LED lighting – business opportunity, design challenge

The market size for high brightness (HB) LEDs is expected to reach $20.2bn by 2015 – a CAGR of 30.6% from 2012 (according to Strategies Unlimited). One of the key application areas driving this significant growth factor are the 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 myriad of handheld communication devices. However, in order to maintain this impressive growth rate LEDs must not only offer enhanced reliability, reduced power consumption and more compact form factors, but they must also provide improvements in contrast ratios and colour accuracy.

Furthermore, in automotive, avionic and marine displays all of these improvements must be optimized while simultaneously being subjected to a wide array of ambient lighting conditions ranging from bright sunlight to moonless nights.

Moreover, according to LED inside, “phenomenal growth has been seen in the high lumen LED lighting system for commercial-use; this is because the LED lighting for household-use is still too expensive for most consumers. Backed by its long-term benefits, energy-saving and environment-friendly attributes, and its relevant tax reductions, there will be a substantial increase in the use of LED lighting in commercial spaces such as the parking lot, office space, factory facility and warehouse.

LED lights can replace not only high pressure sodium lamps, halogen lights, incandescent bulbs but also CFL and fluorescent lights in some areas.”

So, it is no surprise that commercial applications are leading the transition to LEDs as lighting generally represents from 25% to 40% of total energy use in commercial buildings. As these applications require long hours of high intensity light, the economic payback of the saved electrical power is relatively short-term.

Secondly, the long life of LED fixtures dramatically reduces the replacement cost of the bulbs. These replacement costs not only include the price of the bulb itself, but also the labor cost to physically replace them, which in certain applications, such as high bay lighting, are significant. Conversely, at this time, general purpose LED lighting for household use is still too expensive for most consumers.

However, in the coming years, as LED fixture prices decline and become more widely available, the residential sector of the lighting market will also grow substantially. Most analysts expect this segment of the market to accelerate in 2013 and beyond.

LED drivers for commercial building lighting

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 to incandescent lighting, LEDs require less than 20% of the electrical power to provide the same level of light (in lumens).

There are additional advantages which 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 utilises toxic mercury gas to operate. Lastly, LEDs enable new very low profile form factors that other technologies could not.

The ability to drive LEDs from an offline power source enables applications to grow exponentially as this form of electrical power is readily available in both commercial and residential dwellings.

Although LED replacement fixtures are relatively simple for the end user to implement, the new requirements placed on the LED driver IC go up dramatically. As LEDs require a well regulated constant current source to deliver a constant level of light output, powering them from an AC input source requires some special design techniques with very specific design needs.

Depending on location, the offline power will range somewhere between 90VAC and 265VAC with a frequency between 50Hz and 65Hz. Therefore, manufacturing a LED fixture for the worldwide market would ideally have a single circuit design which could enable it to be used anywhere in the world without modifications. This requires a single LED driver IC which can handle a wide range of input voltages and line frequencies.

Furthermore, many offline LED applications require electrical isolation of the LEDs from the drive circuit. This is primarily driven by safety factors and is required by several regulatory agencies. Electrical isolation is generally provided by an isolated flyback LED driver topology which utilises a transformer separating the primary and secondary sections of the drive circuit.

As the driving force behind LED lighting’s adoption is the dramatic reduction of power required to deliver a specific quantity of light, it is imperative that the LED driver IC delivers the highest level of efficiency. Because the LED driver circuit must convert high voltage AC power to well regulated LED current at a lower voltage, the LED driver IC must be designed to deliver efficiencies above 80% so as not to waste power.

Moreover, in order for LED retrofit lamps to be feasible with the large base of installed TRIAC dimmers commonly found in residential applications, the LED driver IC must operate effectively with these dimmers. TRIAC dimmers are designed to work well with incandescent and halogen lamps, which are perfect resistive loads.

However, the LED driver circuit is generally nonlinear and is not a purely resistive load. Its input bridge rectifier typically draws high intensity peaks of current 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 exhibiting any visible flicker and dims properly with a TRIAC.

Power Factor (PF) is an important specification for LED lighting. Simply put, a power correction factor of one is achieved if the current drawn is proportional and in-phase with the input voltage. Because an incandescent lamp is a perfectly resistive load, the input current and the voltage are in-phase and the PF is 1.

PF is particularly important as it relates to the amount of electrical power required by the local power supplier. By way of example, consider an electric power system in which a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred.

The higher required currents increase the energy lost in the distribution system, which in turn requires require larger wires and other transmission equipment. Because of the costs of larger equipment and wasted energy, electrical utility companies will usually charge a higher cost to industrial or commercial customers where there is a low power factor. International standards are still developing for LED applications, but most believe that a PF>0.90 will be required for LED lighting applications.

P268 Short-Fig1Figure 1. Linear’s LT3799 offline LED driver combines single stage active filtering to lower EMI and can provide a Power Factor of 0.98 (top view).

Because a LED driver circuit (which includes an array of diodes, transformers and capacitors) does not act like a purely resistive load, it can have a PF as low as 0.5.

In order to raise the PF above 0.9, either an active or passive PFC circuit must be designed into the LED driver circuit. It should also be noted that a high PF is particularly important in applications which utilise a large number of LED lighting arrays. For example: in a parking garage which uses over several hundred 50W LED fixtures, a high PF (>0.95) LED driver design will be beneficial.

In addition to the importance of a high PF, it is also important to minimise the harmonic distortion levels in LED fixtures. The International Electrotechnical Commission has developed IEC 61000-3-2, Class C Lighting Equipment Harmonics specification to ensure that new LED lighting fixtures meet these low distortion requirements.

Accurate LED current regulation over a wide variation of line voltage, output voltage and temperature is critical in lighting applications as variations in LED brightness must be imperceptible by the human eye.

Similarly, to ensure the longest operation life of the LEDs, it is important not to drive them with current above their maximum ratings. LED current regulation in isolated flyback applications is not always straight forward and often requires an opto-coupler to complete the required feedback loop, or an additional conversion stage can be added.

However, both of these methods add complexity and reliability concerns. Fortunately, some LED IC driver designs incorporate new design techniques that provide accurate LED current regulation without these additional components or design complexity.

There can be no doubt that the combined growth potentials of high lumen LED lighting systems for commercial buildings represent a significant increase in demand from today’s deployment rate for both the LEDs themselves and for the LED driver ICs required to drive them.

Despite numerous obstacles for adoption, the benefits they offer in their end applications are too compelling to ignore. Fortunately for the designers of such lighting systems there are lots of LEDs to choose from, as well as new and innovative IC drivers to power them. Yes, the future is bright indeed for LED lighting.

Tony Armstrong, director of product marketing for power products at Linear Technology

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Courtesy of:
Richard Wilson
Electronics Weekly