LED Journal

The research firm IHS just released a 2014 forecast for the LED general lighting market, which indicated that mid-power LEDs would represent >80 percent of the LED units shipped and roughly 48 percent of the revenue.  This is quite a change from a few years ago where revenue of high power LEDs dominated the general LED lighting market. There are numerous factors that have driven this shift including improved LED efficacy, adoption of lower cost plastic packaging, and a shift from magnetic transformers to mechanical approaches to achieve safety isolation.  This is especially true in integral bulbs and as a result the LED string voltage is no longer limited to 30 to 60 V depending on the regional safety requirements.

In some cases mid-power LEDs are a combination of several low power LEDs in series in a single package.  As a result, high voltage mid-power low current LEDs are available with typical forward voltages ranging from 9 to 100 V. One impact of using these LEDs is that alternate driver topologies instead of the classical isolated flyback can be used that result in lower electronics cost and/or higher power conversion efficiency.

 

Figure 1. Single LED string direct AC drive operation

One of the simplest direct AC approaches involves a bridge rectifier, constant current regulator (CCR) and a string of LEDs as seen in Figure 1. If the LED string voltage is sufficiently high compared to the input AC voltage, the losses in the CCR can be kept to a reasonable level while achieving > 0.9 power factor.  This approach does have lower LED utilization since for a portion of the AC cycle, no current flows through the LEDs as seen by the red line.  In this example there is no electrolytic capacitor in the circuit for energy storage so this type of driver results in 100 percent optical flicker at 100 / 120 Hz (2x the AC line frequency) but the driver circuit is simple, small and can have a long operating life time.   As it is not always practical to match the LED string voltage to the AC line voltage a switching topology is needed.

To better understand how to determine the right switching topology based on the string voltage, we wanted to compare several mainstream topologies and provide some general guidelines to help designers understand how the selection of the LED string voltage impacted the topology choice.  To do this a figure of merit (FOM) was created that combined the maximum voltage stress and peak current through the power switch as a function of the LED forward voltage and the input line voltage.  This FOM is a good proxy for both efficiency and cost in a switching LED driver as the lower the peak current, the lower the losses in the switch, inductor and diodes.

Two general use scenarios were considered. First, applications where high power factor and low THD (<20 percent) were required and the second where high power factor is not required.  As an example for ENERGY STAR LED lamps, there is no power factor requirement for lamps < 5 W and in the EU there is a special input line harmonic exception in EN61000-3-2 for lighting products < 25 W so high power factor is not required.  An added benefit of not needing high power factor is that it is easy to achieve low optical flicker.

The buck, buck-boost, boost and isolated flyback with a turn’s ratio of 3 were all compared based on the FOM (the lower the better) as a function of the V_F to V_in peak ratio.  In the case of the high power factor case, the total harmonic distortion was limited to < 20 percent for analysis and we assumed a 600 V MOSFET (80 percent derating).  For V_F/V_in ratios from 20 to 40 percent, the power factor corrected buck is the best topology. The upper limitation of the buck is related to THD and power factor and not stress on the power switch.  Interestingly it turns out the best topology from the FOM analysis is the boost, which is not the first topology to come to mind for many designers.  An example of a complete high PF 10 W boost design is shown in this design note where a 220 V LED string was driven at 30 mA across an input voltage of 90 to 135 Vac.

 

Figure 2. Topology comparison for high power factor >0.9

 

 

Figure 3. Topology comparison for low power factor (EN61000-3-2 Class C Exception)

 

When looking at applications were it is not critical to achieve high power factor, the clear winner for strings of higher voltage LEDs is the buck topology as seen in Figure 3 as it has the best FOM across a wide range of forward voltages.  An added benefit of this approach is that it is one of the simplest to implement and can have low optical flicker. The reason that the curve ends below 70 percent is due to meeting the requirements of the EN61000-3-2 Class C exception where the input capacitor is undersized to control the input current shape. For applications in the US market where compliance to EN61000-3-2 Class C is not required, this range can be extended. An example of a complete 3.8W buck design is shown in this design note where a 150 V LED string was driven at 25 mA across an input voltage of 200 to 265 Vac and achieved 85 percent efficiency.

