LEDs: Cheaper brighter bulbs to go viral
The first came from Cree, a manufacturer of high brightness LEDs, LED lighting and semiconductor products. They introduced a game-changing series of LED bulbs with a retail price point, which they believe will accelerate their acceptance by consumers. They claim that this new Cree light bulb is designed to last 25,000 hours and has a retail price of $12.97 for the 60-watt warm-light-replacement incandescent bulb, saving 84 percent of the energy compared with traditional incandescents. Furthermore, and more importantly to the consumer, they go on to claim that by replacing the incandescent bulbs with Cree’s LED Bulbs in a home’s five most frequently used light fixtures, consumers can save $611 per year in electric bills.
As if this information is not a game changer in itself, they have also made these LED bulbs available exclusively at The Home Depot. This ensures that the public at large can have an abundant supply of these LED bulbs readily available within a short distance of their homes.
The second release focuses on the suspension section of the famous San Francisco Bay Bridge, where over the past 18 months, 25,000 LEDs, spaced a foot apart has been hung from the top of the bridge’s towers and suspension cables to the deck. For those of you not familiar with this bridge, it spans almost two miles from San Francisco to Yerba Buena Island in the middle of San Francisco Bay. The project was driven by a company called Bay Lights that created algorithms that constantly change and shift to give life to the LEDs. Each light is controlled individually and the patterns that result will be unique for the project duration of two years. The lighting systems was developed by Philips Color Kinetics and has been designed to use 85 percent less energy than traditional (non-LED based) solutions. The specific LEDs from Philips Color Kinetics were the eW Flex SLX product in 4200 Kelvin correlated color temperature. Nearly 4.5 miles of lights were installed.
These two events symbolize the accumulations of technological advances in LED lighting output (measured in lumens per watt, or lm/W), and their commercial affordability due to their annual energy savings impact of the life of the installation.
LED background
There can be no doubt that LED technology has realized significant gains over the past couple of years. Higher brightness levels, higher efficiencies, longer lifetimes and decreasing costs have spun out from the many advances made in terms of heat dissipation, packaging and processing. Unlike incandescent light bulbs, LEDs do not have a filament that will burn out and they tend to run cooler.
A high-power, or high-brightness (HB) LED’s light output has already exceeded the critical milestone of 100 lm/W, with some manufacturers already claiming >230 lm/W in the laboratory. Another added benefit is LED lifetime. Depending on how it is calculated, a white LED bulb has at least a 50,000-hour lifetime while an incandescent bulb’s life is around 1,000 to 2,000 hours.
The primary driver behind the high growth rate of LED lighting is the dramatic reduction in power consumption that LED lighting offers over traditional lighting. Compared with incandescent lighting, LEDs require less than 20 percent of the electrical power to provide the same level of light (in lumens). As can be seen in Table 1, there are additional advantages that LED lighting offers but also some additional challenges. LED advantages include a lifetime orders of magnitude higher than incandescent bulbs, which dramatically reduces replacement costs. The ability to dim LEDs using the previously installed base of TRIAC dimmers is also a major benefit, especially in residential retrofit applications. Instant turn-on eliminates the warm-up period associated with CFLs, and LEDs are not sensitive to power cycling like their CFL counterparts. Additionally, LED lighting fixtures do not contain any toxic materials to manage or dispose of, whereas CFL utilizes toxic mercury gas to operate. Lastly, LEDs enable new very low profile form factors that other technologies could not.
Table 1. For better resolution click here.
As a result, LEDs are now being used in many commercial applications, such as high bay lighting for factories; outdoor architectural lighting for bridges and buildings, as well as sports stadiums and other large event arenas.
LEDs and their driver ICs
An LED is the electrical equivalent of a diode. As such, it is a current-driven device. Apply the correct amount of current to the LED to attain its specified light output in lumens per watt. Supply less than this and you do not get the full amount of light output that it is capable of delivering. For a single LED, this might not seem like a big deal; however, if many LEDs are in a series string and are not getting uniform current, the light output will vary and be noticeable.
Of course, you also have to overcome the LED’s inherent forward voltage drop, which can vary depending on the type of LED you have and the end fixture configuration. For a white LED, this forward voltage drop is usually around 3.5V but can be slightly higher at elevated temperatures. As a result, depending the input power source, a wide range of conversion topologies will be needed. This can be further complicated by the different LED string configurations, which are series (LED forward voltage drop additive), parallel (LED current additive) or a series of parallel string (both LED forward voltage and current additive).
