When it comes to binning LEDs, some like it hot

When it comes to binning LEDs, some like it hot

The human eye’s sensitivity demands that LEDs be sorted in ways that will easily provide uniformity in real-world conditions

BY MARK McCLEAR
Director of Business Development, SSL
Cree
www.cree.com

The human eye is an extraordinarily sensitive optical instrument, and our brains are physiologically wired to detect exceptions and inconsistencies in our environment and analyze them as possible sources of food, physical threats, etc. Most people can readily detect the difference between the points (0.4200, 0.4400) and (0.4203, 0.4403) on the CIE 1931 chromaticity diagram (Fig 1 ) four decimal places of accuracy! Therefore, even tiny process variations in manufacturing can manifest themselves as a problem when it comes to any product that deals with light and color.

Fig. 1: Superimposed on the CIE 1931 chromaticity diagram’s black-body lBlack-Body Locus (BBL) are typical lighting-class LED bins.

We see this phenomenon in lot-to-lot variation in dyes, fabrics and paints but also with traditional light sources like fluorescent, compact fluorescent and metal halide lamps (Fig. 2 ). The LED industry has had to wrestle with this issue as it has rapidly evolved over the past decade, and the way we have chosen to deal with it is called “binning.”

Fig. 2: This bank of 400-W metal-halide lamps illuminating a building façade in Chicago, IL, shows stark lamp-to-lamp color variations.

Binning

Just as the name implies, binning is a physical sorting of LED lamps of similar “brightness” and “color.” The individual bins are then priced and sold commercially based on desirability and availability. Brighter lamps, closer to the black body locus (BBL) are generally more desirable and availability is synonymous with manufacturing yield. The lighting industry has sold, or “binned,” traditional lamps by brightness (connoted for incandescent lamp by wattage) and color (the correlated color temperature, or CCT, expressed in kelvins) for decades, so the LED binning is really only an extension of an existing paradigm.

One of the most difficult concepts for many LED luminaire designers to master is the analog nature of LEDs. LEDs are not one fixed wattage/light output, and they are also not one fixed color/CCT/chromaticity. These parameters vary as a function of many criteria: drive current, operating temperature, number of operating hours, phosphor technology, and the design of the LED lamp itself. Over the years, LED companies have more or less arbitrarily selected standard currents and temperatures at which to bin their products, and they publish standard graphs to form a mathematical framework from which to calculate the performance of the LED lamp under various operating conditions (Fig 3 ).

Fig. 3: On a typical LED datasheet, users can find the mathematical framework in graphical form for calculating luminous flux under real-world conditions.(A): XLamp XP-G relative flux vs. junction temperature (IF = 350 mA). (B): XLamp XP-G relative flux vs.current (TJ = 25C).

The most common binning current for lighting-class LEDs is 350 mA, but this too varies by model and manufacturer. LED lamps designed for general, non-lighting applications (so-called “high-brightness” LED lamps) are often binned at 20 mA. Historically, nearly all LED lamp types have been binned at 25°C, the nominal ambient temperature in the factory at the time of manufacture.

Turning up the heat

Cree was the first LED manufacturer to depart from the 25°C binning convention with the release in February, 2011, of the XLamp MT-G (Fig. 4 ), binned at 85°C. Subsequently, five additional XLamp platforms spanning thousands of lumens in flux range, single- and multi-chip arrays, standard and high-voltage options, and all ANSI chromaticity ranges have been binned at 85°C. This has resulted in the considerable experience with so-called “hot” binning, and a thorough understanding of the strengths and weakness of this approach, from the standpoints of both LED manufacturing and LED applications.

Fig. 4: The XLamp MT-G shown here was the first LED to be binned hot, at 85°C.

As the name implies, hot binning is binning the LED lamps at a higher temperature than the conventional 25°C. The LED manufacturers who have decided to launch new products binned at an elevated temperature have converged on 85°C as the new conventional binning temperature. Though 85°C, like 25°C before it, is somewhat arbitrary, it has one major advantage it is a lot closer to the typical operating temperature of many solid-state lighting luminaires than 25°C.

