Breaking through the bottleneck of offline drive, greatly improving the reliability of home LED lighting system

To meet the increasingly stringent product requirements of the home LED lighting application market, the new generation of offline LED drivers has functions such as electrical isolation, high efficiency, power factor correction and Triac dimming, and provides well-regulated current to maintain consistent brightness. At the same time, various protection functions also improve system reliability.

In the past few years, display backlighting applications have been the main factor driving the growth of the light-emitting diode (LED) market. However, the increasing appeal of LED general lighting in the commercial and residential markets has further accelerated the growth of the LED market. According to a research report by market research firm LEDinside, commercial high-lumen LED lighting systems grew rapidly in 2010. This is because for most consumers, home LED lighting is still too expensive. On the other hand, for commercial areas such as parking lots, offices, factories, and warehouses, LED lighting can bring long-term benefits such as energy saving and environmental protection. Coupled with relevant tax reduction policies, the use of LED lighting in these application areas will grow significantly.

Home LED lighting market will accelerate growth

LED lamps can not only replace high pressure sodium lamps, halogen lamps and incandescent lamps, but also replace energy-saving fluorescent lamps (CFL) and fluorescent lamps in some areas. LED commercial lighting will grow rapidly and be widely adopted, and 2012 is a key year for home LED lighting.

In commercial buildings, lighting generally accounts for 25-40% of total energy use, so it is not surprising that commercial applications are beginning to switch to LED lighting. Since such applications require high-intensity, long-term lighting, the economic payback from energy savings is relatively short-term. In addition, LED accessories have a long life, which greatly reduces the replacement cost of LED lights. The replacement cost includes not only the cost of the lighting fixture itself, but also the labor cost of manual replacement, which is usually very high in some applications, such as the disassembly and assembly of high-end lighting applications, which requires professional-level labor costs.

Overall, from the perspective of most consumers, general household LED lighting is still too expensive, but in the future, as the price of LED accessories drops and LEDs become more widely available, the lighting market will also grow significantly in the residential sector. Most analysts expect that the residential market will accelerate growth after 2012.

LED driver IC design considers many aspects

As mentioned earlier, the main driver behind the high growth rate of the LED lighting market is its ability to significantly reduce power consumption. Compared to incandescent lighting, LEDs require less than 20% of the electrical power required to provide the same light output (in lumens). As shown in Table 1, LED lighting has many more advantages, but also many more challenges. The advantages of LEDs include longer life than incandescent lamps, which greatly reduces replacement costs; especially in the residential lighting market, the ability to dim LEDs using previously installed bidirectional silicon-controlled rectifier (Triac) dimmers is also a major advantage. LED lamps can be turned on instantly, without the warm-up time required by CFLs, and LED lamps are not sensitive to power cycles, which is also different from CFL lamps. In addition, LED lighting accessories do not contain any toxic materials that need to be controlled or handled, while CFLs work with toxic mercury vapor. Finally, LEDs can achieve new and very flat form factors that are not possible with other technologies.

These new LED lighting applications range from 4-watt (W) screw-in replaceable incandescent bulbs to 100W street lamps to high-bay lamps that require hundreds of watts of power. Most of these LED lamps need to operate from an existing alternating current (AC) offline power supply (usually 90 to 265 VAC, depending on the geographic location of the end application). The ability to efficiently drive LEDs from an AC offline power supply creates some fairly unique LED driver IC design challenges.

LED drivers must not only provide the highest efficiency to ensure power savings, but also provide electrical isolation for safety reasons. In addition, LED drivers must also provide power factor correction (PFC) and low harmonic distortion performance to be suitable for a variety of applications. Not only that, in many residential applications, LED drivers must also provide the ability to dim with existing Triac dimmers. Finally, the solution must be cost-effective and the overall area must be small so that there is enough space in the accessory device to accommodate the solution to attract commercial and residential users to replace existing lighting lamps with LED lamps. In short, there must be LED driver ICs that can meet these requirements, which is crucial to the popularization of LED lighting applications.

