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Archive for October, 2010

Philips’ Lamps Make Super Bowl Celebration Glow with Energy Efficiency

26 Oct

Capable of projecting customizable colors onto the underside of the crown of the Tampa, Florida, stadium, 70 programmable floodlights use just 290 W per fixture.

Super Bowl lights

Raymond James Stadium, Tampa’s host venue for the Super Bowl, has been temporarily illuminated with state-of-the-art LED technology as part of a city-wide beautification effort for the championship game on February 1.

From January 27 through game day, the stadium is being illuminated from dusk until dawn with Philips ColorReach Powercore LED floodlights, turning it into a dynamic focal point for Tampa residents and visiting fans.

Design firm Infinite Scale Design was responsible for designing and branding the overall look of the city of Tampa for Super Bowl XLIII. Working with lighting designer Dall Brown, they chose to accentuate the stadium’s exterior crown with colorful, customizable lighting.

Approximately 70 ColorReach Powercore fixtures were used in total, with just two required to evenly illuminate each 40 by 80 foot bay. Mounted on a concrete cross beam from within the stadium, the fixtures project light onto the underside of the stadium’s upper 30 rows.

Super Bowl stadium

Each fixture is individually programmable and can produce millions of colors and color-changing effects, which enables the stadium to display the colors of the opposing teams as well as providing dazzling lighting effects.

Capable of projecting over 500 feet, the ColorReach Powercore fixtures make the stadium visible from the air and from multiple viewpoints across the city. The fixtures were supplied by LED Source and programmed and installed by local Tampa entertainment solutions provider, Bay Stage Lighting.

“The Super Bowl is considered by many to be the most important sporting event of the year, and we wanted to create a visually striking look for the city that matched the excitement of the event,” said lighting designer Dall Brown.

Philips floodlights in action

In addition to generating dynamic effects, ColorReach Powercore supports the National Football League’s recent efforts to make the Super Bowl event more “green.” The temporary lighting scheme requires minimal energy; just 290 W per fixture. Even when operating at full intensity, ColorReach Powercore consumes less than half the energy of a typical coffeemaker. By comparison, the traditional metal halide fixtures typically used in such exterior projects consume 1,600 W each and require gels to produce colored lighting.

Other Philips Color Kinetics installations in entertainment and sporting venues include Boston’s TD Banknorth Garden and Bank of America Pavilion, Los Angeles’ Hollywood Bowl, Milwaukee’s Marcus Center for the Performing Arts and Sweden’s Globen Arena.

 

Light Emitting and Brightness Control Principles of LEDs

20 Oct

1. What is an LED?

When the minority carrier injected into the PN junction of semiconductor material recombines with the majority carrier, the excess energy will be be released in the form of light, which directly converts electrical energy into light energy. When reverse voltage is imposed on PN junction, minority carrier is hard to be injected, it does not light. This kind of diodes which are manufactured by using the principle of injected electricity leading to lighting are called light emitting diodes, commonly known as LEDs.

2. How does an LED light?

The light color and light efficiency of an LED are related to the materials and workmanship used to manufacture LEDs, currently there are three kinds of LEDs, red, green and blue LEDs which are widely used. Since the working voltage of LEDs is low (only 1.5-3V), and LEDs emit light spontaneously and have a certain brightness, meanwhile the brightness can be adjusted through voltage (or current), LEDs themselves are shock resistant with a long lifetime (100,000 hours), so in large display device, currently there is no other display methods to compete with the LED display.

The display screen that use the pixel which is made by putting one red LED and one green LED together is called double color display or color display; the display screen that use the pixel which is made by putting one red LED, one green LED and one blue LED together is called tricolor display or full color display. The pixel size of indoor LED screens is generally 2-10 mm, which often uses the method of packaging several kinds of LEDs that can can emit different primary colors as an entire body, The pixel size of outdoor LED screens is generally 12-26 mm, each pixel consists of several different single color LEDs, the finished product is called pixel tube, and double color pixel tube usually consists of 3 red LEDs and 2 green LEDs, tricolor pixel tube typically consists of 2 red LEDs, 1 green LED and 1 blue LED.

