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Archive for September, 2011

Brighter LEDs Can Simplify Driver Designs

29 Sep

LED vendors are producing components that can operate at higher drive currents and produce more lumens, and system designers can move to a simpler two-stage driver as the number of LEDs in a fixture is reduced, explains BERNIE WEIR.

Much attention in the LED lighting world has focused on two applications that are at polar extremes; low-power incandescent bulb replacement and high-power street lights. While these applications are ubiquitous and iconic, there is also a range of area-lighting applications – such as parking garages, park lighting, stairway lighting – and outdoor commercial lighting – such as wall packs, wall washers, and security, flood, gas-station canopy and pathway lighting – which are hiding in plain sight. These medium- to high-power applications are ideal candidates for solid-state lighting (SSL), and new LEDs that operate at higher drive currents and produce more lumens are a good match. Designing a luminaire with fewer brighter LEDs can also simplify the driver electronics and ease the task of adding adaptive controls.

The applications mentioned above are a good match with LEDs, because they consume large amounts of power, have high hours of annual use, and are used in public spaces that may not be occupied for many hours, thus opening the door for adaptive controls and dimming that can significantly reduce energy consumption. Beyond energy cost, a driving factor of LED lighting is long life and reducing the maintenance cost of bulb replacement.

Traditionally, the aforementioned medium-power applications have used high-pressure-sodium (HPS) and metal-halide (MH) light sources. HPS light sources are actually very efficient but generate a decidedly yellow/orange light that has very poor color rendering since the light source is nearly monochromatic.

MH sources display better optical characteristics but have slightly lower initial lumen output, shorter lifetime, and higher lumen degradation over the useful life. MH lifetime (measured to 50% failure point of the population) is in the 10-20,000 hour range depending on the bulb design and construction, whereas HPS has typical lifetimes of 24,000 hours.

HPS and MH sources are also limited in terms of adaptive controls and dimming. And most of these area-lighting applications require directional light. Although HPS and MH bulbs have high lumen output, 40-60% of the generated light can be lost in the fixture.

LED life and controls

High-brightness white-LED light sources address many of the limitations of the legacy lighting technologies as they are inherently directional in nature, have very long operating lifetimes, provide white light with good color rendering and, given their instant-on characteristics, are easy to dim across a wide range of light levels.


FIG. 1

Moreover LED performance has been rapidly improving in terms of lumen output and efficacy (lm/W) and over the last few years has been making in-roads into these traditional area-lighting applications.

When white HB-LEDs came on the market, the most common configuration was called a “1W” LED because when it was driven with 350 mA, it typically dissipated around 1.2W given a nominal forward voltage of 3.3-3.5V. Today, widely-available 1W LEDs can generate more than 100 lumens at an efficacy of over 100 lm/W. To generate 5000 delivered lumens a fixture needs on the order of 50 LEDs whereas 18-24 months ago the same application may have required 80-100 LEDs.

To power such a large number of LEDs from the AC line, drivers have typically used a three-stage power-conversion architecture as illustrated in Fig 1. A two-stage constant-voltage power supply implements power-factor correction (PFC) in the first stage, followed by a high-voltage DC-DC converter that generates a safe isolated voltage rail typically in the range of 28-60 VDC.

The fixed-voltage output rail supplies multiple strings of LEDs. A third-stage, a dedicated buck DC-DC LED driver, provides regulated current to each string. Depending on the output of the second stage and the forward voltage of the LEDs, each string might include 8-12 LEDs.

Brighter LEDs

Some LED manufacturers have developed advanced processes and improved LED chip designs that deliver more lumens at higher drive currents while maintaining the same forward voltage. For example, the Cree XM-L is rated for up to 3A drive current and has a very low typical forward voltage of 3.1V at 1500 mA drive current. At 1500 mA and 85°C junction temperature, in cool white, each LED generates 440-475 lm. A fixture that uses just 12 such LEDs would have an output in the range of 5200-5700 lm with a typical load power of around 53W, delivering over 100 lm/W efficacy.

Going from 50 LEDs in the previous example to 12 simplifies the optics but it has a more sweeping impact on the driver architecture. Now all the LEDs can be safely driven in a single string, eliminating the need for multiple parallel DC-DC LED buck drivers. This also improves light uniformity as all LEDs are driven at the same exact current.

