Posts Tagged ‘LED’

Weekend Links

October 14, 2011

Energy

Creating a new business model for utilities: a white paper 

* Texas Instruments now offers new integrated circuits targeted at AC power lighting applications such as LED retrofit lamps.

* GE enters the LED retrofit market. The LED chips seen in their video are not GE products; the company sold their LED division years ago.

* Hydraulic fracturing interest groups, both pro and con, have set up a site where you can track individual wells.

Uncategorized

* Lessons we learned in Physics 3C explain why those CERN neutrinos really did not go faster that C.

LED links

March 29, 2011

* Bridgelux has demonstrated a light output of 135 Lm/w from a GaN-on-Si LED, the first achievement of commercial performance from a silicon substrate LED. It may be three years for products based on the technology to come to market. Cree and Boston University want compensation for Bridelux’ use of GaN-on-Si technology.

* Our LED retrofit solution at St. Andrews is featured in LEDs magazine.


Thermal Design for LED Fixtures

April 9, 2010

Watch this video about analysis and design of LED heat sinks.

Energy Links

July 18, 2009

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*Electricians show a marked preference for LED lighting, according to a survey with 2,100 respondents.

* The “New Energy Crisis” is here, more or less.

* Ocean Thermal Conversion is being studied as a way to vitiate the power of hurricanes in the Gulf of Mexico. Any readers willing to study the thermodynamics and feasibility of this scheme are invited to write in. Some smart guys seem to think it has a chance of working.

* Natural gas futures are bouncing off $3.60/ Million BTU.

* A very credible source reports that the NYC Council is considering a Local Law mandating energy audits for buildings over 50,000 SF, with mandatory ECM implementation of all ECM with under a four or five year payback.

* The NYC DoB has begun auditing submitted plans for energy code compliance. An often-used and good program, ResChek, is only applicable to 1 and 2 family dwellings, and as such compliance with ResChek does not signify energy code compliance.

ARMSTRONG

stage-14Tour de France experts have been waiting for Lance and team mate Alberto Contador to finally take control in the Alps. Now, the Alps are finally on the horizon, with the Armstrong-Contador showdown for the yellow jersey nearly upon us. Whether George Hincapie can put up any further challenges after today is unknown. After 14 stages, Nocentini retains the yellow for another day, Lance is fourth at +0:08, conserving energy while relinquishing no time to the leader in today’s stage.

An LED Driver Design for High Currents

June 9, 2009

blockHigh-brightness LEDs are being used in luminaires for commercial downlighting, and industrial, architectural, exterior, and emergency lighting.

To produce more lumens, some LED designs require forward currents to increase. Typical LED forward currents range from 350 mA to 1000 mA; a few of the latest LED require up to 3000 mA.

To address this high current approaching 3 Amps, some driver solutions are utilizing innovative switching.

The majority of LED driver solutions available are based on standard voltage-regulator, using a fixed frequency, current-mode-control buck converter. Control schemes tend to be complex, often having two loops: an outer-loop to regulate the current control and an inner-loop to provide the peak current control. This control technique typically requires external compensation components.

Brightness control is difficult to achieve. Most solutions rely on pulse width modulation frequencies in excess of 200 Hz to avoid flicker. A good dynamic range for brightness control requires the ability to modulate to duty cycles as low as 10%. With a 200 Hz signal, this means the driver has to support the ability to turn on and off in a period of 0.1 × 1/200 seconds = 0.5 ms. As some driver solutions have a soft-start feature in the region of several ms, this can restrict the dynamic control range.

The voltage in the current-control loop can be as high as 1.2 V, which has a large impact on the power dissipation. When driving a single LED with a forward voltage (Vf) of 3.5V, the efficiency drops by 34%, even before other losses are considered.

The diagram shows a circuit built around a driver designed for driving high-current LEDs. With a simplified control scheme, the component count includes two resistors, two capacitors, one Schottky diode, and the power inductor (L). An output capacitor is not required because the peak-to-peak current ripple can be as low as 10% of the maximum LED current. This meets the majority of high bay LED lighting applications.

The design uses an inner-loop that controls the current by sensing the voltage developed across the sense resistor (R2) during the recirculation diode (D) conduction phase. No outerloop is required, removing the need for external compensation components.

Soft-start is not used in the design, so expect the life expectancy of the driver will be less than drivers that do provide soft-start, and as such the driver design is best deployed in applications not requiring repeated starting or on-off switching.

Portions excerpted from a whitepaper by Peter Tod

Scotopic

April 20, 2009

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A traditional efficacy measure for lamps (light bulbs) has been lumens per watt. Recent studies suggest consideration of an added measure for determining efficacy as it relates to human visual acuity for night vision. The Canadian government and ASSIST scientists have determined dual lumen parameters should be calculated to determine visual acuity, Photopic Lumens and Scotopic Lumens.

The eye contains two light receptors, cones and rods. Each has unique spectral sensitivity. At night both are active. For straight-ahead viewing to see objects in your direct line of motion, only cones are active. Detection of objects not in your direct line of motion is done by both rods and cones. Objects are best discerned in central vision. We know not everyone has the same peripheral vision, readers will recall the great Magic Johnson and Larry Bird.

