Technical Editor Margery Conner's PowerSource streams the latest developments in electronic power design and related technologies. Follow Margery on Twitter at: http://twitter.com/margeryc.
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Blog
Tuesday, August 25, 2009
Hot/Cold factor holds key to real-world efficacy for HB-LEDs
Aug 25 2009 10:21AM | Permalink |Comments (4) |
Most high-brightness LED spec sheets give the light output of the LED at 25°C Tj. However, HB LEDs in luminaires spend most of their lives operating at much higher temperatures – typically between 80°C and 110°C. As its junction temperature increases, the LED’s light output and efficacy decreases.
H
ot / Cold Factor compares the light output of an LED at 100°C Tj and at 25°C Tj; Typical hot/cold factors range from 0.80 - 0.85. Philips is claiming that their just-released Luxeon Rebel LEDs now have the top spot for production LEDs with a hot/cold factor of 0.93.
Want to learn more about real-world design considerations for HBLED design? Attend EDN’s free one-day “Designing with LEDs” Workshop in Chicago on October 6. Register now.
Related entries in: Components, Hardware, Interconnect | LED |
Reader Comments
at 8/26/2009 7:22:05 AM, Calvin said:
Another question I would think is that how much the efficiency(optical_output/electrical_input) will change when the HB LED is driven by different level of current, such as 100mA v.s. 1000mA?
at 8/27/2009 11:52:58 AM, Petri said:
Relating to Calvin, what is the difference between running 1000mA LED with 100mA DC or 1000mA at 10% PWM? Average thermal load should be the same, but I think the optical output is not. I think the actual instaneous current does determine the wavelength of the emitted light. And that is the reason why we are told to dim LEDs with PWM, not changing the drive current, as the latter will induce color changes.
at 8/31/2009 9:42:34 AM, Leatherneck said:
Most Power LEDs are actually more efficient when current dimmed, but PWM causes less tint/wavelength shift.
Using a LXHL-BW01 (an old Luxeon emitter from 2004)-
Here is an example, starting from 1,000mA:
www.molalla.net/~leeper/bw1140_3.png
Here is the comparison when you start from 380.1mA:
www.molalla.net/~leeper/bw01dim3.png
PWM dimming can be an issue if the subject, observer, or light source is under motion. You see distinct images, like a strobe light, even at 300Hz- it depends on the speed of the objects in question.
There is some more on this here:
www.molalla.net/~leeper/pwm_cc.htm
at 9/9/2009 5:20:40 PM, jdavidde40 said:
While using PWM to dim an LED load is obvious to "anyone versed in the art", Color Kinetics was STILL granted a patent on that, so users of such schemes beware.
However, Cypress Semiconductor has developed an ingenious way around the above that uses pseudo random pulse train modulation to avoid interference with the CK patent.
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