Whichever kind of screen we use LEDs to make, single color, double color or tricolor screens, to display images, the brightness of every LED that composes the pixel shall be adjustable, and the adjustable degree is the gray scale of the screen. The higher the gray scale is, the more delicate the displayed image is, the more rich the colors are, the corresponding display control system is also more complex. Generally color transition of 256-level gray scale images is already very smooth, while the color transition of 16-level gray scale color images has obvious boundary line. Therefore, the current color LED screen are required to make as 256-level gray scale.

3. Methods to control LED brightness:

There are two methods to control the LED brightness.

1). Take advantage of human beings’ visual inertia and use a pulse width modulation method to achieve gray scale control, which is cyclically changing the pulse width (ie duty ratio), as long as the repeated cycle of lighting is short enough (ie refreshing frequency is high enough), the human eyes can not feel the jittering of light-emitting pixels. Because PWM is more suitable for digital control, and the dispaly content of LED screens is supplied by micro computers, almost all of the LED screens use pulse width modulation to control the gray scale.

2). Changing the current that flows through the LEDs, generally LED tube allows a continuous working current of around 20 mA, except that red LEDs have saturation phenomenon, other LEDs’ brightness is basically proportional to the flowing current.

LED control system usually consists of main control box, scan board and display and control device. The main control box gets the brightness data of different color LEDs in one pixel from the computer’s graphics card and reassigns them to several pieces of scan board, each scan board is responsible for controlling several rows (columns) on the LED screen, and the display and control signals of each row (column) on the LED screen and control signals are transferred with serial manner. There are two serial transfer methods to transfer display and control signals: one is that the pixel gray scale is controlled collectively on the scan board, the scan board decomposes the brightness value of each pixel from the control box. (ie pulse width modulation), then transfer the opening signals of the each row LEDs to the corresponding LEDs with pulse mode and in serial manner (1 for lighting up, 0 for not lighting up) and control whether to light it up or not. This way uses a small quantity of components, but the amount of serial transfer data is large, because in a repeated cycle of lighting, each pixel with 16-level gray scale requires 16 pulses, with 256-level gray scale requires 256 pulses, due to the device working frequency restrictions, this method can only achieve a 16-level gray scale for LED screen.

Another way is that the content that the scan board transfers in serial manner is not the openning or closing signals for each LED but a 8-bit binary brightness value. Each LED has its own pulse width modulator to control the lighting time. Thus, in a repeated cycle of lighting, each pixel under the 16-level gray scale requires only 4 pulses, and under 256-level gray scale requires only 8 pulses, which greatly reduces the frequency of serial transfer. Using this decentralized control method to control LED gray scale can easily achieve 256-level gray scale control.

 

LEDs Go with The Flow in New York City Waterfalls

15 Oct

A series of man-made waterfalls installed around New York Harbor were lit using LED lighting fixtures to simulate the effect of moonlight on the water.

The New York City Waterfalls public art installation, by artist Olafur Eliasson, comprised four man-made waterfalls in the New York Harbor situated along the shorelines of Lower Manhattan, Brooklyn and Governors Island. One of the key features of the waterfalls was their use of LED lighting behind the water flow to simulate the effect of moonlight.

The Waterfalls ranged from 90 to 120-feet tall and were on view from June 26 through October 13, 2008. The project was commissioned by the non-profit Public Art Fund in collaboration with the City of New York.

Each waterfall pumped 35,000 gallons of water per minute from the East River to the top of a scaffold. As the water flowed over the lip of the falls it was illuminated by LED fixtures.

Each waterfall utilized a continuous row of assembled 5 feet sections of LED fixtures, supplied by Boca Flasher Inc, with a mixture of cool and warm white LEDs controlled by 3 separate channels. The total wattage was 300W per 5 ft. section at 100% output, while the actual operating load was 15-25%, depending on the site.

Light from the LED fixtures grazed the back of the water, penetrating the flow and accentuating the effect of wind gusts and changes in water flow rate. A custom baffle just below the LED source prevented glare at normal viewing angles.

