Resources | LpR Article | Special Topics | DC-Grids | Applications | Apr 15, 2019

48 VDC Integrated Drivers Offer New Options

LpR 70 Article, page 102: In the telecommunication and IT businesses, especially, 48 VDC grids have become quite popular. This growing global trend to facilitate direct current (DC) power supply in building "microgrids" presents new opportunities for solid state lighting designers. TE's Lighting Industry Manager, Ron Weber, and Global Product Manager, Scott Hamilton, describe how LED lighting fixture designers can now design luminaires around a 48 VDC constant voltage input, available to take advantage of the benefits of a low voltage DC distribution grid in a building.

DC building power distribution simply has too many benefits to be ignored – nearly 95% efficiency, resiliency in outages, increased safety from shock, compatibility and ease of integration with data and communications wiring, and renewable generation and storage systems…the list goes on. All that adds up to cost savings and convenience in electrical delivery, installation and maintenance, as well as fixture and controls' bill of material (BOM) cost.

Why 48 VDC?

Very likely, the dawn of the 48 VDC age for LED illumination has a lot to do with the work of telecommunications engineering concerns who have, for decades, invested in an entire ecosystem of 48 VDC components, power supplies and cabling. More recently, this resource investment greatly accelerated and advanced as cloud computing and high performance data centers engineers started to standardize around low voltage DC distribution. These systems were first driven by the need to connect to battery back-up systems that operate at 48 VDC. The electrical and mechanical engineering work that has gone into the telecom grade power regulation and distribution components and systems to date seems certain to benefit the lighting and building electrical engineers now taking them up.

Of course, this does require a change in the building electrical when compared to a conventional AC system. DC power servers need to be added to the building electrical design layout and arrayed so one or more servers can efficiently serve an entire office building floor of luminaires. That's a small price to pay given the safety and service cost benefits that come with the easier installation and maintenance of the low voltage system and any lighting fixtures that connect to it.

For a lighting designer working on a new luminaire platform, cost and complexity can be driven out and new functionality and system flexibility can be brought in when low voltage DC (LVDC) at the 48 V level is the starting point. Eye-catching design and aesthetics are often at the forefront of a new fixture designer's requirements. With 48 VDC, the fixture designer is freed from the AC to DC conversion engineering task.

Figure 2: 48 VDC grids require a change in the building electrical system. This is what the new structure could look likeFigure 2: 48 VDC grids require a change in the building electrical system. This is what the new structure could look like

Standards Arise to Support Improvements

The advent of the chip-on-board (COB) package and its inherent low voltage DC power requirement provided a near ideal match to the low voltage DC power distribution system. Of course the early versions weren't without issues, the most important of which is that packages initially came to market with little standardization. Designing with these was no easy or simple task and resulted in a fixture design being beholden to a specific COB LED. This introduced complexity into the design process and in turn, resulted in longer and more expensive product development cycles.

Time has a great habit of sorting out complexities. The LED lighting ecosystem has benefitted in recent years from the standardization efforts jointly driven by luminaire designers, engineers and LED manufacturers. One example of this is under the umbrella of the Zhaga organization. Specifically, the development of Zhaga Book 12 that defines chip-on-board (COB) LED Holders and arrays was a great step in standardizing footprints and LED holders. This, in turn, provided fixture designers with dramatically greater flexibility in light engine design by allowing different COBs from different manufacturers to be interchangeably used in a single fixture design. Customers could now specify preferences and they could be easily accommodated by the fixture manufacturer. The designers were no longer shackled to using a specific manufacturer's LED.

During the early stages of COB development and market introduction, the lack of standardization of these packages tempered connectivity solutions. Those of us with some longevity in the LED industry no doubt recall the early "starboards" that initially entered the market and started the path toward COBs. Customers often resorted to hand soldering to these earlier boardbased LEDs. That posed a whole other list of challenges since soldering to a device that by its nature is designed to pull heat away is a task left to highly skilled solder technicians.

For connector companies such as TE Connectivity, developing COB holders during that time was a challenge due to the wide array of non-standardized COB products out there.

To address this lack of standardization, the first "scalable" holder was marketed that could be adjusted during assembly. While not optimal, it did offer a solderless termination to a wide range of COB packages.

With Zhaga book 12 well codified, the ecosystem has coalesced around standardizing the size and distance between mounting screws, establishing standard light emitting surfaces and establishing preferred COB sizes. The establishment of standard COB sizes included electrical and mechanical "givens" like contact pad location and configuration. That paved the way for the standardized solderless LED holders that were so needed by lighting designers.

The availability of standardized COB holders formed a crucial element to simplify and flexibly integrate COB LEDs into their fixtures. Of course, the issue of driver integration into the fixture still remained a challenge.

However, the holder standardization offered a unique platform upon which to base new solutions that could eliminate this challenge.