LED power saving still has room for improvement

One of the main driving forces behind the rapid growth of the LED lighting market is the significant power savings that LED lamps offer compared to incandescent and even CFL lighting. Therefore, it is important to examine the power savings that can be achieved with existing LEDs and how further power savings will be achieved in the coming years.

Currently, a 60W incandescent lamp provides about 800 lumens, which converts to 13.3 lumens per watt (lm/W). Existing LEDs, for commercial products, have efficiencies between 50 and 170 lm/W. Future LEDs will provide efficiencies in the range of 200 to 230 lm/W. However, the maximum theoretical efficiency is about 250 lm/W.

For lighting purposes, consider an LED with 100 lm/W. The LED itself requires 8W of electrical power. However, the efficiency of the AC-DC LED driving electronics must also be taken into account, which is 85% for high performance LED drivers. Therefore, the total power required for the offline power supply is 8W/0.85=9.4W, which is an 84% power saving compared to an incandescent lamp. If a more conservative figure of 65lm/W is used, the LED power is reduced by 75% relative to the power consumed by an incandescent lamp. Looking ahead, when LEDs providing 200lm/W are available, the power savings will be as much as 92%. To meet current Energy Star requirements, LED lamps must be at least 75% less powerful than standard incandescent lamps to replace incandescent lamps.

Offline power supply design is difficult

The ability to drive LEDs with an offline power supply will allow applications to increase rapidly, because this form of power is very convenient and easily available in commercial buildings and homes. Although the accessories of LED lamps are relatively simple and easy for end users to install, the requirements for LED driver ICs have also increased significantly. Because LEDs require a well-regulated constant current source to provide constant light output, powering LEDs with an AC input source requires some special design methods and must meet some special design requirements.

For example, in different regions of the world, offline power supply parameters may be different, generally between 90 ~ 265VAC, the frequency is 50 ~ 65Hz. Therefore, to manufacture universal global market LED accessories, it is best to have such a circuit design that can make LEDs used anywhere in the world without modification. To achieve this goal, a single LED driver IC is required to handle multiple input voltages and frequencies.

In addition, there are many offline LED applications that require electrical isolation between the LED and the driver circuit, which is mainly for safety reasons and is also strictly required by several regulatory agencies. Electrical isolation is generally achieved by an isolated flyback LED driver architecture, which uses a transformer to isolate the primary and secondary ends of the driver circuit.

The impetus for adopting LED lighting comes from the fact that the power required to provide a specific amount of light output can be greatly reduced, so 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 greater than 80% so that no power is wasted.

And to make LEDs a reality with the large number of Triac dimmers commonly found in residential applications, 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 non-linear and not electrically purely resistive loads. Their input bridge rectifiers typically have to absorb high peak currents when the AC input voltage is at its positive and negative peaks. Therefore, LED driver ICs must be designed to "emulate" a purely resistive load to ensure that the LEDs start up correctly without any noticeable flicker and are properly dimmed with a Triac.

PFC affects driver circuit performance

PFC is an important performance specification for LED lighting. Simply put, if the current drawn is proportional to and in phase with the input voltage, then the power correction factor is 1. Because an incandescent lamp is an ideal resistive load, the input current and voltage are in phase and the PFC is 1. PFC is particularly important because it is related to the power that the local power supplier must provide.

In other words, in an electric system, a load with a low power factor can draw more current for the same useful power transmitted than a load with a high power factor. Because the required current is higher, the energy lost in the distribution system is also increased, so thicker wires and other transmission equipment are required. However, larger equipment and energy waste increase costs, so power companies usually charge higher fees for industrial or commercial customers with lower power factors. International standards for LED applications are still under development, but most people believe that PFC>0.90 will be required for LED lighting applications.

LED driver circuits (including a large number of diodes, transformers and capacitors) do not behave like a purely resistive load and their PFC may be as low as 0.5. In order to increase the PFC to above 0.9, active or passive PFC circuits must be designed into the LED driver circuit. In addition, high PFC is particularly important in applications with hundreds of 50W LED lamps or more, in which case a high PFC (>0.95) LED driver design is beneficial.