The Lighting Designer for the project was Michael Mehl of Jaros, Baum & Bolles. “Working with LEDs offered us very interesting possibilities both technically and aesthetically in realizing Eliasson’s artistic vision,” he said.

Some immediate concerns over the use of LEDs, such as color rendering and heat dissipation, were overcome by the nature of the exterior environment in this project, said Mehl. “Unlike prior projects we have worked on, LEDs were not initially chosen for their promise of extended life, since the project had a limited installation period of only four months of operation,” he added.

Hiding the source was the biggest challenge, so that the water would appear as if lit by moonlight. “The size-to-light output ratio of the LEDs favored them over conventional lamp sources,” said Mehl. “We focused our design attention on the caveats of the technology, such as lamp-to-lamp color consistency and optics.”

The designer also believes that a critical component to using LEDs, beyond the specific source, is the choice of manufacturer. “Architects and lighting designers should choose a manufacturer who understands the technology and can aide the design process considerably. Since LEDs are a compilation of components, the best friend the lighting consultant can have with the burgeoning technology is to work with a manufacturer who understand the nuances of LEDs and can integrate ever-evolving aspects of the technology, while delivering on the conventional design process of conception through installation.”

 

Offline High Power Factor TRIAC LED Dimmer Design

13 Oct

The International Energy Agency (IEA) estimates that lighting power consumption accounts for 19% of total global electricity consumption. Therefore, in recent years, countries all over the world have committed themselves to a higher energy efficiency program to replace inefficient incandescent light sources. As the light-emitting diode (LED) progressed rapidly in terms of lumen output and luminous efficacy, meanwhile the average cost per lumen light output has been decreased gradually, combining with the high directivity, long lifetime and low maintenance costs of LEDs, LED lighting (also known as solid state lighting, or SSL) has become a very attractive alternative solution.

Energy Efficiency Specifications for Solid-state Lighting

To promote energy conservation, government agencies or specification organizations around the world developed different LED lighting specifications, the differences mainly focused on the requirements for power factor (PF). For instance, International Electrotechnical Commision (IEC) in European Union stipulated the total harmonic distortion performance of lighting application with a power more than 25W, This regulation is also applicable to other international standards in certain areas.

In addition, the U.S. Department of Energy developed and released “Energy Star” standards for solid-state lighting lamps. This voluntary standard includes a series of requirements for residential lighting and commercial lighting lamps (such as recessed lights, cabinet lights and table lamps), covering minimum lumen output, the overall light efficiency, reliability target, the light color temperature and a series of other key system-level requirements. It should be noted that this standard does not directly contain the energy efficiency requirements for the power supply, but contains the power factor requirements, which means whatever the power level of the lamps is, residential applications require a PF of more than 0.7, commercial applications require a PF of more than 0.9, while the integrated LED lamps require a PF of more than 0.7.

Of course, not all countries developed absolutely mandatory requiremnets for improving power factor in lighting applications, but some applications may have the requirements. For example, public utilities may vigorously promote the commercial applications of products with a high power factor in public facilities. In addition, when public utilities maitained street lights, they can decide whether the products should have a high power factor (typically more than 0.95 +) according to their will.

13W LED recessed lights design example

1. Refer to the alternative criteria to determine the maximum load design goal

We can take the “Energy Star” solid-state lighting standards as an example, the standards include the overall requirements for light efficiency of lamps; In fact, the standards are system-level standards concerned with the selected LED, on-site working temperature, optical components, driver power conversion efficiency and so on. Developers can thus make their choice in LEDs selection, the use of optical components, thermal management solution, driver topological structure and design to meet the overall requirements. The following table lists the requirements of “Energy Star” residential and commercial solid state lighting application specifications (version 1.1) for the key system of recessed lights.

Opening Size (inch) Minimum Lumen Output Light Efficiency (lumen/watt) Relevant Color Temperature (CCT)
≤ 4.5 345 35 2700K, 3000K, 3500K
> 4.5 575 35 2700K, 3000K, 3500K, 4000K, 4500K, 5000K
Note: Power factor for residential applications is flexible, it can be just more than 0.7, and it can also be as high as more than 0.9.