Now that we understand what an LED needs for correct operation, what can happen to negatively impact their operation and the operation of its IC driver circuit?
The answer is that a lot of things can adversely affect an LED’s output, or even lead to its catastrophic failure. These events consist of overvoltage, usually occurring during an open LED event, or an overcurrent condition, which normally happens during short circuits or the re-plugging of an LED string. Of course, poor thermal environments can adversely affect an LED’s useful life and so a good overall thermal design is critical, too.
Armed with the knowledge of what can harm or even kill the LED and its LED IC driver circuitry, a lighting-systems designer must select design techniques and components for LED IC driver circuits that best safeguard against these dangers.
Some of the more critical aspects are:
(1) Tightly regulated current delivery to the LED. The LED IC driver circuit is key, since it must take whatever the input power source is, which will vary widely due to the broad range of applications, and convert it to the required voltage and the best current levels for LED performance. Since overvoltage or current can negatively impact either LED life or its light output over time, the tighter they can be regulated the more robust the system can be. Therefore, having +/-5 percent voltage and current regulation is going to be beneficial for long and trouble free life.
(2) To protect from over-voltage situations, an LED driver circuit that can handle higher transient voltage conditions above normal operation is a prerequisite. A good example is an automotive environment where load dump might be 42V, or even higher.
(3) Protecting from thermal overstress is a little more challenging. Quite often the LED deployment fixture is small and compact with very little in the way of heat sinking – and probably no fans to provide air cooling. Therefore, most of the heat will have to be dealt with via conduction. Thus, good heat sinking must be taken into account at the design concept. Also, having a LED driver circuit that operates with very high conversion efficiency is a great help. If not obvious, the higher the conversion efficiency, the less heat is produced as part of the conversion power loss. Thus, LED IC drivers with low to mid-90 percent efficiencies will significantly aid good thermal design.
Another interesting way to help with thermals is to have an on-chip temp sensor in the LED IC driver such that if there were a system controller to monitor this temp signal, it could have the overall system throttle back the current so as to allow less heat generation. Of course, this would be at the cost of reduced light output, but this is better than complete system failure. Once the fault condition goes away, normal operation could resume.
Finally, having these same practical approaches as discussed for the LED are also, by extension, applicable to the LED IC driver circuit as well. So having them incorporated in the overall system is a very good idea.
An LED IC driver for high-power lighting
An example of a high-power LED driver IC is Linear Technology’s LT3791 — a rather sophisticated buck-boost LED controller. This device enables a fixed output voltage regardless whether the input voltage is above, below are equal to it. However, unlike a complicated and low-efficiency SEPIC converter, this 4-switch synchronous, single inductor topology not only simplifies the converter design, but also improves the operating efficiency, thereby reducing the power lost as heat.
Figure 1a: 100W LED driver for high bay lighting with 98.5 percent peak efficiency using the LT3791. For better resolution click here.
Figure 1b: Input voltage vs. efficiency
As can be seen from Figure 1a and b, a 100W high bay lighting driver schematic, delivering a 33.3V/ 3A output the conversion efficiency is close to 96 percent across the entire input voltage range.
Some of its key features include a very wide input and output operating voltage range of up to 60V. For optimum and uniform light output of the LEDs within the string, it also provides very tight output voltage accuracy of +/- 2 percent and +/-6 percent LED current accuracy and +/-6 percent input current accuracy (over line, load and temperature).
For ease of use, the LT3791 can use either PWM or analog dimming. Its 4-switch buck-boost topology not only supplies seamless transition between modes, but gives very low noises in all of these modes, including the pass through mode where VOUT approximates VIN – something which is very difficult to do, especially is a load transient event occurs during this mode! Furthermore, it has both input and output current regulation for maximizing system performance and reliability. Since this device is expected to be used in environments that are subjected to wide ambient temperature variations, this part is available in H Grade, meaning that it has an operating junction temperature range of -40°C to +150°C.
LEDs’ bright future
It is clear that the efficacy of LEDs, coupled with their commercial price points, have crossed the inflection point at which they can be embraced for many lighting applications. The two examples given at the beginning of this article are just a few among the myriad of deployments occurring around the world today. The day of the LED is upon us. So I feel justified in saying; “Go LEDs Go!”
About the author: Tony Armstrong, director of product marketing, Power Products, Linear Technology Corporation, tarsmtrong@linear.com
1Based on Cree LED bulb 60W replacements at 9.5 watt, $0.11 per kilowatt-hour, 25,000 hour lifetime and average use of 6 hours per day.
This article by courtesy of EE Times USA
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