Exploring the growth drivers of LED lighting

The market for high-brightness (HB) LEDs is expected to reach $20.2 billion by 2015, growing at a compound annual growth rate (CAGR) of 30.6% since 2012 (source: Strategies Unlimited). One of the key applications driving this phenomenal growth is LEDs used to backlight thin-film transistor (TFT) liquid crystal displays (LCDs). Applications range from high-definition (HD) TVs and portable tablet PCs to automotive displays and a wide variety of handheld communications devices. However, to maintain this impressive growth rate, LEDs must not only offer higher reliability, lower power consumption, and more compact form factors, but also improvements in contrast ratios and color accuracy. Furthermore, in automotive, avionics, and marine displays, all of these improvements must be optimized while withstanding a wide range of ambient lighting conditions, from bright sunlight to moonless nights.

These TFT-LCD applications include infotainment systems, instrument clusters, and numerous instrument displays. Of course, when LEDs are used to backlight these displays, some unique LED driver IC design challenges arise in order to optimize display legibility under a wide range of lighting conditions. It requires the LED driver to provide a very wide dimming ratio range and efficient conversion while withstanding the relatively harsh automotive electrical and physical environment. It goes without saying that such a solution must provide a very flat and compact footprint while also improving overall cost-effectiveness.

In addition, according to LED industry insiders: "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 characteristics, and related tax reduction policy benefits, the use of LED lighting in commercial areas will increase significantly, such as: parking lots, office areas, factory buildings and warehouses. LED lights can not only replace high-pressure sodium lamps, halogen lamps and incandescent bulbs, but also CFL and fluorescent lamps in some occasions."

It is no surprise, then, that commercial applications are leading the transition to LEDs, as lighting typically accounts for 25% to 40% of total electricity use in commercial buildings. Because these applications require high-intensity light for long periods of time, the energy savings can pay off financially in a relatively short period of time. Second, the long life of LED lamps significantly reduces the cost of bulb replacement. These replacement costs include not only the price of the bulb itself, but also the labor costs of actually replacing the bulb in some applications (such as high-bay lighting), which can be significant. In turn, general-purpose LED lighting for home use is still too expensive for most consumers at present. However, in the coming years, as LED lamps become less expensive and more popular, they will also achieve significant growth in the residential segment of the lighting market. Most analysts expect this market segment to grow rapidly in 2013 and beyond.

What factors are driving the increasing adoption of LEDs in automotive displays?

What factors underpin this impressive growth potential in automotive lighting? First, LEDs are 10 times more efficient than incandescent bulbs and nearly twice as efficient as fluorescent lamps (including cold cathode fluorescent lamps [CCFLs]), thereby reducing the electrical power required to provide a given amount of light output (measured in lumens per watt [lm/W]). As LEDs are further developed, their efficacy, or ability to produce light output from a power source, will only continue to improve. Second, ours is an environmentally conscious world, and LED lighting does not require the handling, exposure, and disposal of toxic mercury vapor 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 to the more than 100,000 hours of life that LED lighting can provide.

In most applications, this longer operating life allows LEDs to be permanently embedded in the end application. This is obviously particularly important for the backlighting of automotive dashboards, instruments, and infotainment system displays, which are often embedded in the dashboard of the car because they will not need to be replaced during the working life of the car. In addition, LEDs can be several orders of magnitude smaller and more compact than other lamps, so LED screens can be made extremely thin and flat, so they only take up very little space inside the car. Moreover, through the configuration of red, green and blue LEDs, an infinite variety of colors can be provided. Furthermore, LEDs can also be dimmed and turned on/off at speeds far beyond the perception of the human eye, which can significantly improve the backlighting of LCD displays while providing images with high contrast ratios and higher resolutions.

Barriers to entry into automotive applications

However, one of the biggest obstacles facing 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 supply solutions must be very efficient, rugged and reliable, while also being very compact and cost-effective. Arguably, one of the most demanding applications for driving LEDs will be backlighting for automotive infotainment and instrument TFT-LCDs, as they are in a harsh automotive electrical environment and must adapt to a wide range of ambient lighting conditions while also being able to fit into a very limited, small space. And what remains unchanged is that they must have an attractive cost structure.

Many emerging automotive designs use a single screen to backlight all the display instruments used for driver control. Often, the LED backlighting for the instrument panel is shared with the infotainment system, creating an easy-to-read, all-in-one control panel. Similarly, many vehicles, including cars, trains, and airplanes, have LCD displays in the rear-facing seats that provide passengers with entertainment services such as movies, video games, etc. Historically, such displays have been backlit by CCFLs; however, these relatively bulky bulb designs are increasingly being replaced by very flat arrays of white LEDs to provide more precise and adjustable backlighting and longer service life.

Figure 1: This figure shows a conceptual LCD automotive instrument panel backlight with high-brightness LEDs.