Table 1: The key requirements of “Energy Star” version 1.1 residential and commercial solid-state lighting specifications for recessed lights.

The most common recessed lights are recessed lights with large diameters. For residential and commercial applications, except the differences in power factor, designers can use the neutral and warm white LEDs flexibly. From the minimum requirements in table 1 we can conclude that in order to obtain the minimum output of 575 lumens, the maximum input power threshold shall be around 16.4W.

Because there is no directly applicable energy efficiency standards for LED driver, the “Energy Star” exterior power supply (EPS) standards version 2.0  can be considered as alternative standards. According to EPS 2.0 standards, the minimum energy efficiency requirement for standard power supplies with rated power from 1W to 49W is 0.0626×ln(Pno)+0.622. Therefore, the minimum energy efficiency of a power supply with 12W rated power which complies with the standard is 77.7%, while that of 15W power supply is 79.1%. Since LED lamp standards are based on the input power socket energy efficiency, it is necessary to convert the driver energy efficiency goal into effective LED load. In order to increase design allowance, we set the minimum energy efficiency goal as 80%. As a result, LED load is 16.4W×80%, that is 13.1W.

Thus, we determine the maximum load design goal. LED light efficiency subjects to the manufacturers, LED driver current and operating temperature. ON Semiconductor GreenPoint reference design chooses a constant current of 350 mA, it can support most of the high brightness power LEDs in the market. Another factor that shall be taken into account is that the lamp developers can choose a wide range of LEDs, the higher the light efficiency of the selected LEDs is, the less quantity of LEDs is required. Therefore, the energy efficiency of this GreenPoint reference design at the load range from 50% to 100% of rated load shall be high. As the LED light efficiency improves, you can easily modify the same basic power supply design to drive less power LEDs, thus providing a lamp light efficiency which is much higher than the minimum requirements.

2. Other design requirements

Once determine the basic design requirements, we need to consider other system factors that are related to the needs of end-use applications. For example, although the standards do not require, the dimming solution which can be compatible with existing circuit. Therefore, it is the bidirectional triode thyristor device (TRIAC) wall dimmers that we should focus on to optimize the design. There are many chanllenges to TRIAC dimming, but designers may be easy to ignore one factor, that is, the driver should be able to start and work under low chopper (chopped) AC input waveform conditions. Furthermore, the dimension of the power supply should match the size of junction box of the recessed light fixture. Attention should also be paid to another human factor requirement. Although LEDs can light in an instant, the design of driver shall set aside a specific start time. Whatever the LED lamps are, this aspect shall perform not worse than CFL, even better. Therefore, we can consider CFL as a benchmark. In “Energy Star” CFL bulb requirements, the rated maximum startup time is 1 second, so we will set the design goal of LED driver startup time as 0.5 second. Because this design faces to residential or commercial applications, so the target we set is more challenging. Table 2 summarizes the key design goals of the GreenPoint reference design .

Parameters Design Specifications Remarks
Maximum output power Maximum 15W Vin = 115 Vac
Output current 350mA +/- 5%
Current isolation Required
Forward voltage compatibility > 3:1
Full load energy efficiency > 80%
Power factor > 0.95 Commercial grade, minimum 0.90
Total Harmonic Distortion < 20%
Startup time < 0.5 second Vin = 115 Vac, CFL bulb < 1 second
TRIAC dimming range Minimum 10:1 (35mA)
Open circuit < 58 Vdc UL 1310 category 2 < 60 Vdc

Table 2: Key design goals.

3. The design approach: use single-stage program to provide high power factor

In order to achieve high power factor, power supply energy efficiency target and compact size, it is necessary to use high power factor single-stage topological structure. As power goal is low, the traditional two-stage topological structure (PFC increasing voltage + flyback converter) can not satisfy the request. Therefore, we use CrM flyback topological structure based on ON Semiconductor NCL30000 critical conduction mode (CrM) flyback controller.