To eliminate the third stage, the two-stage constant-voltage LED power supply is replaced with a constant-current LED driver. This provides two fundamental advantages. The overall system efficiency is improved as one power-conversion stage is completely eliminated, and the cost and design complexity of multiple DC-DC converters is eliminated. It also simplifies adding intelligent control as the dimming function can now be integrated in the current-control loop of the constant-current LED driver.

Adaptive controls have been used in indoor lighting for a number of years and involve combining sensors with networked or autonomous microprocessor-based controls to optimize the lighting level to the needs of the environment. Simple examples include adding an occupancy-sensor lighting control to turn off a light source when there is no activity for a prescribed time.

Control scenarios

In area lighting, controls have been mostly limited to on and off at dusk and dawn. But dimming LED light sources is very straightforward compared to HPS and MH light sources that have long turn-on and restart times. LED light sources can be dimmed across a wide range and can rapidly turn on to 100% brightness based on activity or occupancy. In an outdoor park-lighting scene, for example, this means the light along the running path or walkway can be turned down significantly when no activity is detected and instantaneously turned to full brightness if a jogger runs down the trail in the middle of the night.

Fig. 2 illustrates a possible dimming profile through the course of a typical evening. The gray area represents timer-based control and the blue areas indicate those times that the light source is increased to 100% when activity is detected. The energy savings can be substantial since most of the operating hours are at the reduced energy-consumption levels.


FIG. 2

Application trials for such bi-level control performed by the California Lighting Technology Center at the University of California-Davis indicate these control schemes can yield energy savings of as much as 70-90% over traditional HID approaches.

Let’s consider an example of how new high-lumen packaged LEDs can simply the luminaire system architecture and efficiently convert AC power into a regulated current in the 1-3A range (Fig. 3). Such an LED driver design can also incorporate support functions to enable adaptive controls. The addition of a low-cost microcontroller (MCU) opens the door to more-sophisticated control schemes that can monitor ambient light, occupancy/activity sensors, time of day, and day of week to provide appropriate light in a variety of situations while maximizing energy savings.

MCUs and networks add functionality

For centralized control, these smart luminaires can be networked, for example using a power-line-communications (PLC) modem block (Fig. 3). Alternatively, a system can use a wireless interface like Zigbee, a traditional wired interface such as Ethernet, or existing lighting and building controls.


FIG. 3

The block diagram illustrates an AC-DC driver based on a high-efficiency PFC and half-bridge-resonant (HBR) isolated DC/DC step-down controller integrated in the NCL30051 from ON Semiconductor. The basic current-control loop consists of a simple analog circuit which monitors the output current and provides feedback across the isolation barrier via an opto-coupler to the primary-side control IC. The circuit also monitors the maximum output voltage, preventing driver damage in the event the LED string is accidentally opened.

The MCU monitors sensor feedback and network communications to control the light level. Moreover the MCU generates a PWM signal that is used to turn on and off the half-bridge driver, thereby controlling the light level. The PFC block implements a critical conduction mode (CCM) control scheme which achieves high power factor (>0.98 typical) with low harmonic content and is suitable for applications up to 200W.

Efficiency and power factor

This driver architecture can support a wide range of power levels. Fig. 4 illustrates the typical efficiency and power-factor curve of an example driver design based on the NCL30051 driving 2A with a nominal LED load of 55W. Across the 115-240 VAC line-voltage range, the efficiency exceeds 88% and the power factor is greater than 0.94.


FIG. 4

The Cree XM-L LED is ideally suited for this range of current. At 700 mA, it generates 260-280 lm (neutral/cool white). A string of 9 LEDs driven at 2A outputs more than 5000 lm. This lumen level can easily address a range of area-lighting applications such as wall packs and wall washers. Depending on the drive current and LED configuration, this type of driver approach can exceed 90% wall-plug efficiency, as well as supporting a range of dimming methods required for intelligent control.