On the other hand, high PFC is also important to minimize the harmonic distortion of LED accessories. The International Electrotechnical Commission (IEC) has developed the harmonic performance specification IEC 61000-3-2 for Class C lighting equipment to ensure that new LED lighting accessories meet low distortion requirements.

It is imperative to simplify LED drive circuits to meet cost and size requirements

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

However, the biggest obstacle to a rapid transition from incandescent lamps to LED lamps is the cost and size of LED solutions. Although it does make economic sense to replace an LED lamp at this price, considering the reduction in electricity costs and replacement costs during the life of the LED, it is still a big obstacle for consumers to spend nearly $1,000 to replace an LED lamp. Therefore, commercial enterprises that still pay a lot of lighting electricity bills such as warehouses and garages are turning to LED lighting faster, and the cost savings are more obvious. As the cost of purchasing LED lamps decreases, more consumers will be willing to turn to LED lighting in the future.

On the other hand, an equally important factor is the size of the LED lighting solution. Many LED lamps are screwed directly into the lamp holder, so the entire solution must have the same shape and volume as the original incandescent lamp. Since LEDs require heat sinks and many complex driver circuits, it may be a challenge to fit these two things in the same volume. Therefore, consumers need an LED driver IC that can provide all functions in a simple solution with a reduced pin area.

Maintaining current stability, isolated flyback LED controller shows its strength

To solve all the driving problems of these offline LEDs, Linear Technology has introduced an isolated flyback LED controller LT3799 with active PFC, which is designed specifically for driving LEDs in the universal input range of 90 to 265VAC. The LT3799 operates in a critical conduction mode to minimize the size of the external transformer, making it very suitable for a variety of LED applications with drive power ranging from 4 watts to hundreds of watts. The entire circuit for driving up to 20 watts of LEDs is shown in Figure 1. This particular circuit can be dimmed using Triac and can provide up to 1 ampere (A) of LED current in the universal input range of 90 to 265VAC, while providing more than 86% efficiency (Figure 2). With a slight change in external components, this circuit can be further optimized for use in 120, 240, 377VAC or almost any common AC input situation.

Figure 1: Isolated flyback LED controller circuit example

Figure 2 LT3799 efficiency over universal input voltage range

The LT3799 uses a single-stage design, requiring only forty external components for the entire LED driver circuit (including EMI filters), making the solution simple, footprint-saving and cost-effective. The total size of the 20W circuit in Figure 1 is only 30 mm × 75 mm, and the height is only 30 mm, making it ideal for a wide variety of LED applications. In addition, its unique internal circuit provides active PFC up to 0.98, easily meeting the requirements of the U.S. Department of Energy (DOE) and also meeting the harmonic requirements of IEC 61000-3-2 Class C lighting equipment (Figure 3).

Figure 3 LT3799 VIN and IIN waveforms with active PFC

The LT3799 also provides LED current regulation over the entire input voltage, output voltage, and temperature range. In Figure 4, it can be seen that the LED current maintains ±5% stability when the input changes from 90VAC to 150VAC. The component replaces the optocoupler with a unique current sensing circuit to provide a well-regulated current to the secondary side. Not only does it reduce cost, it also improves reliability. Not only that, the component can also be used with a standard wall-type Triac dimmer without any visible flicker. Integrated LED open and short circuit protection ensures long-term reliability in a variety of LED applications. Finally, its thermally enhanced MSOP-16 package helps to form a very compact LED driver solution.

Figure 4 LT3799 regulates LED current as VIN (AC) changes

The general LED lighting application market driven by offline power supply has begun to see unprecedented growth, and the driving force behind it is the market demand for higher performance and cost-effective lighting applications. However, these emerging performance requirements must be met using new LED driver ICs. These LED drivers must provide electrical isolation, high efficiency, PFC and Triac dimming capabilities. In addition, regardless of how the input voltage or LED forward voltage changes, these drivers must also provide well-regulated LED current to maintain consistent brightness, while providing various protection functions to improve system reliability. These LED driver circuits must also be able to form very compact and cost-effective solutions. Fortunately, there are manufacturers that can supply such offline LED drivers.