Development Process for a Metal-Halide Replacement LED Module by Neonlite International Ltd.

The pace of product development in LED lamp technology makes the sector exciting, yet also challenging for fixture manufacturers and designers alike. Already we can see that LED semi-retrofit and replaceable lamps will soon become as commonplace as traditional light sources.

In the professional specification sector, LED lamp alternatives are now commonly used for general lighting solutions with excellent results in terms of color characteristics, longevity and energy efficiency. Thanks to the company’s unique approach of using axial mounted arrays, LED sources are able to imitate the point sources of tungsten halogen in reflector lamps. This provides the professional lighting sector with the intense beams and glare control of a halogen light source, but with the benefits conveyed by using low energy, accent lighting. The pace of change is so rapid that LED halogen equivalents can now perfectly match the light output of halogen reflectors up to 75W.

The challenge now is to move into metal halide territory of 20 W, 35 W and larger and create viable LED alternatives. This has had its challenges for LED light source manufacturers, as most metal halide solutions of this size do not use integral reflectors and have higher lumen packages. In addition, LED equivalents require careful thermal management. A successful LED replacement needs to outperform existing metal halide solutions in terms of longevity and light quality, be available in a range of color temperatures and needs to be of a similar size and shape to fit in existing and newly designed luminaires. Hence a sustainable, flexible LED alternative requires a range of technical challenges to be overcome.

Figure 1: Base to light centre of TECOH® MHx module similar to mounting surface to light centre of ceramic metal halide lamps

The solution has come in the form of the TECOH® MHx module which provides designers with a practical alternative to metal halide capsules, with a similar sized unit. Creating a replacement metal halide solution is not without its challenges. The main hurdles that had to be overcome are:

• Creating a unique format, tubular shape with axial placed LED light emitting unit

• Boosting the output to higher lumen packages while dissipating the heat effectively

• Complying to SELV requirement when using 2 LED arrays in series

• Offering a sustainable solution where the unit can be upgraded, exchanged when required while ensuring good thermal management

The Approach

The principle behind the module is a “capsule” with 2 LED arrays axially mounted on a highly conductive metal substrate. Over the years, traditional lamps have proved that axially positioned light sources, combined with parabolic reflectors offer the most efficient method for directing light and optimizing the use of the lumen output. Through testing, the best light distribution for the MHx module was obtained when two LED arrays were placed back to back on a vertical construction, the so called ‘capsule’.

The TECOH® MHx module is in essence an evolution of the company’s AR111. The development team took the thermal conductive highway of the AR111 and the back to back chip mounting on the lamp’s bridge and, after stripping away the AR111 reflector, sought to use these technologies as a basis on which to create a viable LED module.

During the development process, two 18 W LED arrays were placed in series. However this posed electrical challenges on top of the thermal management issues as the initial prototypes had a voltage Vf of 72 V (due to long term safety requirements for SELV, the maximum voltage for VDE certification is set at 60 V). With this in mind, a redesign of the array was required and optimization of both the module and driver were carried out to fulfill the SELV criteria.

Solving the Heat Dissipation Challenge

The major challenge during the product development of the module was to provide lumen packages to compete with ceramic metal halide, whilst ensuring sufficient heat was drained from the LED array in order to provide long life and high performance. After extracting the thermal conductive highway and back to back chip placement from the AR111, the product development team had to redevelop the lamp’s thermal interface to accommodate the reduced dimensions of the LED module.

Although the module’s product design was based on the company’s patented Thermal Conductive Highway™ (TCH) Technology, which has proven its robust effectiveness in the company’s reflector lamp series, a range of metals had to be tested to find the correct combination for the ultimate in heat dissipation for this specific construction.

A combination of metals was tested for the heat bridge, including a copper core with aluminum layers, to achieve the correct temperature distribution. The final module design uses a ‘heat drain’ across a highly conductive metal substrate, together with a unique combination of materials to dissipate the heat efficiently.

Throughout its product development it was decided to design the module as a 2-part, socketable solution for interchangeability reasons. However this poses another challenge as the thermal conductivity of such additional interfaces needs to be maximized in order reduce its impact on the heat drain from the LED arrays. With this in mind, the thermal conductivity between the base and capsule has been optimized by the use of a highly conductive Thermal Interface Material (TIM).

Table 1: Thermal conductivity for different materials

The heat finally needs to be dissipated away from the base. A heat sink which optimizes this dissipation and secures compliance with the maximum temperature requirements will ensure an excellent lumen maintenance and product life within the fixture. The TECOH® MHx has a long lamp life (L70) of up to 40,000 hours. This is more than double that of the most efficient ceramic metal halide lamps, which have maximum average lifespans of up to 20,000 hours.