New LEDs that have high source-lumen capability can greatly simplify the LED driver architecture by reducing power-conversion stages and increasing overall power-conversion efficiency. Adding smart controls can yield further energy savings and opens the door to new capabilities such as constant-lumen-output operation modes. Here, the driver can control the LED current to maintain constant lumen output over lifetime to combat lumen depreciation. Smart dimming-control techniques have an additional benefit of extending the operating lifetime of the LEDs as well as the driver electronics by reducing the average operating temperature.

 

Sealed LED Lighting Units from MHA Reduce Hospital-acquired Infection Rates

21 Sep

Sealed LED lighting units protect patients from harmful bacteria, while providing uniformly-distributed dimmable light.

MHA Lighting Ltd. (Manchester, UK) has worked with the National Health Service (NHS) Trusts in the UK to combat hospital-acquired infection rates using a new technology involving sealed lighting units.

MHA Lighting’s patented LED technology has been designed as a fully-sealed unit to stop dust, bacteria and deadly superbugs from gathering around warm light fittings.


Hospital room with sealed LED lighting units.

The long LED lifespan (estimated at 60,000 hrs or seven years) also eradicates the needs for routine bulb replacement, which stops harmful bacteria from being distributed into the atmosphere.

MHA Lighting MD Tom Harrison said, “Not only is the NHS saving money on operational cost for routine light maintenance, but our LED’s burn 20% of the energy of traditional fluorescents.

“The Carbon Reduction Commitment Carbon Tax on large organizations such as the NHS means for every tonne of carbon saved, hospitals receive £12 ($19.50). This money can be directly ploughed back into front-line patient services,” said Harrison.

Hospital lighting needs – 4000 K, dimming

The Manchester-based lighting specialist has completed numerous lighting refurbishments for the NHS as well as the private health care sector.

MHA recently completed an installation in North Devon District Hospital in Barnstaple, which is part of the North Devon Healthcare Trust (NDHT).

There MHA replaced traditional 72W fluorescent lamps with 4000 K Tilite 20W and 30W LED units in wards (see photo), corridors and reception areas.

The correlated color temperature of 4000K is the color required by the NHS to provide ideal light quality for examining patients or the cleanliness of the facilities.

Harrison described MHA’s technology, “Unlike other LED technologies, the light is not shone directly out, it is shone sideways and reflected out in a uniformly distributed manner,” he said. In this way, he added, the photometric performance of traditional lamps can be achieved while providing the energy and maintenance savings of LED technology.

MHA Lighting also developed a custom dimming solution for NDHT to allow lights to be dimmed down to 5%. It was fundamental for the Trust to create a more pleasant and healing environment where patients and staff are in control of ward lighting levels during sleeping hours, for example.

The Trust reduced its energy usage from 127,910 kWhr/yr to 33,044 kWhr/yr. Overall carbon emission reductions with the integration of dimmers have exceeded 75%.

Moses Warburton, re-development manager of NDHT, said: “NDHT already has an enviable standard on carbon reduction and this is part of our continuing commitment to reduce our carbon footprint.

Warburton added “It [the new lighting] has created a much better atmosphere for our patients and staff. Even light and less flicker is a huge improvement for those patients with sight problems.”

 

Are LEDs Really a Suitable Replacement for 40W Incandescent Bulbs?

16 Sep

In consideration of whether or not LED light bulbs are truly a proper replacement for 40W incandescent bulbs, as major manufacturers claim, brightness is an important factor, as we have previously mentioned. What other standards must be met in order to make the transition to LED bulbs?

According to a relevant survey on brightness of LED light bulbs, major brands that lived up to replacement specifications include European company Philips, U.S. lighting giant GE, Japanese manufacturers Sharp and Toshiba, and Korean electronics maker LG. While these companies’ products meet brightness standards, do they also adhere to other standards necessary for incandescent light bulb replacement?

The research further analyzes light bulbs from major international LED makers that met brightness standards to replace 40W incandescent lamp.

Color Rendering Index

First, in terms of color rendering, the CRI of LED lighting is not as high as that of incandescent bulbs (CRI = 100). Both the U.S. Department of Energy (DOE) and Korea’s Korean Industrial Standards (KS) have CRI standards for the replacement of incandescent bulbs with LEDs. The DOE requires a CRI of 80 or above, while the KS standard is 70 CRI or higher. While LED products from the major brands do not match incandescent bulbs in terms of CRI, they still meet various national standards. Uniquely, Sharp does not list CRI values for their products. While the research believes Sharp’s products have ratings of 70 CRI or above, it is somewhat disappointing that they do not explicitly label their products as such.

