same with either method!

I do notice that it appears the die is bonded directly to the slug. Which is different than a blue/green/white luxeon, where the die is floating on a few solder balls, which connect it to an ESD protection diode, which is then bonded to the slug. I'm guessing the thermal time constants between the red and B/G/White parts are different.

Anyone venture to guess about the other mechanisms that would cause the difference?

 

[Canuke]

I'd wonder about capacitance or some other mechanism that "smooths" PWM enough to make it resemble CC dimming... but I speculate. I know nothing about parasitic capacitances in the different LED architectures.

 

[cgpeanut]

This is awesome work guys thanks for posting, Let me read this thread again give me a chance to digest.

 

[andrewwynn]

Well.. i already built my prototype dimming nano.. http://rouse.com/nano (look for varinano).. and i was definitely NOT unsatisfied by the color shift when i went down as low as 1mA (well.. it doesn't really output any light at that level.. it's just a cute green square).. in any event, once the light is turned up into double digit miliamps.. it might shift around a bit, but i was very happy to see that it's more efficient at lower power levels than PWM which i would definitely normally perfer.

I picked up a few sample TI chips that use PWM for higher power and FSM (frequency shift) for lower levels.. it's only designed to work from like 50-60mA to 400mA, but that's perfect for my little AAA light.. the beauty is that efficiency actually goes up at the lower power levels.. up to 95% at like 70mA.. i'm dying to get a circuit built into one of my prototypes.

Love the details that went into this test, even though i'll be using the more sloppy color-correct way to dim my lights, very nice to see the details.

-awr

 

[NewBie]

Canuke said:
I'd wonder about capacitance or some other mechanism that "smooths" PWM enough to make it resemble CC dimming... but I speculate. I know nothing about parasitic capacitances in the different LED architectures.

No, when driven properly, the capacitance doesn't cause a smoothing of the PWM. I've actually made sub microsecond pulses that have a nice square shape to the optical output.

There is only 800pf of capacitance, that changes as the forward voltage changes.

I actually exchanged some emails with Mike Krames of LumiLEDs Advanced Laboratories.

It has nothing to do with the thermal time constants or anything.

"Efficiency increases with decreasing current for InGaN (blue/green/white) LEDs, to reach a maximum at very low currents (1-2 mA or so for standard small chip used in 5 mm lamps). So, decreasing the dc current level gives you better efficiency, than pulsing at a high current. However, blue shift also depends on the drive current level, so the peak wavelength will change under "current dimming" but not under PWM. This color shift is not due to dimming but is related to some details in the InGaN/GaN electronic bandstructure.

For AlGaInP (red), the blue-shift effects are not present so all you get is red-shifting due to Joule heating, at ~ 0.16 nm/K (which apparently you are avoiding, which is good). Also, for red the efficiency is relative flat with current (it actually *decreases* at very low currents), so you don't observe the effects as with InGaN/GaN. The differences between AlGaInP and InGaN/GaN all relate to bandstructure properties and active region designs, which get into many details."

 

[pokkuhlag]

Free Bump /ubbthreads/images/graemlins/smile.gif Thanks for the info.