In practice, when taking into account the installed luminous flux (survival rate x lumen maintenance) for the installation, the usable life span (when the overall light output has dropped 30%) for ceramic metal halide lamps becomes less than 2 years (3000 hrs/ year), since other than the lumen drop of the individual products, a percentage of failures must also be taken into account. In comparison to this, the useful lifespan of the TECOH® MHx is expanded to 10 years.

Figure 2: The unique geometry allowing optimum thermal control with the TCH technology

AR111 lamps and the Thermal Conductive Highway™ technology
The patented Thermal Conductive Highway™ technology uses a unique design of ‘heat drain’ across the reflector to dissipate heat efficiently and prevent deterioration of the LED and other components. The technology also gives the lamps a longer life with lumen maintenance, resulting in 70% of initial lumens being available even at the end of the lamp life. Thanks to careful thermal management, these LED’s combine the higher efficiency, lifetime, and reliability benefits of LEDs, with the light output levels of many conventional light sources.

With the first AR111 LED reflector solutions the goal was to replace halogen and still achieve good quality accent lighting with a sustainable solution using LED for 80% energy saving and long life.

Table 2: TECOH® series electrical characteristics and general data

The technical challenge came from the need to use LED to approximate the axially mounted conventional incandescent tungsten halogen filaments. Sources mounted in this way allow the light to be controlled much more effectively by a parabolic reflector. Most of the light is then directed by the reflector minimizing uncontrolled direct light which leads to uncomfortable glare and poor beam quality. Thermal management of the resulting back to back LED arrays is extremely difficult but the breakthrough was the thermal conductive highway which provides a heat drain of sufficient capacity while still allowing the arrays to be close enough to approximate a point source. With the AR111 reflector lamp the heat sink is so efficient that the temperature difference between the centre of the bridge and the edges is only 5°C. The good approximation to a point source is demonstrated by the ability within the 111 mm footprint to produce intense beams as narrow as 8° with up to 20,000 cd, a power consumption of only 15 W and little stray light.

Figure 3: Color consistency

With this problem solved the move to separate the reflector from the source was a logical next step allowing the fixture maker to use their own expertise in reflector design and for the company to provide the first alternatives to metal halide capsules.

Table 3: Photometrical characteristics

Addressing Light Quality over Lifetime

The secret to maximising light output and lumen maintenance lies in sound thermal management. As can be seen with the company’s patented TCH technology, the efficient dissipation of heat has led to significant lumen maintenance over the module’s considerable lamp life. However, the question of color consistency is always a key focus for end users and designers alike.

The key to creating an LED lighting scheme that looks good for years to come is in ensuring that, over their lifespan, all of the lamps are performing within an acceptable tolerance in terms of color deviation. The modules have high color uniformity, with a consistency of < 3 SDCM (Standard Deviation of Color Matching) initially and < 5 SDCM throughout the lamp’s rated life. This high level of color consistency is thanks to the control of the phosphor/LED blend and the lamp’s optimized control.

The color consistency of the module is superior to existing metal halide lamps, whose colour characteristics tend to change during their life (color and output of metal halide lamps is specified after the lamp has burned for 100 hours or ‘seasoned’, according to ANSI standards). This is even compared to newer metal halide lamps, which encompass a technology referred to as “Elite, Superia or Ultra”, which have improved color rendering and more controlled kelvin variance of ±100 to 200 kelvins.

Thanks to the MHx modules excellent lumen maintenance and stable color, individual product replacements become feasible whereas with ceramic metal halide, individual replacements lead to different colors and intensity impressions, resulting in unbalanced lighting. To avoid this, only group replacements are advised with metal halide products.

Figure 4: The TECHO® MHx modules allows for interchangeability

Provide Flexibility with Interchangeability

One of the primary aims when creating the module was interchangeability (for serviceability) to promote a more sustainable future for LED lamps and fixtures.

The 2-part base and head design of the module ensures that its head can be replaced when it comes to the end of its life or earlier if an upgrade is wanted for a more up-to-date version. This interchangeable design therefore offers intrinsically a more sustainable approach when compared to dedicated solutions.

The interchangeability of the module design allows for the late addition of the light source into the fixture and the desired color temperature to be put in as required for the specific application.

Next to the common 3000 K and 4000 K color options, the module is also offered in R9 technology. The R9 series is optimized for food illumination as it maximizes the visual impact of meat, fruit and vegetables by increasing the product’s red rendition. The R9 modules not only have a high red color rendition value R9 of >75 (compared to the even for this application specially designed ceramic metal halide lamps having R9 values below 60), but also have high values for regular CRI and other ‘saturated’ colours R10 to R14.

Conclusion

The TECOH® MHx is the first LED solution that directly competes with ceramic metal halide lamps. With no UV and low maintenance costs, its better color characteristics, superb lumen maintenance throughout life, instant start/hot re-strike and dimming capabilities, the module’s design brings the future of LED lamp technology to the high power accent lighting market.