This FOM comparison is a good reference tool at the beginning of the LED selection and architecture definition.  In some cases, especially at more narrow V_F/V_in, there are several options that should be considered since the differences between different topologies is relatively small but in many cases this can give clear guidance in what topology is best for lowest system cost and better efficiency.

The accelerating transition to LED-based lighting is changing the dynamics of luminaire design. Traditional approaches in which components are evaluated separately to arrive at an optimal BOM are being replaced by more synergistic, system level thinking. Several trends are driving this evolution in system design, which is supported by greater collaboration between providers of LEDs and power electronics components.

The first trend is the emergence of many new competitors, concurrent with rapidly falling prices for LEDs. While LEDs historically represented a large portion of a fixtures’ cost, other electronic components – including driver, power stage ICs and LED protection devices – are becoming a more significant portion of the overall BOM. And of course, there is relentless pressure to reduce the total end product cost.

Another major trend affecting the industry is increased demand for energy efficiency. This is best addressed by a system level approach and not by focusing solely on component selection. To respond to the efficiency challenge, the on-going migration to digital control of driver ICs and power stages for LEDs provides fixture designers with much greater ability to optimize power conversion.

Lastly, as LED-based luminaires become pervasive across multiple segments and different lighting sub-applications, there is enormous pressure to simplify the design process. To best address the widest number of market opportunities, fixture designers need to shorten development cycles. This is addressed by the emergence of simple and easy-to-integrate LED and power modules that reduce the level of complexity in design of the lighting element.

The flexibility to drive LEDs at different drive currents and different configurations, thus forming luminaires in different shapes and sizes than possible with traditional light sources, provides an opportunity for disruptive innovation in lighting. Yet this flexibility poses unique challenges for driver design. Until some level of standardization takes hold, each individual design requires custom drivers. The relative immaturity of control schemes related to dimming and other feature requirements can make driver design overly complicated and expensive. The application itself poses several challenges, whether it is outdoor wall-washing lights, or indoor down-light solutions driven from AC mains or from a DC bus or power-supply.

With these concerns in mind Infineon developed .dp, a Digital Power 2.0 generation of digital and intelligent LED driver ICs used in combination with state-of-the art power stages. The internal structure of the controller is built around a programmable Digital Signal Processor core, combined with optimized power-management and protection peripherals (Figure 1).

 

Figure 1. High-level digital LED driver IC architecture

High level integration as well as programmability offers several advantages, beginning with flexibility. Designers can employ several different drivers at different drive currents, and take advantage of features such as support of 0-10V interfaces and DALI interface compatibility. The LED driver can handle AC mains variants as well as DC input voltages in several different topologies such as boost, buck, fly-back, etc. Designs can easily be adapted through firmware changes, supporting fast time-to-market for variants of an original design for product line or market segment extension. Diagnostics and command-and-control (e.g. read out sensors, NTS, etc.) are greatly improved due to the digital nature of the part. Advanced controls support time based correction to prevent lumen degradation of a fixture and temperature compensation during color mixing to obtain desired colors or tunable white light. Fixture designers can add customized control code to support unique features, with the added code protected from reverse engineering efforts by security features that prevent readout of memory content.

With continued evolution of designs and manufacturing advancements, LED components will continue to increase in efficacy and light output. Manufacturers such as Philips Lumileds continue to innovate in response to the dynamic demands of the market. The vast selection of options and the introduction of more and more application specific LED components offer luminaire manufacturers multiple opportunities for innovation in such areas as energy consumption, light output, and lifetime claims to name only a few.