To assure a lighting system meets standards for object detection/recognition, it would seem that the recommended light levels should be given in both Photopic and Scotopic photometric quantities, which vary by light source.

090420-field-of-vision1

The graph below shows the luminous efficiency functions of the eye at various wavelengths of light, normalized at 683 lumens at 555 nanometers. The cone achromatic channel for photopic vision has a maximum sensitivity at the green color of 555 nm. Rods for scotopic vision have maximum sensitivity illustrated by the peak of the Scotopic sensitivity function at the blue-green color of 507 nm. The differing peaks of the two sensitivity functions are due to the normalization of 683 lumens at 555 nm and not innate differences in rod or cone light sensitivity.

090420-wavelength-v-lumens1

The Scotopic and Photopic lumens seen by the eye associated with a lamp with a given spectral density should be determined by weighting that spectral density with the Scotopic and Photopic spectral sensitivity. The integrated result over the range of visible wavelengths of the weighted spectral density yields the Scotopic and Photopic lumens. The ratio of Scotopic to Photopic light for a particular lamp (S/P) is independent of absolute light level to the extent that the spectral density remains constant.

Comparison of a tested high pressure sodium lamp with a tested LED lamp array shows LED to have a significantly higher S/P values, about 2.65 times greater.

090420-100w-hps-scotopic-v-photopic-lumens090420-88w-led-scotopic-v-photopicThe Photopic lumen output of both sources, 100W HPS and 88 input W LED, is equal. The Scotopic lumen output advantage of the LED is 11,179 to 4,226 Scotopic lumens for the 100 W HPS. At a lamp output 0f 6650 Photopic lumens, the LED has an efficacy of 70 to 94 lumens per input watt (plug load) and the 100W HPS has an efficacy of 48.5 lumens per input watt (plug load), including a standard magnetic ballast with a power supply efficiency of 73%. Though efficiency of the HPS could be improved with a more efficient but more costly and less robust electronic ballast, the  advantage related to the S/P ratio cannot be matched by the 100W HPS source.  In applications such as walkway or roadway lighting where both straight-ahead and peripheral vision are required, the LED array has distinct electrical energy and visual acuity superiority.

Photo of red LED sculpted rear lights courtesy Porsche USA.

LED life

March 27, 2009

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LED lumen maintenance and mortality needs to be understood in similar terms to those of conventional incandescent and gas-discharge lamps.

Incandescent lamps display little change in light output until the bulb fails catastrophically. In large lighting installations, each lamp failure reduces the overall light output of the system, and in many such installations, a minimum illumination level is mandated by prevailing codes. When the light level falls below the code minimum, relamping is required. This is an expensive process involving administrative and manual labor. The total cost of relamping can be 16 times higher than the purchase cost of the lamp, particularly for high bay or municipal lighting installations. The issue is magnified when both lamps and ballasts are systemic.

090327-ave-rated-life-all-lamp-types-defines-b10-and-b50-valuesThe graph above is generic to conventional lamp types and illustrates the definition of B10 and B50 lamp failure for any multiple lamp installation.

LED sources do not tend to fail catastrophically, the light output degrades gradually over time. The level of degradation can be controlled by two variables available to the design engineer, input current (how hard the chip is driven), and junction temperature at the LED, a function of heat sink design and other variables. The power source can be selected to control the forward DC current, and resulting thermal management controls the operating temperature, yielding extended LED life.

The useful operating life of a power LED is extremely long, and often exceeds the lifetime of the product in which it is embedded. In a large installation, the effect of lumen degradation over time may be to reduce the overall light output level below a specified minimum. However, as relamping is so infrequent, the total cost of ownership is reduced.

The power LED industry group, the Alliance for Solid-State Illumination Systems and Technologies (ASSIST), has found that 70% lumen maintenance is close to the threshold at which the human eye can detect a reduction in light output for isolated light sources. For side-by-side lamps compared one to the other, the ASSIST threshold figure is 80%. ASSIST maintain that 30% reduction in light output is acceptable to the majority of users for general lighting applications. ASSIST propose that two co-ordinates be used to express the useful life of an LED component. These are L70, the time to 70% lumen maintenance, and L50, the time to 50% lumen maintenance.

090327-how-changes-in-forward-current-affect-useful-life-of-an-led

The graph above illustrates how varying the forward current from the power source will affect LED life. 700 mA is a good reference for power sources used to drive retrofit LED fixtures, although in the graph below, we see the junction temperature stress tests have been run at 1500 mA. Note that driving the LED at a level below its maximum rated forward current will extend its useful life, thereby increasing the quoteable L70 and L50 lifetimes.

090327-how-changes-in-junction-temp-affect-useful-life-of-an-led1In the graph above we see that limiting junction temperature to 115C will greatly impact LED life. Using the ASSIST L70 parameter, the LED lasts 80,000 hours without detectable lumen degradation.

