By Emmanuel Gardette
Field Application Engineer, Future Electronics (France)
LED street-lighting designs are entering a new phase. Lighting equipment manufacturers are attempting to maintain all the benefits of LED street lamps – their long operating lifetime, low power consumption, and pleasing light output with good colour rendering – while at the same time substantially reducing the Bill-of-Materials (BoM) cost of the luminaire.
One way to reach this goal appears to be the use of alternative types of LED, replacing today’s high-power LED light sources with cheaper mid-power LEDs. This article describes the technical problems that OEMs face in implementing this change, and the efforts that LED manufacturers are making to support the change.
High-reliability, high-power LEDs
High-power LEDs are a proven light source in street lights installed in some of the hottest as well as some of the coldest parts of the world. Highly dependable, their construction is robust, featuring materials which are immune to corrosion and which tolerate extreme ambient conditions. Normally supplied with ESD protection integrated in the package, high-power LEDs are today in operation in thousands of installations, casting bright white light in precisely shaped beam patterns.
The robustness of high-power LEDs is a crucial part of their appeal to street-light OEMs, since the luminaire might be exposed both to extreme temperatures and to air polluted by vehicle exhaust fumes, industrial emissions and other airborne contaminants, as shown in Figure 1.
Despite the high unit cost of high-power LEDs, which offer typical price:performance ratios in the 500lm/$ range, street-light OEMs have until now continued to depend on the devices because of their long-term reliability in harsh outdoor conditions. This continued reliance on high-power LEDs, however, could now be about to stop.
Mid-power LEDs: cheaper, less robust
The balance that street-light OEMs previously had to strike was that with a typical value of around 1,000lm/$, mid-power LEDs provided a lower priced alternative to high-power LEDs, but brought with them a higher risk of premature failure.
That is because mid-power LEDs have a less robust construction than high-power LEDs: a plastic case rather than a ceramic substrate, and a silver-plated leadframe. These components are inherently less resistant to chemical contamination and thermal stress than non-corrodible material; and indeed contamination is one of the new risks associated with the introduction of mid-power LEDs in street lights. In the benign and stable conditions in which indoor lights operate, this risk does not arise, and so mid-power LEDs are very widely used in indoor luminaires. The broad adoption of mid-power LEDs has led to the development of a rich ecosystem of compatible components, such as optics, drivers and connectors, supporting the leading manufacturers’ families of mid-power LEDs. This ecosystem, as well as their low unit cost, help to make the mid-power LED an attractive choice for manufacturers of street lights.
Outdoors, however, conventional mid-power LEDs are prone to suffer impairment or failure in ways that manufacturers of indoor lighting are not familiar with:
• Contamination (sulphuration) of the silver-plated leadframe, which can cause a reduction in flux and colour shift due to oxidation of the metal, as shown in Figure 2
• Degradation of the plastic case material due to thermal or mechanical stress
• The wire bonding used in mid-power LEDs can break. This effect depends on the composition and concentration of the corrosive gas.
At a system level, of course, street-light OEMs design their products for a certain Ingress Protection (IP) rating, typically as high as IP68. It might be thought that this gives sufficient protection from contamination. But this is to gamble on the lifetime of the luminaire:
• Contamination might reach the LED in the luminaire’s assembly or maintenance process
• Materials used to make the luminaire might contain corrosive gases. These might be found for instance in the gasket, glue or paint.
• Components installed in the luminaire can emit gases in operation
The risk of impairment or failure is clearly worrying for manufacturers of street lights: municipal authorities and other buyers will normally specify extremely long operating lifetimes of 15 years or more, and set tight conditions governing long-term lumen maintenance and, in some cases, colour shift. This means that shifts in CCT, flux reductions or catastrophic failures attributable to contamination could trigger claims against the fixture manufacturer’s warranty.
How manufacturers are hardening mid-power LEDs
There is, then, clearly a market need for an alternative to the high-power LED in street lights, one which can both withstand harsh conditions and which is cheaper.
Now LED manufacturers are responding to this need by introducing new families of mid-power LEDs which are hardened for use in street lights. These new devices tend to be produced as two-die packages with a 3030 outline. For example, LG Innotek’s 3030N LEDs have excellent EMC characteristics, and feature silver terminations which are coated to protect them against corrosion, as shown in Figure 3. The 3030N products are valued at approximately 1,000lm/$.
