Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Light output of LED with various current waveforms (but same average current)

treez

Advanced Member level 5
Joined
Sep 22, 2008
Messages
7,602
Helped
560
Reputation
1,123
Reaction score
536
Trophy points
1,393
Location
cambridge
Activity points
76,156
Hi

We want to drive 350mA (average) through a Cree XT-E white LED. We want as much light output as possible.

What will be the difference in light output if we drive the LED with…..
..a) 350mA constant DC current (as in the red waveform attached)
..b) 350mA average but it’s a 500Hz triangle wave (as in the green waveform attached.)

The light_Flux vs Current graph at the top of page 27 of the LED datasheet (below) appears to suggest that the maximum “Light flux per Amp” comes at around 100mA.

Do you know what this curve looks like from zero Amps to 100mA?

Datasheet: Cree XT-E Lamp (350mA LED)
https://www.cree.com/led-components/media/documents/XLampXTE.pdf
 

Attachments

Last edited:

KlausST

Super Moderator
Staff member
Joined
Apr 17, 2014
Messages
17,435
Helped
3,938
Reputation
7,874
Reaction score
3,811
Trophy points
113
Activity points
115,599
Hi,

you might just linearely extrapolate to 0/0.

Thus you might not have much difference in brightness (at least an eye won´t recognize the difference). Even for a luxmeter it should be marginal.

Since there is a resistve component in the LED the power dissipation will be higher at triangle.


Klaus
 
  • Like
Reactions: treez

    treez

    points: 2
    Helpful Answer Positive Rating

BradtheRad

Super Moderator
Staff member
Joined
Apr 1, 2011
Messages
13,473
Helped
2,667
Reputation
5,328
Reaction score
2,572
Trophy points
1,393
Location
Minneapolis, Minnesota, USA
Activity points
100,744
An old article gave a comparison to the effect that subjective brightness is greater when pulsing an led, say at 20mA 50% duty cycle, as compared to 10mA continuous. Power is the same.

This was before 350mA or bright white led's but it should apply today as well.
 
  • Like
Reactions: treez

    treez

    points: 2
    Helpful Answer Positive Rating

treez

Advanced Member level 5
Joined
Sep 22, 2008
Messages
7,602
Helped
560
Reputation
1,123
Reaction score
536
Trophy points
1,393
Location
cambridge
Activity points
76,156
Thanks, i have read that due to "Auger recombination", then when below a certain level of LED current, the light photons just cant all properly escape from the LED silicon, and so at low currents, there is proportionally very much less light output....so in other words, if you view page 27 of the datasheet in the top post (top graph)......the unshown bit below 100mA is a region of very very low light flux output....thats why they dont show it.

Would you agree......if the bit below 100mA was just a plain extrapolation...then they would have shown it...but because it is in fact a region of (proportionally) very poor light output...they dont show it...for marketing reasons. Would you agree?
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
46,940
Helped
13,925
Reputation
28,098
Reaction score
12,568
Trophy points
1,393
Location
Bochum, Germany
Activity points
273,761
The region below 100 mA isn't shown because it's beyond the normal application of this LED. Other manufacturers show efficiency at lower currents, see e.g. this graph for a 350 mA Osram LED. I added the red line of constant efficiency.

1592329157156.png

Efficiency is dropping below 100 mA, but not abruptly. Mapping your waveform to this or a similar efficiency curve suggests that you produce most additional losses compared to constant current operation in the range above 350 mA and less below 100 mA. But for an exact calculation, you need to refer to a efficiency curve for pulsed operation (at constant chip temperature) which is rarely given in datasheets. Without it, you don't know which part of efficiency drop at higher currents is caused by temperature increase and which part by instantaneous current.
 
  • Like
Reactions: treez

    treez

    points: 2
    Helpful Answer Positive Rating

treez

Advanced Member level 5
Joined
Sep 22, 2008
Messages
7,602
Helped
560
Reputation
1,123
Reaction score
536
Trophy points
1,393
Location
cambridge
Activity points
76,156
Thanks FvM, that is very interesting.....it in fact shows how at 700mA, the light flux is signficantly less than 2x what it was at 350mA......i'd say about 1.6x.

So in fact, having a constant 350mA current would be definetely best, better than the triangle wave current
 

Toggle Sidebar

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top