Beam Angle

Beam angle, or light distribution, is another important standard in LED lighting. Unlike CRI, product specifications vary quite a bit by manufacturers: beam angles begin at 120 degrees and range to more than double that, at 260 degrees. Looking at things from an incandescent lamp standpoint, most LED products are not up to par as the light distribution of incandescent bulbs is general and non-directional, with a beam angle of over 300 degrees. As LED light bulbs are inherently directional, a different standard is required for measuring LED beam angle.

At present the U.S. DOE and JELMA (Japan Electric Lamp Manufacturers Association) are both working on new standards for LED light bulbs replacing incandescent bulbs. Bulbs that meet current national standards include: GE’s 62180, Philips’ 90048400 PHI (250 degrees), and Toshiba’s LDA8L-G (260 degrees). These products are a cut above the rest due to their adherence to beam angle standards.

Cost-Performance Ratio

After sifting through various LED products, a few winners have emerged. However, the products that met required specifications for replacement of 40W incandescent bulbs also have accordingly high prices.

Taking a step back to look at the total economic benefit analysis, the lifespan of a single incandescent bulb is 1,000 hours. At 3 hours of use per day, one bulb will last one year (3 hours x 365 days ≈ 1,000 hours), with total expense at approximately US$5.50 (initial purchase price plus cost of electricity). All of the previously mentioned LED bulbs, while boasting low power consumption, have a higher initial purchase price than their incandescent lamp counterparts.

However, such an analysis is unfair to LED bulbs that each has a lifespan of 25,000 hours. Looking at it from a different angle, the lifespan of a single LED bulb is equal to that of twenty-five 40W incandescent bulbs. Thus, it is clear that whether we are looking at GE’s 9W 62180, Philips’ 8W 90048400 PHI, or Toshiba’s 7.7W LDA8L-G. Besides, the total expense (initial purchase price plus cost of electricity) for one LED bulb is approximately 40% ~50% lower than that of 25 40W incandescent bulbs (based on U.S. electricity rates).

Since electricity rates vary by country, total accumulated expense will vary as well. In countries with higher electric prices such as Japan, total expense for one LED bulb is only 30% or so less than that of 25 incandescent bulbs. In other words, once a consumer has bought four incandescent bulbs, their total cost has exceeded that the LED equivalent.

Summary

While many manufacturers claim that their LED light bulbs are suitable substitutes for 40W incandescent bulbs, in actuality there are few products that meet replacement standards. As for those that pass the test, although initial cost is much higher than incandescent lamp purchases, factoring in longer lifespan and higher efficiency, LED bulbs still come out on top in regards to cost-performance ratio.

Still, as LED light bulb inspection standards are not fully developed, LED product specifications are not yet uniform. Different LED manufacturers place emphasis on different product features, causing consumers much confusion in the purchase of LED lighting. The fog will only clear when governments set clear standards and LED makers automatically disclose full product information.

 

GE Lighting’s Irick Says Reliable, System-level SSL Solutions Will Prevail

13 Sep

At a recent press conference, Jamie Irick, GE Lighting Solutions CEO talked about a system-level approach to solid-state lighting.

When Thomas Edison developed the first light bulb in 1879 it lasted less than 14 hours. Today’s LED lamps are rated for 25,000 hours – that’s 22.8 years when the lamp is operated for three hours a day. “You need to make sure you can count on the company that supplies that lighting solution,” said Jaime Irick, president and CEO of GE Lighting Solutions. In a recent press conference at the company headquarters at Nela Park in Cleveland, Ohio, Irick outlined a future of lighting that is dominated by solid-state solutions, making all other technologies obsolete, eventually.