Single-stage topological structure does not need the specialized PFC voltage increasing stage, which will help reduce the quantity of components, and decrease total system cost. However, using single-stage topological structure will also cause influence to the system, for instance there is no initial high-voltage energy storage, the output voltage maintaining time is short. In addition, the output ripple is high, we must use more low voltage output capacitance to meet the maintaining requirements, and also the single-stage topological structure responses slowly to dynamic load. The advantage is that it will not trouble various kinds of LED lighting, because the LED lighting applications require no system maintenance time, and the ripple will flow into the average light output, human eyes can not notice.

Designing high power factor (PF > 0.95) is conducive to easily meet the requirements of SSL commercial lighting requirements, and can make the input current waveform look like the waveform of a resistor type load. This is very important for being compatible with TRIAC dimmer, because the TRIAC dimmer is designed to use with incandescent lamps, and the role of incandescent lamps in the circuit is like a resistor, which acts as a resistor type load. The waveform recorded by an oscilloscope  shows that the basic current waveform of optimized design single-stage CrM flyback power supply maintains the same phase as the input voltage waveform.

Figure 1 shows simplified function block diagram of single-stage high power factor flyback topological structure which is based on ON Semiconductor NCL30000. As we can see from Figure 1, isolated flyback secondary terminal has constant current and voltage (CCCV) control module. This module has two main functions, one is steady regulating a 350mA constant current, and providing feedback to initial terminal, this function is mainly used to adjust the conduction time and stablize the constant current that passes through the LEDs; the other is entering into constant voltage control mode and generating stabalized fixed voltage under malfunctions when the open circuit incident happens. In addition, when the output short circuit happens accidentally, it can also limit the power to prevent LEDs from being damaged.

Figure 1: Simplified function block diagram of single-stage high power factor flyback topological structure which is based on NCL30000.

4. Test results

The test results show that the performance of this reference design has exceeded all the design goals listed in Table 2, see Figure 2. Figure 2 shows the power factor and input current total harmonic distortion of LED driver under the voltage range from 90 to 135Vac, we can see that the power factor of the reference design is high (exceeds the minimum 0.9 power factor requirements for commercial lighting), the total harmonic wave distortion is low (<20%). Figure 3 shows the LED energy efficiency under different load conditions. If we make an average calculation of energy efficiency of four working points 25%, 50%, 75% and 100%, the total average energy efficiency shall be 80.7%; while in the key work areas from 50% to 100% load, energy efficiency range is from 81.1% to 82%. This is not only beyond the 80% energy efficiency target that the reference design set, but also more than the 79.1% energy efficiency requirement that EPS 2.0 standards set for the 15W power supply. the energy loss also contains the energy consumption of a 15 ohm current limit resistor which is necessary to input EMI stage to support TRIAC dimming.

Figure 2: The power factor and input current total harmonic distortion of LED driver under the voltage range from 90 to 135Vac.

Figure 3: Energy efficiency under different load conditions when the input voltage is 115Vac.

Summary:

There are many chanllenges for designing an offline LED driver that can meet all the requirements of next generation solid state lighting products. This reference design document shows that reference design of ON Semiconductor’s single-stage CrM TRIAC LED dimming driver GreenPoint which is based on NCL30000 has achieved all key performance targets, such as “Energy Star” version 1.1 power factor requirement for solid-state lighting in commercial and residential applications, and even achieved the energy efficiency requirement for exterior power supply under critical load conditions in version 2.0. This reference design also provides flexibility to system developers, enabling them to increase or decrease power and meet different application requirements. This approach allows designers to respond flexibly to the improvement in LED light efficiency, and enables them to design lamps with fewer LEDs but still providing the expected light output.

 

Relationship Analysis Between LED Lamp Quality And Driving Power Supply

08 Oct

Because LED is eco-friendly light source which does not contain toxic materials, and it has a long lifetime with high efficiency, the applications of LEDs in various industries have developed fast in recent years, theoretically, LED’s service life can reach about 100,000 hours, but in the practical application process, as a result of some LED lamps designers’ inadequate knowledge or improper selection of LED driving power supplies, the lifetime of LED lamp products is greatly reduced.

