LED lighting: More intelligent, more beautiful, more affordable

-December 01, 2015

LEDs lead the way

Until now, a disproportionate share of the decline in the cost of LED lighting has come from lower LED prices which have been falling at 20+% per year for several years. Many of the cost savings come from steady improvements in efficacy which allow LED makers to produce the same number of Lumens from a smaller chip. Many manufacturers are also introducing LEDs which can operate at higher currents. While their efficacy remains the same (or declines slightly), high-current LEDs are able to deliver dramatic increases in output. But manufacturers must also find a way to help the devices survive the higher operating temperatures resulting from the increased power levels.


OSRAM's OSTAR Stage high-performance LED. (Image courtesy of OSRAM)

For example, OSRAM's OSTAR Stage products are based on new devices optimized for high current operation. Increasing the maximum forward current per chip from 1 to 2.5A boosts input power from 15 to 30W, with a roughly proportional increase in light output. This boost is made possible by a new ceramic based phosphor layer that can survive the higher operating temperatures, as well as packaging with enhanced thermal performance.  

Cree has also been extremely active in developing technologies that allow their LEDs to operate at higher power levels in applications such as roadway and outdoor area lighting as well a track, stadium and high bay lights. Their XLamp MH and XHP LED series, for example, use high reliability ceramic package technology to operate at much higher temperatures than their earlier products with no reduction in rated lifetime.  As we'll see shortly, high-current, high-temperature LEDs can also provide other cost savings for LED lighting manufacturers.  

Other manufacturers are achieving important savings through the use of more efficient manufacturing techniques and moving to larger, lower-cost substrates. For example, Samsung's CSP3 process, scheduled for introduction in 2016, uses GaN-on-Si techniques to produce high-performance flip-chip LEDs on standard 8"silicon wafers. Moving to a silicon-based process on large-format wafers should dramatically reduce the materials and processing costs of a LED die in comparison to SiC or sapphire substrates.

Improving the lumens/$ a LED delivers is not the only way it can reduce total cost of lighting products.  Although LEDs are no longer the most expensive portion of an LED lighting system, they fundamentally determine the overall system performance and cost. "We've applied several innovations to deliver LEDs that directly address the increased burden that thermal, mechanical and optical elements now place on total system cost," says Paul Scheidt, Cree's leader of product marketing, LED components.

As an example, Scheidt explained that Cree's high-reliability high-temperature ceramic package technology allows its MHD-G LEDs to achieve an impressive 60 percent reduction in heat sink size, enabling it to deliver 10,000 lumens while being cooled by only $5 of metal.

Cree, Samsung, and most other manufacturers are also transitioning to chip-scale packaging (CSP) technology, which can reduce packaging costs and to scale down the size of an LED package. A CSP LED's smaller dimensions help designers to create more flexible and compact designs for lighting modules or fixtures, thereby lowering the manufacturing costs of the lighting system.


The advanced integrated primary optics used in Cree's XLamp XP-L provide a narrower beam to the secondary optics of a directional lighting fixture. (Image courtesy of Cree)

The smaller, brighter LEDs being introduced by Cree, Osram, Philips, and other manufacturers also reduce the number of devices required for a given application. Reducing the number of LEDs translates directly to savings from fewer optical components, a smaller printed circuit board, a smaller housing and less handling. "Better technology and innovation in turn enables a better and lower cost system by allowing the use of less ‘stuff’ -- less metal, smaller PCB, fewer optic, and lighter weight," says Cree's Scheidt.

Advances in primary optics, the lenses integrated with the LED itself, will also help reduce overall solution costs in directional applications such as track and architectural lighting or stadium lights. By reducing optical source size, LEDs with advanced primary optics, such as Cree’s XLamp XP-L, can help designers reduce the size, complexity and cost of secondary optical components.

Drivers and power elements

Driver IC manufacturers are also feeling price reduction pressures (although not as heavily as LED makers), especially those involved with the incandescent replacement market. TI's John Perry observes that much of the cost of conventional diver architectures has already been beaten out of them, as evidenced by fewer new LED driver ICs for the retrofit LED lamp market coming from legacy semiconductor companies.  

"Much like LEDs," says Perry, "the IC suppliers have been lowering IC prices or releasing new products with better cost structures for the switched mode power supply (buck, flyback) topologies. Transformers and inductors (magnetic components) are likely cost optimized already due to their ubiquity in cell phone wall adapters."  Perry also notes that there are a number of lamps sold today which do not use an IC at all. Instead they rely on self-oscillating power circuits using discrete (bipolar/MOSFETS, diodes, resistors, capacitors and magnetics).

Another alternate power architecture gaining a lot momentum is the “AC linear” or “tapped linear” driver. These designs trade off magnetic components for high voltage ICs which attempt to drive segments of a high voltage sting of LEDs directly from an AC line. This is not a perfect solution however, as most AC linear drivers suffer from high LED ripple currents. Unless controlled, the ripple creates stroboscopic effects which can be unpleasant or, in some situations, even dangerous. TI’s TPS92411 floating switch architecture was developed to address the trend toward low cost linear LED drive solutions while allowing the designer to decrease LED current ripple and 120 Hz flicker with the addition of some storage capacitance.  

Despite the market's emphasis on cost reductions, there is still a place for performance and innovation in driver ICs being used in luminaires, as well as higher power and higher end applications (commercial/industrial lighting as well as street lights and other outdoor lighting). Electronics companies such as Infineon, ON, and Texas Instruments are helping lighting manufacturers address these applications with efficient, intelligent drivers that support value-add functionality, such as flicker-free dimming and precise color mixing. Many of these devices will be digitally controlled, or even have some level of embedded intelligence, to accommodate the growing use of Smart Lighting and IoT-driven lighting systems. For example, Infineon's recently released ICL8105 and ILD2111 digital LED Driver ICs combine analog and digital functionality to provide features like extended dimming capability, 0-10 V dimming with a configurable dimming curve, and intelligent thermal management.

We can also expect to see lower-cost solutions for the electronics used fluorescent tube replacement/retrofit applications which represent a large and growing market. For example, Microchip has recently introduced the CL8800, a sequential linear LED driver designed to drive a long string of inexpensive, low current LEDs directly from the AC mains. In fluorescent replacement applications, it can deliver up to13W at 85% efficiency and a power factor of over 95 with no additional capacitors, EMI filters, or power factor correction circuit components. Similarly, the low solution costs of devices like Infineon's ICL8201 AC/DC buck controller are expected to help LEDs quickly displace the metal halide systems currently used in green houses and indoor farming.

Thermal management

Even the humble heat sink is playing an important role in slimming down the solution costs of LED lighting products. Although the new generation of high-temperature LEDs can dramatically reduce the amount of aluminum required to cool them, there are still additional savings to be had from innovations in the passive side of the thermal path.

Many of these gains are being achieved through smarter application of materials, such as adhesives and thermal interfaces. Berquist/Henkel, a leading manufacturer of thermal materials, says that these savings can be achieved by substituting using liquid thermal interface materials for traditional thermal pads. Liquid thermal conductors offer excellent thermal resistance and can be applied using automatic dispensing equipment. In many applications, they can also be used as adhesives, eliminating the costs involved with mounting hardware.


Reference
  1. "LEDs Continue to Gain Adoption" A&V Elettronica, April, 2015, Tony Armstrong, Linear Technology Corp.
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