High-power in a 1 mm square chip: the origins of the 350mA "one watt" LED
The term "one watt LED" is widely used in the industry, but the designation was not conceived through deliberate engineering decisions, as Steve Landau of Philips Lumileds explains.
For years, high-power LEDs were defined by two specs that seemed to be engraved in stone: that each chip was 1mm square and could be driven at 350mA. This "one watt" LED launched the use of solid-state illumination in general lighting applications and became the de facto standard in the industry.
Ironically, however, these specs were the product of serendipitous design decisions, rather than deliberate engineering choices that were calculated to generate a specific level of performance. The design team simply put a stake in the ground and the rest of the industry followed.
The establishment of the 350mA and the 1mm square chip specs are related and date from the mid to late 1990s. Both originated with the 1998 introduction of the first high-power InGaN LEDs, with the brand name LUXEON®, building on work that began four years earlier.
In 1994, Hewlett Packard’s Optoelectronics Division (OED) adopted the vision that LEDs would have a place in the lighting market and embarked on work that would ultimately lead to the products available today. Key to future success was the understanding that the LED die (chips) would have to get larger and be driven at higher current levels in order to deliver the light output that would make their use in lighting applications feasible.
In addition, HP engineers recognized that they would need to offer reliability based on the lighting industry’s standards, not those of the semiconductor industry. Semiconductor lifetime measurement was based on a 50% lumen depreciation level, while the lighting industry used a stricter 30% standard.
The first high-power LED products that resulted from OED’s efforts were AlInGaP high-power LEDs that were driven at about 280mA, a drive current that allowed Lumileds to meet the higher reliability and lifetime requirements of the lighting industry. The chip used for these first products is the now familiar tip-chip (inverted pyramid) that is still in use today for AlInGaP LUXEON® LEDs.
In late 1996 and early 1997, the company’s development of InGaN chips accelerated. The materials science dictated a different chip design. The team assigned to the project ultimately decided that a 1 x 1 mm square chip would be a good “even” size to work with. There was no scientific reason that the chip couldn’t be larger or smaller; the size was a simple matter of convenience.
As work proceeded toward release in 1998, a drive current had to be determined. The AlInGaP products were already being driven at 280mA, and there was serious discussion about using that drive current. But the team’s analysis of the materials science and LUXEON® package for the chip allowed for higher drive currents. Drive current limits would be related to heat management and the ability to respect the lighting industry’s lumen maintenance parameters.
Ultimately, the team selected a 350mA drive current because they felt it was a conservative number that would allow the new InGaN LEDs to comfortably deliver the necessary light at a cost that could be accepted in the traffic signal and signage markets – these were the target customers at the time.
While the 350mA LED is generally considered synonymous with a one watt package, wattage played no part in the decision. In fact, an LED driven at 350mA generates approximately 1.23 W (depending of course on the forward voltage). If the goal had been to deliver 1 W, a lower current would have been used. In the end, the "one watt" designation was simply a handy way to market the product.
With the release of the 1mm square chip and 350mA InGaN LED, the Lumileds team decided to raise the specifications of the earlier AlInGaP products to the same 350mA drive current for consistency across the product line. The switch was also based on reliability data. Thus was the 350mA, 1mm square standard born.
It is important to understand the genesis of this standard in light of the assertion by some LED manufacturers that 350mA is the "sweet spot" for the lighting industry. There is certainly a place for 350mA LEDs, but the history of high-power LED development shows that 350mA is not a magic number. The "standard" could just as easily been anywhere between 280mA and 450mA.
Moreover, sticking with the 350mA standard will limit the potential applications for solid-state lighting. Being able to drive an LED harder and thereby increase light output can be critical for certain types of lighting products.
For these and other reasons, Lumileds is expanding market development and lighting opportunities by increasing drive currents to 700mA and even 1000 mA while still observing the lighting industry’s long-established requirements for reliability.
Lumileds will continue to support the 350mA, 700mA and new Lumileds-exclusive 1000mA LEDs with its LUXEON® I, III and K2 products. There will be uses for all three, at least in the short term.
With ongoing development, however, the 350mA "standard" may someday be a distant memory, like Intel’s first-generation Pentium chip. If that happens, it may go down in history as the benchmark that launched a lighting revolution.