To ensure that designers meet established criteria for light output and relamping intervals, ASSIST recommend understanding that LED performance depends on many parameters, including heat sink design, ambient temperature, thermal resistance of package and leads, and voltage and current applied to the fixture.

To enable a simplified graphical approach, more information than that expressed by the L70 or L50 figures is required. An equivalent to the traditional B-lifetime figure (illustrated at the top of today’s post) is necessary for engineers to understand the percentage of LED in which the lumen output falls below threshold, remembering that an LED exhibiting “lumen failure” below L70 still provides illumination.

To ease the challenge for design engineers who need to factor drive current and thermal management needs, testing data should be expressed in terms of L-lifetimes and B-lifetimes and presented graphically with reference to driving current and junction temperature. That graph is illustrated below, the B50 (equivalent) limiting the LED “life” to 60,000 hours.

090327-b50-l70-how-changes-in-both-tj-and-current-affect-life-of-ledFrom a design engineer’s perspective as it relates to product design for real-world customers, the data is most usefully expressed as functions of LED life as junction temperature and forward current are varied. The heat sink design and the AC-DC power supply choices become critical. Factors of thermal performance, first-cost, and weight and size of both heat sink and power supply become key to successful fixture design, low cost of installation, and low life-cycle cost.

The photo at the top of Dean Kamen’s folly illustrates an RGB LED application; testing was all performed on white LED.

ASSIST is sponsored by NYSERDA and Sylvania.

NYSERDA

March 24, 2009

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NYSERDA have traditionally funded demand reductions (kW) in preference to usage reductions (kWh), the theory being that reduction in use of transmission and distribution infrastructure, obviating T&D build-out, was more important to the participating utilities than reduction in generation.

NYSERDA have now reversed that bias.

Below is a letter we received last week from NYSERDA related specifically to LED retrofits, some of which involve exterior lighting, never a peak-demand reduction energy conserving measure. The bracketed words and designations are added for clarity.

“As per our conversation, this is our current stance on LEDs:

“LEDs are now eligible technology and they are being aggressively adopted by many of our lighting contractors.  This is exciting for everyone, but we want to make sure that applicants are installing reliable technology.  As a result, we are requiring the criteria listed below for performance projects including kWh savings from LED lighting.

“These criteria are based upon the standards addressed by the DOE’s Energy Star program for solid-state lighting.  (The applicant will probably need to request this info from the LED manufacturer(s))

  1. Independent IESNA LM-79 test data to verify Light Output, Luminaire Efficacy, CCT, and CRI
  2. Independent LM-80 test data from LED Manufacturers at 55 C, 85 C, and a 3rd temperature of the manufacturers choosing, usually higher temp.
  3. IES File in LM-63 format to verify photometrics.

“Note:  The DOE actually has a 4th criteria, but this is not required under our (NYSERDA) program.

  1. L(M)-70 Lifetime and written explanation of how it was determined, and a complete description of the thermal management of the luminaire, or something similar

“Here’s a link to the (DOE) Energy Star page for Commercial LED Lighting:

http://www.energystar.gov/index.cfm?c=ssl.pr_commercial

“As the Energy Star standards evolve, it is likely that our requirements will as well.  In general, please discuss any planned LED projects with us and expect that these applications may take more time to process than usual.

“Related to run hours, as we discussed, our programs are now entirely kWh based.  The run hours (do) not have to coincide with (facility) peak demand to receive incentives (cash rebates).”

This is a very encouraging development. The caveat related to “more time to process than usual” is the one hindrance to LED deployment with NYSERDA assistance.

Photo of Dean Kamen’s retreat in Long Island Sound lit with LED

Lighting

March 6, 2009

gabriel-led

Light-emitting diodes (LED) are compound semicondutor devices that convert low-voltage electricity to light. General Electric scientists invented the first application of LED in the 1960s. Unlike conventional lamps that can shatter,  LED are resistant to shock and vibration. The solid-state nature of LED means no filaments to break or moving parts to fail.

The advantages of the technology vary with the application. Features of LED include 90% energy savings over similarly-bright incandescents, lamp-life minimum of 50,000 hours, and excellent cold weather start-up and performance. The disposal issue faced with mercury vapor and fluorescent lamps and ballasts is obviated.

Early applications included traffic signals replacement; the technology offers color rendering choices and significant lamp replacement and energy usage advantages over legacy incandescent traffic signals.

Applications have expanded to include parts for televisions, building interior and exterior lighting, signs, focused retail displays, flashlights, elevator call buttons, commercial and residential fixtures (those changing colored lights in your hot tub are LED), and transportation and street lighting.  The LED industry is estimated to have grown 50% year on year between 1995 and 2004, and for the period 2004-2009 the US market is expected to grow from US$3.7 B to US$7.3 B, the highest growth being in transportation uses (projection courtesy Oppenheimer research).

Use in transportation infrastructure continues, South Korea and Los Angeles, Califonia having recently announced major LED initiatives.

Remote monitoring capability facilitate “smart grid” applications; flashing street lights on the curb will one day signal emergency responders regarding the location of a call.

Photo of exterior fixture