LED lighting development prospects and challenges

According to LED industry sources: "Amazing growth has occurred in high-brightness LED lighting systems for commercial use; this is because in the eyes of most consumers, LED lighting is still too expensive for home use. With its long-term advantages , energy-saving and environmental protection features, and related preferential tax cuts, the use of LED lighting in the commercial sector will have a substantial increase, such as: parking, office areas, factory buildings and warehouses. LED lights can not only replace the high pressure sodium lamp , halogen and incandescent bulbs, and in some occasions can replace CFLs and fluorescent lamps."

Thus, it is not surprising that commercial applications dominate the transition to LEDs because lighting typically accounts for 25% to 40% of total electricity usage in commercial buildings. Because such applications require long periods of high-intensity light, saving power can provide a satisfactory economic return in a relatively short period of time. Second, the long life of LED lamps significantly reduces the cost of lamp replacement. These replacement costs include not only the price of the lamp itself, but also the actual cost of replacing the lamp in certain applications (eg, high-bay lighting). In turn, general-purpose LED lighting for home use is still too expensive for most consumers. However, in the next few years, as the price of LED lamps decreases and the popularity rate increases, the residential sector in the lighting market will also achieve substantial growth. Most analysts expect that this market segment will accelerate growth in 2013 and beyond.

What are the factors that support this impressive growth potential in automotive lighting? First, LED's luminous efficiency is 10 times that of incandescent bulbs, and it is almost twice that of fluorescent lamps (including cold cathode fluorescent lamps [CCFL]), thus reducing the amount of light output that can be provided (in lumens/watt [lm/W]). Measure) the required electrical power for the unit. With the further development of LEDs, their efficacy or the ability to produce light output from the power supply will only continue to increase. Second, we are a world focused on environmental protection, and LED lighting does not require the handling, exposure, and disposal of toxic mercury vapors common in cold cathode fluorescent lamps (CCFLs). Finally, incandescent bulbs often need to be replaced after about 1,000 hours of use, while fluorescent lamps can last up to 10,000 hours. However, these figures pale in comparison with the fact that LED lighting can provide more than 100,000 hours of life.

In most applications, this longer operating life allows the LED to be permanently embedded in the final application. This is obviously particularly important for the backlighting of car dashboards, meters, and infotainment displays, which are often embedded in the car's dashboard because they will not need to be replaced during the car's working life. In addition, the shape of the LED can be several orders of magnitude smaller than other lamps, and is also more compact, so that the LED screen can be made extremely thin and thin, so that it takes up only a very small amount of space inside the automobile. And, with the red, green, and blue LED configurations, an infinite variety of colors can be provided. Furthermore, the LEDs can also be dimmed and turned on/off at speeds far beyond the perceived ability of the human eye, which can significantly improve the backlighting of LCD displays while providing images with high contrast ratio and high resolution. .

However, one of the biggest obstacles faced by automotive lighting system designers is how to optimize all the features and advantages of the latest generation of LEDs. Because LEDs generally require an accurate and efficient current source and a dimming method, LED driver ICs must be designed to meet these requirements under a variety of operating conditions. In addition, their power solutions must be very efficient, rugged, and reliable, yet they must be very compact and cost-effective. It can be said that one of the most demanding applications for driving LEDs will be backlighting of automotive infotainment and instrumentation TFT-LCDs because they are in a harsh automotive electrical environment and must adapt to a wide range of changes in ambient lighting conditions. At the same time, it must also be able to fit into a very limited and narrow space. And, always, they must have an attractive cost structure.

Many emerging automotive designs use a single screen to provide backlighting for all display instruments used for driving control. In general, the LED backlight of the instrument panel is shared with the infotainment system to form an easy-to-read integrated control panel. Similarly, many vehicles, including cars, trains, and airplanes, also have rear-facing seats equipped with LCD displays for providing passengers with entertainment services such as movies and video games. Historically, such displays used CCFL backlighting; however, there has been an increasing use of very flat white LED arrays to replace these relatively large bulb designs to provide more accurate and adjustable backlighting and longer Useful life.

LED driver for commercial building lighting
The main driving force behind the high growth rate of LED lighting is that, compared with traditional lighting methods, the power consumption of LED lighting is greatly reduced. Compared with incandescent lighting, to provide the same light output (in lumens), LEDs require less than 20% of the electric power of incandescent lamps. As can be seen from Table 1, LED lighting has many other advantages, but there are also some challenges associated with LED lighting. The advantages of LED lighting include several orders of magnitude longer working life than incandescent lamps, which greatly reduces replacement costs. The ability to dim LEDs with previously installed TRIAC dimmers is also a major cost advantage, especially in residential lighting retrofit applications. LEDs can be switched on instantly, unlike CFLs that require warm-up time, and LEDs are not sensitive to power cycling, which is also different from CFLs. In addition, LEDs do not contain any toxic materials that need to be managed or disposed of, while CFLs require toxic mercury vapors to work. Finally, LEDs can achieve new and very flat form factors that are not possible with other technologies.

