The dawn of a new age in street lighting

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-ofMaterials (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-rel, 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, such as in Almaty, Kazakhstan.

Mid-power LEDs: cheaper, less robust

The problem for street light OEMs used to be that the cheaper alternative to high power LEDs (mid-power LEDs) are typically valued at around 1,000lm/$ – and also posed too high a risk of premature failure.

That is because mid-power LEDs have a less robust construction than high power LEDs, consisting of a plastic case rather than a ceramic substrate, and a silver plated lead frame. 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 midpower LEDs in street lights. In the benign and stable conditions in which indoor lights operate, this risk does not arise, and so midpower LEDs are very widely used in indoor luminaires. The broad adoption of midpower 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 lead frame, which can cause a reduction in flux and colour shift due to oxidation of the metal. In separate detailed application note, Nichia and LG have explained the process and consequences of silver sulphuration.
• 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 ingredient 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. However, 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 Correlated Colour Temperature (CCT) – a measure of the relative yellowness or blueness of a fixture’s white light output – flux reductions or catastrophic failures attributable to contamination could trigger claims against the fixture manufacturer’s warranty.

MIid-power LEDs are toughening up

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 midpower 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 has carried out intensive research to understand the process by which hydrogen sulphide (H2S) and sulphur oxide (SOx) gas contaminates LED lamps, and this underlies the design of its mid-power 3030N LEDs - their package has excellent EMC characteristics, and features silver terminations which are coated to protect them against corrosion (see Figure 3 - below). 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 IEC60068-2-43 and IEC60068-242 standard, the LEDs’ performance is measured when operating in the presence of air contaminated with sulphur.

Nichia is another LED manufacturer which aims to support the demand for exterior mid-power LEDs, and has done numerous studies on the contamination and oxidation risks affecting its LEDs. Tests based on different classes of corrosive gas (according to ISO11844), and field tests to measure the increase in mass of the silver elements of the LED attributable to oxidation, led Nichia to develop a new approach to the testing of sulphur contamination - as defined by IEC 60068-2-60, Nichia now performs accelerated testing in a mix of H2S and nitrogen dioxide (NO2) at 40°C and a relative humidity of 75%. All its 757 family of 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 HR30 (HR stands for ‘High Robustness’) series of LEDs are described by the company as a bridge between high power and mid-power LEDs in outdoor and industrial applications. They feature a gold plated lead frame (eliminating the liability to contamination that mid-power LEDs with a silver lead frame suffer from) 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 (above) 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 lead frame performs well in these tests, whereas silver can react with Cl2 to form silver chloride (AgCl).

Ecosystem supporting 3030 format

As described here, 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 (www.ledil.com/stradella) accommodates the increased number of points of light in a mid-power design, while still producing beam patterns suitable to a streetlight.

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.

Driver implementation for mid-power designs should not be difficult. Even though the number of LEDs in a midpower design is higher, the light manufacturer should be able to use the same driver as for a high power LED system, using a parallel series connection between the LEDs (high power LEDs are usually connected all in series).

Mid-power LEDs also give system designers the opportunity to think afresh about the power supply. In some cases an AC direct solution, in which the power management circuit is implemented directly on the light engine board, and which offers space and cost savings over a conventional switch mode power supply, might be a valid choice for street lights.

New LED choices

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 (downtown) 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 midpower 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.