Let’s take the example of LED-based downlight fixtures. In the recent past, downlights were mostly constructed with a serial string of high-power LEDs, which meant relatively high drive current at relatively low forward voltage. Some manufacturers evolved this design by adopting CoB (Chip on Board) solutions with various current/voltage configurations depending on the serial/parallel configuration of the particular product. Today it is possible to construct downlights using an array of mid- or even low-power LEDs, introducing yet another long list of configurations and options. In all cases, the traditional design approach for these solutions quite likely involved re-design of the LED driver. This introduces cost and time-to-market complexity in terms of establishing design reliability, and conforming to fire and safety regulations around the globe. This complexity is eliminated with newly available, second generation digital power and power stage devices. Reconfiguration now is managed by simply re-programming the DSP based system to provide optimal efficiency at many Vf/If configurations.

Another example would be a luminaire manufacturer’s effort to expand across market segments, say from residential to professional and even industrial applications. While system geometries may be similar, retail price, light output performance or warranty requirements may be different. To address each segment successfully, it is desirable to minimize the number of fixed system sub-components and have the ability to simply change operating parameters. Taking the previous example of the downlight, let’s compare a residential with a professional solution. The usage profile in a private residence is completely different compared to a hotel hallway. The residential user is mostly interested in the initial cost to purchase whereas the hotel manager focuses on total cost of ownership. In this scenario the luminaire manufacturer may choose two different light engines – one low cost, the other high efficacy and high reliability – which usually require different driver configurations as well. With a programmable base product, the same driver subsystem with the appropriate firmware can be deployed to satisfy both scenarios with minimal design and qualification effort.

Yet another value of DSP-based programmable LED driver sub-systems is end of line calibration. Consider a luminaire that is exchangeable with its own kind, but due to uniformity reasons can’t differ from others in light output over its expected lifetime. In retail store lighting, where merchandise is illuminated by highly specialized lamps and luminaires, it is essential that newly added or replaced light sources have the same visual output as those already installed. There are two factors to consider in this case. First is the light output degradation over time. This can be compensated for by ever so slowly increasing the drive current based on either a pre-programmed or default current compensation curve, or by custom compensation patterns based on specific applications or needs. Secondly, as LED components keep improving in efficacy later versions of the same luminaire may require less current to achieve the same light output. At the same time lower currents mean lower stress and aging which in turn will have an impact on the long term lumen output stability – hence the light output compensation algorithm may need to be slightly adjusted as well.

In conclusion, market trends and the wider adoption of LEDs pose new challenges and open the doors to new market opportunities in the lighting industry. The trends include increased pressure to reduce system costs, the continuous increase in demand for energy efficiency and growing complexity of system design. To address these needs, the industry requires solutions to drive the LEDs that are flexible and that enable modular designs. The flexibility enabled by parameter setting or firmware changes can be leveraged at different stages of the product development or at different stages of production flow. Advanced controls such as time based correction, temperature compensation or end of line calibration can be implemented either through predetermined characteristics curves of LEDs or by sensing relevant parameters at the application level. Lighting designers should look to both LED and driver-IC manufacturers for components and modules that support a system level approach. This will allow them to take advantage of the full potential for innovation that LEDs bring to lighting at different levels of the supply chain and ultimately in the hands of the final customer.

In the early years of solid-state lighting, manufacturers focused on ensuring LED luminaires and sources were first and foremost good illuminators. Manufacturers focused on efficacy, reliability, color and luminance. These early LED solutions were designed to behave and look like the traditional products they were replacing. As LED lighting matures, the focus is shifting toward designs that leverage the extraordinary flexibility of the LED. 

LEDs are semiconductor light sources that can be controlled like any other electronic components. The ability to control the LEDs, coupled with their inherently small size, opens the door to endless flexibility in lighting designs and features. For example, the lighting industry has seen an abundance of products with new dimensions of control such as intensity and color tuning.

While important, changing the hue and color temperature of lighting are not the only desired features to control. A critical dimension of light output control is the spatial distribution. Where the lighting is aimed, the beam shape, angles, and distribution are critical parameters.

What architects and designers really want is the ability to put bright white light exactly where they want it to create a perfect design. What owners really want is the flexibility to easily change that distribution as space needs change.

In multifunctional and reconfigurable spaces—such as retail shops, entertainment, hospitality, meeting rooms, museums, galleries and residential spaces—focal points and tasks can change frequently, so flexibility in the light distribution is valuable. A growing number of spaces are joining this category, such as modern classrooms and offices.