LED Drivers for Commercial Building Lighting

The main driving force behind the high growth rate of LED lighting is the significant reduction in power consumption compared to traditional lighting methods. Compared to incandescent lighting, LEDs require less than 20% of the electrical power required to provide the same light output (in lumens) as incandescent lamps. As can be seen in Table 1, there are many other advantages to LED lighting, but there are also some challenges associated with LED lighting. The advantages of LED lighting include an operating life that is several orders of magnitude longer than incandescent lamps, which greatly reduces replacement costs. The ability to dim LEDs using previously installed TRIAC dimmers is also a major cost advantage, especially in residential lighting retrofit applications. LEDs can turn on instantly, without the warm-up time required by CFLs, 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 vapor to operate. Finally, LEDs enable new and very flat form factors that are not possible with other technologies.

Table 1: Comparison of LED, CFL, and incandescent light sources.

Things to note for offline LED drivers

The ability to drive LEDs with an offline power supply has led to an exponential growth in applications, as this form of power is readily available in both commercial and residential buildings. While replacement accessories for LED lamps are relatively simple and easy for end users to install, new requirements for LED driver ICs have been greatly increased. Since LEDs require a well-regulated constant current source to provide a constant light output, powering LEDs with an AC input power supply requires some special design approaches that meet some very specific design requirements.

In different regions and cities, the parameters of offline power supply vary, generally between 90VAC~265VAC, and the frequency is 50Hz to 65Hz. Therefore, to manufacture LED accessories for the global market, it is best 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.

Additionally, many offline LED applications require electrical isolation between the LED and the driver circuit. This is primarily for safety reasons and is required by several regulatory bodies. Electrical isolation is typically provided by an isolated flyback LED driver topology that uses a transformer to isolate the primary and secondary portions of the driver circuit.

Since the driving force behind the adoption of LED lighting is that the power required to provide a specific amount of light output can be greatly reduced, it is imperative that the LED driver IC provide the highest efficiency. Because the LED driver circuit must convert the high voltage AC power to a lower voltage and well-regulated LED current, the LED driver IC must be designed to provide efficiency above 80% to not waste power.

Furthermore, to make LED retrofit lamps possible when TRIAC dimmers common in residential applications are already in place in large numbers, LED driver ICs must work efficiently with these dimmers. TRIAC dimmers are designed to work well with incandescent and halogen lamps, which are ideal resistive loads. However, LED driver circuits are generally nonlinear and not purely resistive loads. Their input bridge rectifiers typically draw high peak currents when the AC input voltage is at its positive and negative peaks. Therefore, LED driver ICs must be designed to "mimic" a purely resistive load to ensure that the LEDs start correctly without any noticeable flicker and are properly dimmed with a TRIAC.

Power factor (PF) is an important performance specification for LED lighting. Simply put, if the current drawn is proportional to and in phase with the input voltage, the power correction factor is 1. Since an incandescent lamp is a perfect resistive load, the input current and voltage are in phase and the PF is 1. PF is particularly important because it is related to the electrical power required to be delivered by the local power supplier. For example, in an electrical system, a load with a low power factor draws more current than a load with a high power factor for the same useful power delivered. Because the required current is higher, the energy lost in the distribution system is also increased, which in turn requires thicker wires and other transmission equipment. Larger equipment and wasted energy increase costs, so power companies often charge higher rates to industrial or commercial customers with lower power factors. International standards for LED applications are still under development, but most believe that for LED lighting applications, PF>0.90 will be required.

Figure 2: Linear Technology's LT3799 offline LED driver integrates single-stage active filtering to reduce EMI and provide a power factor of 0.98 (top view).

The LED driver circuit (including a large number of diodes, transformers and capacitors) does not behave like a purely resistive load, so its PF may be as low as 0.5. In order to increase the PF to 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 especially important in applications that use a large number of LED lighting arrays. For example: In a parking garage using more than a few hundred 50W LED lamps, a high PF (>0.95) LED driver design is beneficial.

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 the IEC 61000-3-2 Class C lighting equipment harmonic specification to ensure that new LED lighting accessories meet these low distortion requirements.

In lighting applications, it is critical to accurately regulate the LED current over a wide range of input line voltage, output voltage, and temperature variations, as changes in LED brightness must be imperceptible to the human eye. Similarly, to ensure the longest operating life of the LED, it is important not to drive the LED with a current higher than its maximum rating. In isolated flyback applications, regulation of the LED current is not always straightforward, often requiring an optocoupler to close the required feedback loop, or perhaps an additional conversion stage. However, both of these approaches introduce complexity and reliability issues. Fortunately, some LED driver IC designs have adopted new design methods that can accurately regulate the LED current without these additional components or without increasing the complexity of the design.

in conclusion

There is no doubt that the combination of HB LEDs in automotive applications and the growth potential of high-brightness LED lighting systems in commercial buildings has led to a significant increase in demand for both LEDs themselves and the LED driver ICs needed to drive them, which are now widely used. Although their popularity faces their own barriers, the advantages they provide in these applications are simply too great to ignore. Fortunately, for designers of such systems, there is a wide range of LEDs to choose from, as well as new and innovative IC drivers to power them. Yes, the future of LED lighting is bright indeed.