What you should know about HBLEDs – and nobody will tell you – Part 3

Technology News |
By eeNews Europe

In part 2, ‘Light from Chips’ we looked at what makes LEDs work, how they’ve evolved, and some of the design challenges modern HBLEDs present to the unwary engineer.

Part 3 – From Theory to Practice

It is well recognized in the lighting industry that fluorescent lamps can often equal the efficacy (Lumens per Watt) of the very best LEDs and are dramatically lower cost. But the Lumen-per-Watt (LPW) figures for LEDs on manufacturers’ data sheets only tell half the story. If an LED is rated to deliver 120 LPW when driven with 350mA at a junction temperature of 25C, it is unlikely to have what is called a "delivered" LPW of no more than 80-90 after all system losses for temperature effects, optics, power supplies and current drop-off are factored in.

Meanwhile, LEDs’ arch-nemesis (fluorescent lamps) can deliver very comparable performance at a cost that’s less than one/fifth the cost of LED lamps, and still last 6-10 times longer than incandescent bulbs. This means that, barring any technical breakthroughs or market disruptions, fluorescents will dominate many areas of wide-area lighting for a few more years. For our purposes , wide-area lighting serves applications which don’t require light to be focused or directed toward specific areas and can include certain down-lights and high-bay warehousing lighting.

These hard realities mean that LEDs and fluorescents are likely to coexist quite a while longer, each doing what it does best. That said, LEDs will displace their mercury-laden cousins more quickly in applications where fluorescents have difficulty, such as operating in temperatures below 32 F, performing full-range dimming, operating without any noise or vibration, or being constantly subjected to very frequent on/off cycles. LEDs will also quickly edge out fluorescents in locations where breakage is an issue.

In addition, there are some sizable segments in the vast global market where fluorescent technology is ill-suited to replace the many incandescent lamps which are likely to be phased out to meet economic or environmental goals. These segments include applications requiring "directed" lighting, such as commercial down lights and the many spot lights of varying beam angles used in homes, stores, museums, entertainment, etc…

Optics and the relevance of directed lighting

Since LEDs are, by their nature, directional light sources, it’s easier to focus the light they produce than the near-spherical radiation patterns of incandescent filaments – and significantly easier than even the smallest fluorescent tube. In addition, the lens used to collect the light from a tiny LED device is dramatically smaller than for an incandescent of equivalent output. Because directed-lighting applications are one of the largest potential markets for LED lamps or fixtures, it goes without saying that having appropriate lenses and reflectors for such LED lamps and fixtures and knowing how to best integrate their use in systems is critical to market success.

But, as usual, it’s not quite that simple. A single tiny LED chip is what makes it easier to collimate (i.e focus) the LED output into a 10, 20 or 40 degree narrow beam. But there’s been a trend within the industry to make large LED chips and even larger multi-LED/chip-on-board (COB) arrays, which can be treated like a single large chip. Theoretically at least, COBs make fixture design simpler, more like just installing an old fashioned light bulb. But the large radiant area of a COB or large LED produces a broad stream of light that’s extremely difficult to focus, defeating one of the primary advantages originally offered by LEDs. No matter how tightly they’re packaged, an array of 30 LEDs, treated as a single source will still require a single reflector that’s 2X to 3X the diameter and height as an array of small conical plastic lenses, one for each LED.

What this means is that as the LED lighting industry continues to grow and moves into higher power mainstream applications, many of the assumptions we made four or five years ago will have to revised – or replaced – as greater levels of hands-on experience bring new considerations to light. At present, down-lights and directed lighting for commercial general illumination (under 40 watts), represent the two most important potential markets for white HBLEDS. The growing use of LED-based products in these applications is forcing suppliers of optical device and services to become much more familiar with the nuances of LED lighting.

While industry is full of optical design consultants who are eager to assist with modeling and producing solutions to meet your needs, very few of them have much actual experience with commercially successful LED products. Also be on guard when you encounter the small number of LED lens companies with a molding affiliate which offers a family of standard lenses. My experiences have shown that, despite the manufacturer’s claims, most of them aren’t optimized to meet market needs or for how people actually use them. Until these problems are solved, the industry’s R&D efforts will suffer as lighting equipment designers struggle to obtain properly designed standard lenses which are priced effectively and accurately/thoroughly documented.

The LED lighting designer must also cope with the fact that the industry hasn’t matured sufficiently yet for standards to emerge which define how lens makers specify beam angles, mounting methods or the efficiency of their products. There’ not even an agreed-upon protocol within the industry for specifying and making samples available to potential customers.

LED Power Supplies and Dim-ability

While most LED lamps and LED fixtures operate from line voltage (typically 120VAC or 220VAC) the LEDs inside of them require a special power supply (a little black box if you will) because, unlike regular light bulbs, these lamps and LEDs will blow up when connected directly to 120VAC. Their requirement is almost identical to that for fluorescent or HID lamps, which have always required a ballast, located somewhere within the fixture.

The recommended type of power supply or ballast for nearly all LED lighting systems used in illumination applications is a so-called "constant-current" supply. There really is nothing special about constant current supplies except that 98% of the switching power supplies used in the world are "constant voltage" types, with most of the other 2% used in LED illumination and laboratory power supplies. (Note: most LED systems used in indicating/messaging applications are driven by conventional constant voltage power supplies.)