Even with the most advanced LED solutions, it was relatively difficult to change the pattern of light. For example, in a retail shop a typical solution is to use track lighting to complement the general lighting. This requires the addition of an accent lighting layer with appropriate luminaires. Each time the space changes, the track lighting must be re-aimed. The typical approach is to do this on a ladder, though some manufacturers offer motorized luminaires. This is labor intensive and expensive.

Using aimable lighting, we can direct the light where we want it, but the beam angle is fixed. Since objects and tasks not only change in terms of location but size and shape, the ability to adjust the light pattern becomes important. The industry responded with adjustable optics. This allows designers and owners to adjust the beam spread within a defined range, again using a ladder to individually access each luminaire

OSRAM SYLVANIA talked to designers, architects, retailers and specifiers about the flexibility they need to easily control lighting.  We attempted to address these different needs in a single LED innovation.

This new luminaire is extremely flexible.  Light intensity, beam shape and angle can be simply and easily controlled remotely using a wireless Android app.  The app allows the user to take a picture of the space using a wireless camera and then touch the image on the screen to aim light. Lighting conditions can be tuned and transformed instantaneously without the use of a ladder.

The recessed luminaire, called OmniPoint, consists of an array of LEDs that are focused through an aperture about the size of a five-inch downlight. Each LED is individually controllable enabling a virtually unlimited number of light patterns. The luminaire can provide ambient and accent lighting at the same time, which can be directed almost anywhere in the space in real time.  This flexibility may reduce the overall number of luminaires that are needed in the space resulting in a clean ceiling look.

This LED innovation is the result of natural evolution marrying new technology with longstanding professional lighting needs. At the recent LIGHTFAIR International 2015, it was recognized with an Innovation Award for the Most Innovative Product of the Year and also won the Innovation Award in the Recessed Downlights category. You can see it in action here.

Smart, connected LED lighting solutions that allow us to easily adjust parameters such as intensity, color temperature, hue, beam shape, angle, and distribution are rapidly emerging. These solutions are redefining how we think about, design, specify and use lighting everywhere.

In my last article, I reviewed examples and case studies on how glass (as luminaire lens material) can be successfully employed in various LED lighting applications to both optimize lighting efficiency and economic paybacks (ROI, TCO, etc.) in standard operating environments (i.e., temperatures > 32°F and 0°C).  In this article, the final entry in this series, I’m going to discuss ways in which a fabricated conductive/heatable glass lens can enable LED lighting to be effectively used in extreme environmental conditions that, to this point, have represented a hurdle towards wider market adoption.

LEDs offer high levels of flexibility and customization (output levels, color temperature, etc.) for lighting applications and, as such, have been and will continue to be adopted in almost every lighting market segment (as shown in the following diagram from the McKinsey Global Lighting Report of 2012).

 

As you can see, most areas in lighting are quickly migrating towards LED technology.  However, three segments seem to be lagging slightly behind:  Office, industrial and outdoor.  For office lighting, this can be rationalized by the still highly competitive position (in price and energy efficiency) of fluorescent (linear and compact) technology for this market.  For industrial and outdoor, though, there is a different roadblock stagnating LED adoption and that is simply the environmental conditions where these lighting applications operate.  In industrial and outdoor lighting applications, luminaires are seeing extreme weather conditions ranging from extreme hot to extreme cold (i.e., temperatures < 32°F and 0°C) with rain, snow, ice and hail exposure. These conditions, as you will see, prove to be an inherent problem for LED-based lighting.

In traditional outdoor lighting technology like Incandescent and  High-Intensity Discharge (HID) high levels of Infrared (IR) energy (see spectrum below) or “heat” are generated by the light source itself.

 

Diagram courtesy of Guardian Industries

With this IR heat comes higher energy consumption and lower levels of efficiency/efficacy, which has allowed LEDs to become a more-attractive technology long-term.  However, the absence of this IR heat generation from LED lighting vs. Incandescent and HID technology (as shown in the following output spectra) makes it difficult to use LEDs in lighting applications where it is necessary to remove snow and ice from the lens surface – such as outdoor and industrial.