GE has made a stronger investment in its LED sector recently, increasing its R&D personnel by 2.5X in the last three years. In Cleveland, they have 30 LED technologists researching next-generation optics, thermals components and electronics, while 90 LED engineers investigate performance, timing and reliability. This group developed the 40W equivalent 450-lm LED lamp on which subsequent 60W, 75W and 100W equivalent LED lamps will be based. In the mean time, customers will also have the choice of high-efficiency halogen bulbs, CFLs or LEDs to meet the energy-efficiency standard requirements. “It’s all about choice for our customers, but they were looking for more energy efficient alternatives, even before the legislation was enacted,” said Irick.

When asked when LEDs are likely to penetrate the general indoor lighting market, Irick said he expects that to still be 10 years in the future. However, LED costs are coming down at approximately 20% per year, and the introduction rate for LED lamps has accelerated. “We have 70 Energy Star LED products now and will have 120 by the end of the year,” said Irick.

Irick points out that customers are dictating the form that lighting will take. “While other companies may have product expertise, GE’s will offer the best system integration platform, bringing together the LED module, thermals, optics and drivers to provide the most efficient lighting platform for the application.” One example is GE’s LED edge lights, which are based on backlighting technology, and use LEDs around the perimeter of the panel and Rambus MicroLens technology to distribute the light uniformly across the panel.

Irick said that people are just beginning to envision new lighting applications. For instance, on airplanes, controlled RGB LEDs are being tested in cabin lighting to help passengers sleep during night flights.

Irick indicated that the industry is in a period of transition. He said that GE is behind the efforts of the FTC with the new Lighting Facts label. “The DOE has found that nearly thirty percent of the products tested in its Caliper studies do not meet manufacturer’s performance claims. In fact, we have already seen some products being pulled from retail shelves,” he said. “This will help level the playing field.”

GE is already seeing a shake-out of manufacturers. John Strainic, global product general manager at GE Lighting claimed this is similar to the consolidation they saw with the compact fluorescent lamps when they began to replace incandescent bulbs, which took 5-7 years to complete.

Product reliability is a cornerstone of GE’s operations. While the company does not manufacture LEDs, their reliability laboratory performs incoming qualification and accelerated lifetime testing on LEDs and LED modules. “Not all lighting manufacturers test their LEDs, but we have rigorous testing requirements and over ten years of field test data,” said Cherian Jacob, systems manger of GE Lighting Solutions.

 
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Peru’s Stadium Façade Lighting Responds to Football Fever

13 Sep

Cinimod Studio and an international design team developed an interactive LED lighting system for the Peru National Stadium that captures the audio levels of the stadium crowd and depicts it visually on a dynamic façade.

Cinimod Studio, a London, UK-based architecture and lighting design firm, has delivered an interactive lighting-control system for the Peru National Stadium in Lima. The system gathers the crowd’s noise levels in real-time and translates the audible signal into a visual map that is depicted on the façade’s lighting display.


LED-lit façade

Cinimod Studio worked as part of an international design and delivery team including lighting designer CAM and software designer ArquiLEDS, both of Lima, Peru; e:cue, a lighting control company based in Paderborn, Germany; and Traxon Technologies, a lighting designer based in Hong Kong.

The façade-lighting system begins with a network of customized microphones deployed along the stadium’s roof line. This data is then processed by Cinimod’s custom processing hardware and software located in the stadium’s main communications room. The audible data is analyzed using mathematic calculations and self-calibrating algorithms. The software then communicates a “mood state” to the e:cue lighting controller, which transmits the relevant DMX control signal to the lighting fixtures on the building’s facade.

The external lighting scheme is designed to integrate seamlessly within the architectural framework of the building. The majority of the lights are laid out as fans of flames that wrap upwards around the form of the structure. The façade’s patterns vary in color, speed, brightness and scale.

The software runs perpetually, constantly evaluating the mood, which varies between celebration at one end of the spectrum to disappointed at the other.


The façade’s patterns

The main mood states include: 1) Boring, a neutral mood; 2) Excitement, accompanying a surge in crowd noise and pitch; 3) Celebration, typically triggered by a goal and followed by a further rise in noise level; and 4) Disappointed, triggered by an excited state followed by a rapid decline in noise level.

The scale of the stadium façade necessitated a very large array of color- and pixel-addressable fittings. The lighting controller provides 62 universes of DMX lighting control output.

 
 
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