Throughout the 90’s, and up until about 2004, 99% of high brightness LEDs were red, green, yellow and blue used for indicators, signs .and graphic displays. The dominant system architecture used a somewhat conventional power supply, followed by LED driver chips, typically using PWM, to control brightness or mix colors appropriately. Such PWM brightness and color control of low and medium power LEDs became the basis for the massive patent portfolio of Color Kinetics (now part of Philips).

Such a complex pairing of constant voltage supplies with PWM driver chips is not generally compatible with the needs of a general illumination white-light lamp or light fixture. It is now a given the power supplies for these types of products must be of a constant current type and there is growing market pressure that they be "triac dimmable".

There are over 125 million installed triac-type dimmers in the US, They are cheap, easy to install, and have an extraordinary 40 year history. Unfortunately, they have several characteristics which make it difficult for a simple LED driver to derive its power from the SCR’s chopped output and dim smoothly across its entire output range.

Why then, are so many LED products compelled to work with primitive triac-based dimmers? Mostly, it’s basic economics. A simple, single-pole triac dimmer costs 10-$12 (often even less) and doesn’t need an electrician to install it. On the other hand, a 30-40W low voltage dimming supply suitable for general illumination LED applications (often called an "ELV" dimmer) costs well over $50-60, plus the cost of an electrician to install it. In addition, its two-pole control input is not readily compatible with very simple wall box installation.

All of this has brought increased visibility for the need of power supply vendors, for levels up to about 75 watts, to offer "triac dimmable" versions. It turns out there are many nuances involved in creating a truly dimmable supply, free of anomalies at low brightness levels. Addressing any of these issues properly would be best done in a separate article, but suffice it to say that the industry is finally starting to address these things. Several manufacturers of LED retrofit lamps (the term for a screw-in LED lamp) are leading the way and have recently incorporated custom triac dimmable power supplies into their lamps.

The rules change again for higher-powered LED illumination systems which require large power supplies (above 30-40 watts). They must be compatible with two different non-triac dimming systems. One scheme is referred to as analog (often called 0-10V dimming) and the other is a PWM-based method. Both schemes offer better control without producing unwanted changes to the supply’s power factor or EMI emissions.

Design Considerations for LED Bulbs & Luminaires

There are many important issues a designer must address when creating an LED lamp or fixture. While the decisions to me made are somewhat dependent on the particular application, there are at least 10 issues which appear on nearly every design’s critical path:

– What are the physical size/space guidelines the design must meet?

– How much light is actually needed on the target surface?

– What’s the best number of LEDs to use to produce that amount of light?

– What’s the best way to interconnect them?

– What junction operating temperature needs be maintained within the HBLED?

– What range of ambient temperatures will the lamp or fixture be operating in?

– Which type of cooling system should be used to meet those requirements so the LEDs remain cool enough to deliver the light we need?

– Is there a need for dimming or occupancy control electronics?

– What kind of lenses or reflectors are needed to create the illumination pattern required by the application? (This is especially important in spot lighting applications.)

– What are the application’s weight and cost limits?

Unfortunately, after the designer has made all these decisions, s/he will discover that the linkages which cause each change to affect other factors within the design. Often, this requires numerous cycles of successive adjustments (i.e. an iterative process) until all the design parameters are met and each choice reflects a sensible engineering decision. This means that the designer may not know what operating current and power levels will best suit the application until s/he has completed assessing all the factors to determine what is desired, what is achievable and what is ultimately acceptable to the company’s marketing department.

Trends to Watch in 2014

One never knows which of the many "breakthrough" announcements coming out of universities labs and corporate R and D labs might have relevance down the line. But history has shown that it’s best to take all these breathless announcements and "expert" predictions with a grain of salt.

Nevertheless, certain trends in "real world" technologies and commercially available products have emerged, aided by dramatic improvements in dollars per watt or dollars per lumen which occurred during the 2013/2014 time frame.

Chip on board (COB) LED arrays: These pre-integrated multi-LED assemblies have rapidly become the component of choice for downlights under 40 watts where collimation under 40 degrees isn’t needed.

Low/midpower SMD LEDs: With outputs ranging between 1/3—1.0 watt, the very significant recent price reductions achieved by SMD makers are making them the first choice for LED 10-15 W bulbs and for glare free low bay/high bay luminaires up to 300 watts

Wireless control: As prices drop and more and more types of LED lamps and luminaires become "commoditized", manufacturers are feeling greater pressure to find new products which offer greater profitability. One promising way to add value (and profit ) to a product is to offer a wireless control feature (with or without smart phone compatibility). Exactly where, how and to what extent protocols like Wi-Fi, Bluetooth, and various flavors of ZigBee will be embraced by consumers and lighting professionals in the next few years remains to be seen. While many complex or interesting capabilities "can be done", the jury is still out as to whether they will pass muster when it comes to return on the investment required to both implement and to maintain in other than very large installations.

Driverless (aka "AC LED") lamps and luminaires: Technology has become available which eliminates the traditional LED modular power supply as we know it. While not presently exhibiting all the features, versatility or wattage of traditional drivers/power supplies. this alternate methodology can significantly reduce LED "power regulation" cost and size in select applications where the pluses outweigh the minuses.


Linked Articles
eeNews Europe