 

Diagram courtesy of Guardian Industries Corp.

As you can see, the LED output is highest in the visible range and low in the Ultraviolet (UV) and Infrared (IR) ranges of the spectrum.  This means great visible light quality and low levels of UV damage and heat being generated which is good in terms of safety and efficiency.  However, this is bad news when you need that IR energy to remove snow and ice from your luminaire, which is “built into” HID light sources.  This issue has limited the market space potential for LED lighting in outdoor and industrial lighting applications.

LED luminaire OEMs have a number of ways to resolve this problem including:

• Redirect heat from the heat sink into a cavity in the area between the light source and lens;
• Integrated a conductive laminate interlayer (tungsten “wiggle wire”) on the lens;
• Integrate a conductive coating on to the lens surface itself.

Let’s now assess each of these options. The redirection of the heat from the heat sink into the optical cavity is an easy fix but compromises the lifetime of the LEDs junction (one of its major commercial advantages).  Integrating a laminated conductive interlayer is another easy fix but increases luminaire cost and weight and compromises optical integrity and clarity.  Putting the heat directly where it needs to go, on the lens, clearly makes the most sense for LED lighting applications.

Now the question is on the lens material itself to use to accomplish this.

• Plastics (PMMA Acrylic, Polycarbonate, etc.) are commonly used in LED luminaires but have limited ability to conduct heat or survive long-time exposure to it without degradation.
• Glass is a proven material in this regard with its ability to be thermally stable > 1,100 °F and 600 °C.

By using a Transparent Conductive Oxide (TCO) coating on glass, we can provide a highly transparent yet conductive (15 – 20 ? /square) lens surface to create heat but still maintain high levels of optical efficiency (> 85 percent) with long-term thermal stability (because the coating itself has seen stable > 1,100°F and > 600°C in the tempering process).  The electrical interconnection can then be easily made through silk-screening buss bars with a conductive paint (such as Ag) onto the surface which are fired into the TCO coating during the tempering process.  By adding an Anti-Reflective (AR) coating, which has been covered in my earlier articles, you can raise the efficiency to > 90 percent.  The following diagram shows the configuration of such a fabricated monolithic heatable glass lens component.

 

Diagram courtesy of Guardian Industries Corp.

This monolithic heatable glass configuration would provide the following optical performance vs. traditional TCO technologies (Pyrolitic Fluorine Doped Tin Oxide) allowing for > 90 percent optical efficiency in the visible to be met with a single monolithic 5 mm glass lens while providing a conductive surface to heat the glass up to > 200°F (100°C) in temperatures down to -67°F (-55°C).

 

Diagram courtesy of Guardian Industries

In terms of heating the glass with this TCO coating, the following parameters and options are available for consideration and customizable during the luminaire design stage:

• Input Power/Supply Voltage up to 100 V AC/DC
• Operating Power Density range of 0.1 to 9 W/in² which, in consideration, with the following other attributes will determine the maximum surface temperature and heating ramp rate:
  • Input Power/Supply Voltage
  • Surface Area and Shape (rectangle, square, circle, etc.)
  • Terminals (Buss Bar) Size and Distance/Location
• Electrical and Mechanical Interconnects
  • Terminal/Wires
  • Toggle Pins
• Manual or Automatic Heating
  • Manual:  Simple ON/OFF control
  • Automatic: Thermocoupler with Feedback Loop

Last, because this heatable glass lens is tempered glass, it enjoys all the historical benefits associated with glass lenses that I mentioned in my first article including mechanical durability, chemical durability, environmental durability, and strength/impact resistance while providing the heating function.