The advent of a 48 VDC distribution system and the standardization of COB holders offered up a "perfect storm" of sorts. Unshackled from the need to incorporate bulky AC/DC conversion components, the next step in the evolution of COB holders started to gel. The empty space under a Zhaga-compliant COB holder offered an ideal location to integrate a compact constant low voltage input / constant current output DC/DC convertor. The resulting integrated driver COB holder provides luminaire designers with an option that lets them eliminate unattractive "squirted plastic and bent metal" bricks that they have been required to integrate into their fixtures for many years. Interestingly the more compact fixture designs that result from using integrated driver COB holders and the space freed up is allowing for more innovation around fixture connectivity. The added space simplifies integrating wired or wireless data connectivity modules in their lighting systems and is vital for manufacturers' future product plans and roadmaps. The more work luminaire engineering and manufacturing teams can do to support connectivity integration, the better.

Figure 3: The integration of 48 VDC drivers in LED holders or compact moldules must not compromise qualitiy aspects like durability and low flicker, even under dimming conditionsFigure 3: The integration of 48 VDC drivers in LED holders or compact moldules must not compromise qualitiy aspects like durability and low flicker, even under dimming conditions

In Practice - Track Lighting

Let's look at a conventional track lighting system design. On a conventional LED track lighting head, a heat sink is likely necessary which is integrated into a base that clips into the track. A COB is mounted to the heat sink and an optic that the designer has chosen is attached to the front. Next comes the challenge of integrating an AC/DC driver into the fixture. In order to house the LED driver, a separate, unsightly rectangular enclosure is most likely hanging off the lighting head.

Not only can that make the track light heads very unsightly, it adds a level of complexity to the manufacturing assembly. With a 48 VDC system that has integrated the LED driver into the Zhaga-compliant holder, the external driver box is eliminated. A track head fixture simply becomes the heat sink, COB and integrated driver holder, the track attachment base, and optic. The simplicity of this design is clear.

The aesthetic benefit of using an integrated driver COB holder is not the only consideration. By using an integrated driver COB holder, the manufacturing process is dramatically simplified because it's solderless. It also eliminates the connection between the driver and the COB thereby offering additional assembly efficiencies. Design phase choices like these can make or break a product's success in the market, because added assembly or manufacturing expense drive up the product sales price.

Product line customization is possible since you can switch the COB without any other industrial design change. If the customer wants a different color temperature or a different light output, the COB LED is fully modularized and changeable. This would certainly not be possible when the manufacturer is hand-soldering the COBs.

48 VDC without Compromise on Controllability and Flicker

The benefits of 48 VDC power in a building microgrid are clear, and the LEDs in such luminaires clearly benefit in terms of thermal characteristics, efficiency and longevity. But it is clear, too, that luminaire makers should not be asked to sacrifice on light controllability. This is a first "must have" from a fixture designer's point of view. In Europe, that means digitally addressable light interface (DALI) compliance. In North America, 1 to 10 V dimming is a must.

To simplify connectivity of power and dimming, four wires are needed for an integrated driver COB holder - two for current and two for dimming.

Given the miniaturization of lighting fixtures and a trend away from the 18AWG wires traditionally used in lighting fixtures, finer wire (such as 20, 22 or even 24AWG) can be used in these newer LED fixtures. Fine wires do pose handling challenges in manufacturing but this is easily solved by connectorizing the interface to the COB holder. That's where a fine pitch connector such as TE Connectivity's mini CT connector comes in. Properly done, this single plug and play connection of the four wires is keyed and polarized, so the assembler cannot terminate the wires to the luminaire together in any way but the correct one and no manual wire crimping errors are introduced.

The second "must have" is high performance on flicker. The end customer demands a fixture that must deliver very low flicker. There can be no "weak link" in the luminaire components causing flicker since changes in illumination quality are perceptible (and often highly annoying) to the human eye. It's one of those fundamental defects that illumination engineers have sought to overcome since the very first meetings of the Illuminating Engineering Society (IES) in 1906 and 1907 [1]. These "problems involved between the production of light and the physiological effect produced by the light on the eye" are at the heart of the LED professional's engineering task.

Clearly, the quality of the LED driver circuit regulating voltage to the LED array is important, as flicker from LED sources is a direct result of variations in the current being delivered. Well-engineered and tested COB holders with integrated drivers would deliver less than 2 percent flicker at minimum dim, which as is well known, the industry considers zero flicker.

Figure 4: Quick sketch of a possible 48 VDC grid office solutionFigure 4: Quick sketch of a possible 48 VDC grid office solution

Engineering Alignment Benefits LED Lighting

Now is a fascinating time for the LED lighting professional. Standards have been sufficiently defined so component suppliers can deliver interchangeability, modularity in platforms, and attractive pricing and options for light sources and optics. 48 VDC constant current infrastructure is at hand. Integration at the holder and driver level brings design and assembly benefits. As exciting as the industry was ten years ago when LEDs first came to market as incandescent and fluorescent replacements, the promise today of new levels of illumination quality, luminaire sophistication and greater than 95% energy efficiency is just as exciting, if not more so.

References:
[1]    Charles Steinmetz, "Light and Illumination," transactions of the
        Illuminating Engineering Society meeting, 1907.
        http://media.ies.org/docs/research/100papers/002.pdf

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