Binning at 85°C makes the initial part of the design process slightly easier and more intuitive. For example, if a designer were working on an LED system that needed 1,000 lumens at an 85°C temperature, then he or she could simply select 10 LED lamps with a luminous flux of 100 lumens per LED, binned at 85°C. Thus, hot binning makes it easy to estimate the performance of these LED lamps in this real-world situation. On the other hand, if the LEDs were binned at 25°C, the same 10 LED lamps would need to be binned at 114 lumens each and de-rated per the LEDs mathematical framework (Fig. 3) to arrive at the same 1,000 lumen goal at the system level.

So, the good news is binning at 85°C makes the first-pass math more intuitive. The bad news is you still have to do the same math if your system runs or ever runs at any temperature other than 85°C. Examples of this would be outdoor luminaires (60° to 65°C is much more common) or freezer cases (20° to 25°C is typical) or downlights in insulated ceilings or almost any retrofit bulb (often over 100°C). In each of these cases the value of binning at 85°C is lost and the designer is back to doing the same math from a new mathematical framework where, arbitrarily, 85°C is now set to equal 100%.

Don’t get burnt

As we pointed out earlier, LED lamp performance can vary considerably as a function of drive current and temperature. Since an LED supplier can never know exactly what application an LED will be applied to in the field, minimizing this variation across all possible drive currents and operating temperatures is very important. Minimizing this variation also poses extreme challenges to LED chip design, phosphor technology, process control, and package/lamp construction.

There are hundreds of ways to cut corners on these LED lamp design parameters this is one of the main differences between lighting-class and “high-brightness” LEDs and the results are often not readily apparent from a typical LED datasheet. Hot binning can mask some of these issues even further, so hot binning can be misused to mask enormous color variation of low-quality LED lamps over the full range of operating temperatures and drive currents.

For example, Fig. 5 shows LED lamps binned at 85°C from two different manufacturers. In each case, the LED lamps were driven starting at the binning drive current (350 mA) and then stepped up to the datasheet maximum (1,500 mA). The lamps were mounted on a thermal chuck to control the temperature from 35°C and allowed to run up to 105°C at each current step, and the chromaticity points (x, y in Fig. 1) were recorded.

Fig. 5: This diagram, of LED color shift over maximum current and temperature, shows how hot binning can be used to mask massive color shift for some LEDs.

In Fig. 5, LED A has a very large color variation over 400K CCT as a function of drive current and temperature. We also note that this variation is more or less horizontal across the CCT range. Over the allowable operating range of the device, the LED lamp actually changes color from the starting point in the 2,700K ANSI quadrangle and crosses over well into the 3,000K ANSI quadrangle. This would not be considered acceptable color stability performance for most lighting applications.

Binning this lamp “hot” is convenient for the manufacturer of LED A, who is betting that the lamp will be used in or around 85°C in the field, and also near the nominal binning current of 350 mA. If this is the case, the customer will more or less get the flux and chromaticity that was ordered. Problems arise when the actual application (for example, outdoor, freezer, downlights, and bulbs, as noted previously) calls for almost any temperature or drive current other than the hot-binning parameters selected arbitrarily by the LED manufacturer.

LED B, on the other hand, shows only 31K variation across this same range of drive currents and temperatures, and remains in the ANSI 3,000K quadrangle under all datasheet operating conditions. This lighting-class LED lamp system was engineered to manage the variation more in the vertical direction — along constant CCT lines — regardless of end use application.

In summary, binning is how LED manufacturers reconcile manufacturing process variations and the exacting sensitivities of the human vision system. Binning at 85°C is quickly becoming the convention in the Lighting-class LED market segment because binning at elevated temperatures can make the initial design efforts a bit more intuitive. There is no freedom from LED binning at this point in LED technology development, or from the math that must be done in designing with these analog components in most real-world applications. Hot LED binning is not always a magic fix-all designed to help LED system designers — it can also be a marketing fix-all designed to help mask an underlying design weakness of an LED component. ■