In summary, we can form the following key takeaways about the use of a heatable glass lens to optimize optical efficiency for LED lighting in applications in extreme cold environments:

• It further opens up the available market space for LED lighting into segments like Industrial and Outdoor where extreme environments requiring the melting of snow and ice prevented implementation and adoption;
• It allows heat, to be focused on the lens area itself which reduces the risk to the lifetime of the LED light source itself;
• It can heat the lens and, with the addition of an AR coating, can still reach > 90 percent optical efficiency; and
• It offers a high-level of design flexibility and customization in terms of performance (optical and heating temperature and ramp rate) as well and serviceability (mechanical and electrical interconnections).

This is my final installment for the series on using glass in LED lighting and I’m confident that you will now look at glass in lighting a bit differently now than before. I hope that you will consider using glass as a lens material in LED lighting applications where you need to increase performance and differentiate your luminaires for competitive advantage.

LEDs have been replacing traditional lighting at a rapid pace, offering longer life times, more efficiency, lower cost, etc. OLED lighting promises to complement LED lighting by offering better color performance, power efficiency, unique shapes and designs, as well as a thin, lower weight and elegant profile. To enable OLED technology in this emerging lighting market, there is a need to replace traditional materials with new, better-performing materials such as silver nanowires.

 

Figure 1. Image of Silver Nanowires at 70° tilt

Silver Nanowires
In an OLED device, the top electrode is made of a transparent conductor and plays an important role in light transmission/efficiency. A transparent conductor made of silver nanowires allows for high conductivity with excellent transmission and acts as the top electrode/anode in the case of an OLED lighting system. Silver nanowires in the top electrode are used in the form of a network of wires (see Figure 1) that are a few nanometers thick and a few micrometers in length. With silver being the best conductor on the planet, the network of overlapping nanowires offers conductivity less than 10 ohms/sq while allowing 94 percent of the light to go through the percolated network.

 

Figure 2. High transmission of silver nanowires at low resistance (Photo courtesy – ClearOhm silver nanowire material by Cambrios Technologies)

Aesthetic Lighting
OLED lighting is not a point source and, therefore, does not need to be diffused or set at a distance from the area that needs to be lit. OLEDs emit light all through their surface and can be used to create aesthetically pleasing lighting structures of various forms and sizes. Imagine an elegantly shaped lampshade emitting light instead of the shade diffusing the light from the lamp within. Lights in any form factor that needs to be bent, curved or flexed, needs flexible transparent conductors. This can be achieved easily through the use of silver nanowires rather than thick and brittle conductors.

 

Figure 3. OLED lighting tiles. Photo courtesy – Panasonic

OLED devices can serve a dual purpose of being a window or a light. They can be made transparent – a window that you can see thru during the day and that would emit light at night. The skylights and car sunroof could not only allow ambient light to enter, also could become lamps at night. In such applications, it is important to have very transparent layers of OLED materials, another area where silver nanowire networks play a huge role.

In addition, OLED lighting can offer shades of color previously not possible with conventional lighting. Better color tuning is possible with silver nanowires, and OLED lights can offer a more precise shade of color for premium lighting applications.

2.5D Lighting
OLED lights can be produced on plastic substrates and coated with silver nanowires. These types of flexible and rugged form factors can be deployed on non-flat surfaces, such as the dashboard of a car. This type of 2D and 2.5D lighting systems are very desirable in high-end consumer devices, retail stores and museums where any surface can provide changeable color. The traditional transparent conductor materials like ITO (indium tin oxide) are brittle and don’t lend themselves to bending or shaping, hence newer materials such as silver nanowires, which are malleable and ductile, are preferred.

Thinner, Lightweight, Rugged
Silver nanowires complement OLED in that both materials are thin, lightweight and very rugged. The thin and lightweight features make them suitable for applications such as aircraft lighting and illuminating skyscrapers, and the rugged nature of the system (both OLED and silver nanowires can be coated on plastic or thin glass) makes them suitable for outdoor public venues, which typically require lighting that is not only bright but also unbreakable.

Conclusion
LED and OLED technologies will reshape the lighting market with more efficient devices that require less power and offer flexibility in design at lower overall cost. While LED technology is way ahead in replacing conventional lighting technologies, OLED lights will follow and offer lighting solutions that we have never had before. Silver nanowire technology has begun to enable these emerging applications.