LG is also demonstrating the effectiveness of its design with extended product testing to which standard mid-power LEDs are not subjected. Tested in accordance with the IEC 60068-2-43 and IEC 60068-2-42 standards, the LEDs’ performance is measured when operating in the presence of air contaminated with sulphur.
Nichia is also supporting the demand for exterior mid-power LEDs, and has done numerous studies on the contamination and oxidation risks affecting its LEDs. Nichia now performs accelerated testing in a mix of hydrogen sulphide (H2S) and nitrogen dioxide (NO2) at 40°C and a relative humidity of 75%. All its 757 family devices are tested under these conditions for 240 hours to determine the impact of oxidation and to inform lumen maintenance models.
Lumileds has taken a different approach to solving the problem. Its High Robustness (HR), HR30 series of LEDs is described by the company as a bridge between high-power and mid-power LEDs in outdoor and industrial applications. The LEDs feature a gold-plated leadframe and a high-reflectance SMC housing. According to Lumileds, the HR30 devices offer ‘superior corrosion and chemical resistance to all mid-power solutions, including those specially treated for that purpose’. In contrast to high-power LEDs, however, the HR30 devices feature a lower maximum current rating of 240mA. The HR30 devices are valued at between 500lm/$ and 1,000lm/$.
Again, testing in the presence of harsh chemicals demonstrates the robustness of these hardened LEDs. The results shown in Figure 4 are drawn from tests conducted in accordance with the IEC 60068-2-43 Method 4 standard, with a 15ppm concentration of H2S in the air at a temperature of 45°C and relative humidity of 75%. Lumileds has also introduced tests performed in the presence of dichlorine (Cl2) gas. The gold leadframe performs well in these tests, whereas silver can react with Cl2 to form silver chloride (AgCl).
Ecosystem supporting 3030 format
As described above, the 3030 format is the favoured package outline supported by manufacturers of mid-power LEDs for street lighting. To help speed adoption of these 3030 packages, optics manufacturers have developed various arrays which are compatible with street lighting specifications for illuminance, uniformity and glare.
The avoidance of glare is made easier by the use of mid-power LEDs: since they emit less light than high-power LEDs, they need a larger Light-Emitting Surface (LES) for any given illuminance value.
Besides reducing the effect of glare, the optic must also produce the precise beam patterns required in street lighting. LEDiL’s approach has been to match the mechanical dimensions of its STRADA family of optics for street lights that use high-power LEDs. Its STRADELLA family accommodates the increased number of points of light in a mid-power design, while still producing a beam pattern suitable to a street light.
Carclo, by contrast, has developed an array specifically for mid-power LEDs. Its 12821 product is for an array of 84 mid-power LEDs, and produces a beam pattern suitable for P and S Class street lighting. Carclo has also characterised its street-light optics when used with LEDs in the 3030 package.
New LED choices for street-light OEMs
There is, then, a new opportunity for manufacturers of street lights to substantially reduce the BoM cost of their products by using a new generation of hardened mid-power LEDs. Equipment designers should, however, bear in mind the optical properties of mid-power LEDs. When producing mid-power LEDs, manufacturers use a cheaper process for phosphor-coating the die than for their high-power LEDs, instead embedding the die in a rectangular bath of phosphor. This means that the phosphor coating is thicker at the sides than at the centre of the die, resulting in inherent non-uniformity of CCT across the beam. By contrast, high-power LEDs have a coating of a uniform thickness over the entire light-emitting surface, producing an almost perfectly uniform CCT across the entire beam.
In very many street-lighting applications, the lumen depreciation attributable to environmental conditions, coupled with any colour shift, will be of negligible concern to specifiers and users. In some high-traffic areas, however, such as city centre pedestrianised streets, specifiers set very strict criteria for quality of light, and here the use of mid-power LEDs might be inappropriate.
It is also worth noting that street-light manufacturers are looking not only to mid-power LEDs but also to Chip-Scale Package (CSP) LEDs as an alternative to high-power LEDs, in their efforts to improve performance and lifetime and reduce system cost.
Certainly the new generation of hardened mid-power LEDs from Lumileds, LG Innotek and Nichia promises good performance at a much reduced cost compared to high-power LEDs, and looks set to make future street-lighting products more competitive and more attractive to potential buyers.