Due to the particularity of LED manufacturing, the current and voltage characteristics of LEDs produced by different manufacturers or even the same manufacturer in the same batch are of great differences. Now we can take 1W high power white LED with typical specifications as an example and do a brief description in accordance with the LED current and voltage change law, the general application forward voltage of 1W white LED is around 3.0-3.6V, in order to ensure the lifetime of the 1W LED, most LED manufacturers suggest the lamp factories to use 350mA current to drive, When the forward current which passes through both ends of LED reaches 350 mA, a small increase of forward voltage to both ends of LED will cause a significant increase of LED forward current and a linear increase of LED temperature, thus speeding up the LED light degradation, and shortening the lifespan of LEDs, or seriously burning out LEDs. Due to the particularity of LED’s voltage and current changes, the industry makes strict requirements to the power supplies that drive LEDs.

LED driving power supply is the key point to an LED lamp, it is just like a person’s heart, in order to manufacture high-quality LED lamps for illumination lighting, we must give up the constant voltage mode to drive LEDs.

Currently, LED lamp products produced by many manufacturers (such as rail lights, lamp cups, spotlights, garden lights etc.), use resistance or capacitance to reduce voltage, then add a voltage stabilizing diode to stabilize the voltage, and supply power to LEDs, such driving method has significant defects, first, the efficiency is low, much electricity will be consumed on quenching resistor, which may even exceed the LED’s power consumption, and can not provide large current driving, as when the current is large, consumption of energy on the quenching resistor will be high, and can not guarantee that the current passing through the LED does not exceed the work requirements of LED, when designing products, the designers use the method of reducing voltage of both ends of LED to supply power and drive LED, which is at the expense of sacrificing LED brightness. Using resistance or capacitance to reduce voltage and drive LED will cause unstable brightness of LED, when the power supply voltage is low, LED brightness becomes low, when the power supply voltage is high, LED brightness becomes high. Of course, biggest advantage of resistance or capacitance reducing voltage to drive LED is the low cost.

Some manufacturers, in order to reduce product costs, use constant voltage to drive LED, which also brings a series of problems when doing mass production, for instance, the brightness of LEDs is uneven, LEDs can not achieve their best performance and so on.

Constant current driving is the optimum driving method for LEDs. When driven by constant current, LED current will not be influenced by the change of external power supply voltage, environment temperature, and the discreteness of LED parameters, thus it can maintain the current constant, and fully demonstrate various excellent features of LED.

Utilizing constant current driving mode can avoid the current variation caused by LED forward voltage changes, meanwhile a constant current will stabilize LED brightness, also facilitate the LED lamp factory to implement mass production, therefore many manufacturers have been fully aware of the importance of driving power supply, many manufacturers have chosen constant current mode to drive LED lamps.

Some manufacturers worried that power supply driving board choosing electrolytic capacitors may affect the lifetime of power supplies, which is actually a misconception, for instance: if we choose 105 degree Celsius high-temperature electrolytic capacitors with a lifetime of 8,000 hours, according to the electrolytic capacitors life estimation fomula “Temperature drops by 10 degree Celsius, litime increases by 100%”, then when it works under the environment of 95 degrees Celsius,its lifetime is 16,000 hours, when it works under the environment of 85 degrees Celsius,its lifetime is 32,000 hours, when it works under the environment of 75 degrees Celsius,its lifetime is 64,000 hours, if the actual working environmental temperature is lower, then the lifetime will be longer! Therefore, as long as the manufaturers select high quality electrolytic capacitors, the lifetime of driving power supply will not be influenced!

There is another issue that should be noted: since LED will emit a lot of heat when working, which causes a rapid rise in junction temperature, the higher LED power is, the greater heating effect is. LED chip temperature rise will lead to changes in light-emitting device performance and the reduction or failure of electro-optical conversion efficiency, according to the experimental tests: when LED temperature increases by 5 degrees Celsius, luminous flux will decreases by 3%, so the thermal control of LED light source itself is very important, if possible the heat dissipation area of LED itself shall be enlarged in order to decrease the working temperature of LED.

 
 
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