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Advances in 1-W White LEDs Yield Higher Efficiency



A new generation of 1 W type white LEDs delivers improved features that translate into higher efficiency for a variety of lighting applications. They combine high luminous flux, reduced forward voltages, and lower thermal resistance. All of these characteristics can impact LED life and LED brightness, and the overall result is higher performance than previous generation product.

These new devices also feature improved temperature stability (which helps ease thermal management) and smaller package sizes, including lower height profiles, for greater design flexibility.

LED manufacturers attribute improvements in component technology to enhancements in such areas as chip manufacturing, phosphor combinations, and packaging. This article will examine all of these advances and present some representative examples, along with links to product pages on the Hotenda website.

Characteristics of 1-W emitters

Let’s start by defining 1 W emitters for general lighting applications. These are typically characterized by the industry as 350 mA/mm2 area. However, it is important to note that in some cases, 1 W (1 mm2) devices can be driven up to 2.5 W.

One example is OSRAM Opto Semiconductors' new-generation OSLON SSL LEDs, which are said to offer an efficiency increase of about 25 percent, together with higher temperature stability. This was partly achieved through the use of a new phosphor system to improve conversion efficiency of the light output and color stability over temperature.

With improvements in the chip and phosphor system, these white LEDs achieve 97 percent efficacy even at 100°C, said Brian Terao, director of SSL for OSRAM Opto Semiconductor. "Typically, you could see between a 10 to 15 percent light output drop as the temperature moves up from room temperature to 100°C," he added.

Terao said this is an important characteristic to consider especially when typical applications run in the 65°C to 95°C range.

So, in theory, if the product is more stable at temperature, it can runner hotter while maintaining its light output, which, in turn, translates into less thermal management required for the application.

In addition to quantum efficiency, which indicates how well the chip converts electrical power to blue light, another factor that needs to be addressed is extraction efficiency, how much of that blue light "escapes" from the chip, said Terao. To address this issue, OSRAM chose a top emitter design that allows for more light out of the chip.

Another feature of the OSLON SSL is its white reflective surface (Figure 1) on top of the ceramic substrate. This improves reflectivity, which can enhance light by another 3 percent, said Terao.

Figure 1: OSRAM's OSLON Square (above) and OSLON SSL next-generation white LEDs both employ white reflective surfaces to enhance light output (Courtesy of OSRAM Opto Semiconductors).

"It's all of these things together — the chip, the phosphor system, and package — that continually raise the benchmark in terms of lumens and efficacy,” Terao added.

The OSLON SSL has a thermal resistance (Rth) of 7 K/W. The lower thermal resistance means less heat at the LED chip. It offers a luminous flux of typically 98 lumens (lm) in warm white (3,000 K), with an operating current of 350 mA at an application temperature of 85°C in the chip.

The OSLON SSL LEDs are available in warm white (3,000 K color temperature, color rendering index (CRI) 80 min), targeting interior lighting applications, and cold white (5,000 K, CRI 70 typ), aimed at exterior lighting. All devices are available in a 3 x 3 mm package.

With a typical luminous efficacy — the ratio between luminous flux and electrical power — of 96 lm/W at 85°C (at 3,000 K color temperature) OSRAM claims the OSLON SSL LED is one of the most efficient 1 mm2 chip LEDs available. It also touts a reduced forward voltage of 3.1 V. Together, this translates into an efficiency increase of about 25 percent when compared to the company’s previous generation, and for designers, it means fewer LEDs are needed to meet the same luminous flux.

Designers also could consider 2 W type devices like the OSLON Square high-power white LED. At an operating current of 700 mA, which is a little over 2 W, it can be driven up to 1.5 A, and like the OSLON SSL, it features the same white reflective surface, and is housed in a 3 x 3 mm package.

By using the same package size, designers now have the option to choose a 1 or 2 W device in the same footprint.

The OSLON Square can be used in a number of ways. For example, the LED offers twice as much light when it's driven at 700 mA in the same package compared to the OSLON SSL, but it can also be driven at 350 mA to boost efficiency.

At an operating current of 700 mA, the OSLON Square offers an efficacy of above 90 lm/W and at 350 mA, efficacy is above 100 lm/W.

"Although the OSLON Square is characterized at 700 mA (max current 1.5 A), some choose to run at lower currents to maximize efficiency," said Terao. "This, of course, would increase the $/lm, but customers would balance the difference of maximized efficacy with cost per lumen."

The luminous efficacy of the OSLON Square (700 mA) is 96 lm/W at 85°C (3,000 K color temp) and 115 lm/W (5,000 K color temp). It features a low thermal resistance of 3.8 K/W.

Aimed at outdoor applications, the OSLON Square PC and UW LEDs can be operated with currents from 200 mA to 1.5 A, opening up other applications, including downlights and spotlights. According to OSRAM, with a CRI of at least 70 (4,000 K) and 65 (6,000 K), these LEDs combine good quality of light with high efficiency.

Although the package size is somewhat larger, Vishay Intertechnology's 1 W white LED series in the Little Star family of power LEDs also can be used in a variety of applications. The white LEDs offer a low thermal resistance of 10 K/W and high luminous intensity (brightness).

According to Vishay, these cool, neutral, and warm white 1 W LEDs use nitride phosphor for enhanced color stability over temperature. Typically, Vishay noted, phosphors, including YAG, TAG, silicate, or nitride-based phosphors, along with special mixtures, are used for white LEDs. The company selected the nitride phosphor due to its extremely good color stability versus ambient temperature (Figure 2).

Figure 2: By using a nitride-based phosphor, Vishay's VLMW712 series delivers better lumen maintenance compared to existing silicate and YAG-based solutions at higher temperature (Courtesy of Vishay Intertechnology).

Housed in a 6 x 6 x 1.5 mm package, the VLMW712 series of LEDs feature a drive current of up to 350 mA, bright luminous intensity of 33,500 millicandela (mcd) typical, and a typical luminous flux from 67.2 to 113.6 lm. This makes them suitable for many lighting applications ranging from traffic signal lights and indoor/outdoor video and advertising lighting, to architectural lighting and general lighting.

The white LEDs offer a luminous flux of 87 to 113 lumens (lm) for the cool white (33,500 mcd), 76 to 113 lm for neutral white (29,400 mcd) and 67 to 87 lm for the warm white (25,000 mcd). Maximum forward voltage is 4 V.

Summary

When it comes to LEDs, there are a number of characteristics that designers have to consider. These include luminous flux, efficacy, CRI, color temperature, operating current, forward voltage, and thermal resistance. All of these specifications impact LED brightness and LED life.

To select the best solution for your lighting applications, LED manufacturers recommend that designers define their requirements (particularly lumens and CRI) for their specific lighting application, while keeping in mind other key considerations, such as electrical constraints or particular drivers that they want to use.

Finally, also keep in mind that higher efficiency can ease design by reducing the number of LEDs needed to meet the same luminous flux. While there will always be trade-offs and cost considerations to find the right balance for your lighting application, there are several choices of white LED families that offer a wide range of specifications that can deliver high quality of light with high efficiency.
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