LED lighting development prospects and challenges Need to pay attention to off-line LED driver

The ability to drive LEDs with off-line power supplies allows applications to grow exponentially, because this type of power supply is readily available in commercial buildings as well as in residential buildings. Although the LED light replacement attachment is relatively simple and the end user can easily install it, the new requirements for the LED driver IC have been greatly improved. Because LEDs require a well-regulated constant current source to provide a constant light output, the use of AC input power to power the LED requires special design methods that meet some very specific design requirements.

In different cities, the offline power supply parameters will be different, usually between 90VAC ~ 265VAC, and the frequency is 50Hz to 65Hz. Therefore, to manufacture LED attachments for the global market, it is desirable to have a circuit design that allows LEDs to be used anywhere without modification. This requires a single LED driver IC to handle multiple input voltages and grid frequencies.

In addition, many offline LED applications require electrical isolation between the LED and the driver circuit. This is mainly due to security considerations and is required by several regulatory agencies. Electrical isolation is usually provided by an isolated flyback LED driver topology that uses a transformer to isolate the primary and secondary parts of the driver circuit.

Since the driving force for using LED lighting is that the power required to provide a specific light output can be greatly reduced, it is imperative that LED driver ICs provide the highest efficiency. Because LED driver circuits must convert high-voltage AC power to lower voltages and well-regulated LED currents, LED driver ICs must be designed to provide greater than 80% efficiency without wasting power.

Moreover, in order for LED retrofit lamps to be made possible when TRIAC dimmers, which are common in residential applications, are already installed in large numbers, the LED driver ICs must operate efficiently with these dimmers. TRIAC dimmers are designed to work well with incandescent and halogen lamps. Both lamps are ideal resistive loads. However, LED driver circuits are generally non-linear and are not purely resistive loads. Its input bridge rectifier typically absorbs high-intensity peak currents when the AC input voltage is at its positive and negative peaks. Therefore, the LED driver IC must be designed to "mimic" a purely resistive load to ensure that the LED starts properly without any noticeable flicker and uses a TRIAC for proper dimming.

In the case of LED lighting, power factor (PF) is an important performance specification. In short, the power correction factor is 1 if the current drawn is proportional and in phase with the input voltage. Since the incandescent lamp is a perfectly resistive load, the input current and voltage are in phase and the PF is 1. The PF is particularly important because it relates to the electrical power that local power suppliers need to provide. For example, in a power system, a load with a low power factor draws more current than a load with a high power factor when the useful power delivered is the same. Because the required current is higher, the energy lost in the power distribution system is also increased, which in turn requires thicker wires and other transmission equipment. Larger equipment and energy waste increase costs, so power companies often charge higher fees for industrial or commercial customers with lower power factor. International standards for LED applications are still under development, but most people believe that for LED lighting applications, PF > 0.90 will be required.

LED lighting development prospects and challenges
LED driver circuits (including a large number of diodes, transformers, and capacitors) do not behave like a purely resistive load, so their PF may be as low as 0.5. To increase PF above 0.9, active or passive PFC circuits must be designed into the LED driver circuit. It should also be mentioned that high PF is particularly important in applications using a large number of LED lighting arrays. For example, high PF (>0.95) LED driver designs are beneficial in garages that use hundreds of 50W LED lamps.

In addition to the importance of high-PF, it is also important to minimize the harmonic distortion of LED lamps. The International Electrotechnical Commission has developed IEC61000-3-2C Class C Harmonics for Lighting Regulations to ensure that new LED lighting accessories meet these low distortion requirements.

In lighting applications, it is important to be able to accurately adjust the LED current over a wide range of input line voltages, output voltages, and temperature changes, because the LED brightness changes must be invisible to the human eye. Similarly, in order to ensure the LED has the longest operating life, it is important not to use a current higher than its maximum rating to drive the LED. In isolated flyback applications, LED current regulation is not always straightforward, often requiring an optocoupler to close the desired feedback loop, or an additional conversion stage may be added. However, both methods bring complexity and reliability issues. Fortunately, some LED driver ICs are designed using new design methods that do not require these additional components or adjust LED current accurately without increasing design complexity.

in conclusion
Undoubtedly, HBLEDs combine the growth potential of automotive applications and high-brightness LED lighting systems in commercial buildings, making the LED itself and the LED driver ICs needed for its driving widely used today, resulting in significant demand. increase. Although their popularity faces individual obstacles, the advantages they provide in these applications are so great that people cannot ignore them at all. Fortunately for designers of this type of system, the choice of LEDs is large and there are as many new and innovative IC drivers that can power them. That's right, the future of LED lighting is indeed bright.