Constant Current vs. PWM dimming Revealed

NewBie

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This chart is the CIE color chart with the Constant Current dimming and PWM dimming, with the Luxeon bins shown:

bw01dim2.png



Here is an exploded view of just the data, note the dimming % is listed, % for PWM is the PWM duty cycle, % for Constant Current is the % of the max current:

bw01dim.png


In the above chart, take note of the percent where the constant current and PWM dimming split, about 50% for the Constant Current. A person could dim with constant current to 50%, then utilize PWM, to minimize color shift, yet pick up the efficiency increase.



Here is the efficiency difference:

bw01dim5.png




Yet that chart only shows half the story, here is the efficiency gain at each step, I was rather surprised here:

bw01dim3.png



Basicially it all boils down to Current Dimming is alot more efficient than PWM dimming (so you get considerably more run time on down to 0.5% current dimming, but PWM dimming results in less color shift. A combination of the two approaches could sure be interesting indeed.

FYI, the current level this Luxeon was ran at is 380.1 mA = 100% Also, the Luxeon emitter slug had a hole drilled into the side and a type-K thermalcouple inserted, the slug temperature variation over the whole test was a maximum of 2.3 degrees Celcius (yes, a large aircooled heatsink was utilized for this). The same emitter is utilized for all sets of data.
 

modamag

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Awsome!

What's circuit size? Will it fit inside Mag AA, C, or D host? /ubbthreads/images/graemlins/naughty.gif
 

idleprocess

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PWM requires power for switching. I'm surprised the efficiency delta isn't just a constant all the way up to 100% power, where they're both just constant-current situations.

Switch power on, switch it off. The hertz should be the same no matter what the pulse width, so the penalty seems like it should be linear.

...but I'm thinking electrically, not optically. I'd think that the optical difference would be negligible with a pulse width that's long enough to reduce LED response time penalty to a negligible function of the pulse.
 

NewBie

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Right, but the Luxeon emitter becomes significantly more efficient at lower current levels.

Where with straight PWM, at the same current, you are hitting the emitter with full current.

The info is right on the Luxeon datasheet, if you look carefully.

Next to measure the difference utilizing 1 Amp.

I am expecting even a larger efficiency difference, we will see.

The efficiency gain of current dimming of up to 136% was rather surprising, I was not expecting that large of a gain, more like 15%, but the datasheet doesn't go down to 11.4 mA...
http://www.lumileds.com/pdfs/DS45.PDF

It's kinda cool, how the PWM dimming turned out so flat, just like it should.

BTW, each individual measurement point was taken 15 times and the 15 samples were averaged, to increase accuracy. (in otherwords, 270 measurements were taken, thank goodness for computers).

For the technogeeks that wanna be in the know, the frequency that the PWM circuit was ran at was 133 Hz, which means the pulse width at the lowest level (0.05%) was 0.000003759 seconds (or 3.759e-6 seconds).
 

McGizmo

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Wow! There's detail for you! /ubbthreads/images/graemlins/bowdown.gif
 

idleprocess

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[ QUOTE ]
NewBie said:
Right, but the Luxeon emitter becomes significantly more efficient at lower current levels.

[/ QUOTE ]

I knew I was missing something.

I wonder if the numbers would change much with a higher pulse rate?
 

NewBie

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Okay, I ran some numbers at 1140mA, such as you'd have with a Li-Ion, and using a PWM to dim the LED.

First off, at the higher current level, the tint variation increases substantially:

bw1140.png




A close up at 1140mA:

bw1140_1.png



And we have the increased light output efficiency of current dimming vs. PWM dimming:

bw1140_2.png



It really adds up alot at the lower dimming levels:

bw1140_3.png



On a side note, I noticed that the curve fit for what is shown on the LumiLEDs datasheet (output vs. current), when measuring a blue emitter, ends up with nearly an exact match , but the curve is more for a white emitter.
 

HarryN

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Hi Newbie - Great info as usual. Also as usual, I am trying to really grasp some of the more subtle aspects of your data and analysis.

I always try to compare the data and value of an advancement to the most basic way to manage the current of an LED driver (a set of resistors at 2, 10, and 25 ohms in line with 2 x 123 cells.)

Would it be correct to interpret from the information that at dimming near the 10 % x 1100ma point, a resistor setup would be more efficient than PWM ? Of course, color control is something else entirely.

Sorry if this question is entirely missing the point of your post. /ubbthreads/images/graemlins/confused.gif
 

NewBie

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HarryN said:
Hi Newbie - Great info as usual. Also as usual, I am trying to really grasp some of the more subtle aspects of your data and analysis.

I always try to compare the data and value of an advancement to the most basic way to manage the current of an LED driver (a set of resistors at 2, 10, and 25 ohms in line with 2 x 123 cells.)

Would it be correct to interpret from the information that at dimming near the 10 % x 1100ma point, a resistor setup would be more efficient than PWM ? Of course, color control is something else entirely.

Well, if you mean 1140mA * 10% = 114mA for current dimming,
vs. a 10% PWM duty cycle with 1140mA pulses, yes, most definitely. If 47.5% of the power to the LED went up in the resistor, you'd be equal for efficiency, with more color tint drift. You'd probably pick up a tad of additional efficiency, since the I^2*R losses in the battery would be lower too.

If you have more than 47% of the power going up in the resistor, the PWM would be more efficient.

If you utilized a switcher that was over 53% efficient, you'd have less losses than a PWM, and most switchers are considerably more efficient than that, even the simple Zetex ones do better than that.

Even at 85% On duty-cycle and below, the NextGen converter that dat2zip did, and setting it for a lower output current level, would do better than a PWM curcuit, due to the efficieny losses emitter when you whack it with 1140mA using a PWM circuit.

So you are going to have to figure out your resistor value, and compute the power you have to burn up as heat, to know for sure.
 
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Doug S

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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.
 

McGizmo

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Are there any lumen maintenance considerations between a PWM dimmed LED (from a high level of current) as compared to a constant current set up that is driving at a lower current level, comparable in output to the dimmed PWM circuit? If heat is the primary antagonist to the lumen manitneance then I am assuming that the only difference would be due to the delta in efficiencies between the two systems. However is there additional degradation to the life of the LED as a function of the current level beyond the thermal issues?

EDIT: I recall PK stating that there were a number of reasons and merits for going with the various constant current levels on the U2 instead of going with PWM but he did not elaborate. Even though there are the short comings in color shift, SF chose to go with the variationin current level over a duty cycle. /ubbthreads/images/graemlins/icon3.gif /ubbthreads/images/graemlins/thinking.gif
 
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