Philips Lumileds has announced the Luxeon Q family of phosphor-converted white LEDs in a 3.5×3.5-mm package, based on the blue die that the company announced last February. The company said that the epitaxy technology combined with light extraction and conversion enhancements at the die, phosphor, and package level deliver industry-leading efficacy for solid-state lighting (SSL) products.
The Luxeon Q family spans the range of 2700K to 5700K CCTs. The warmer-white LEDs have a CRI of 80 whereas the members with CCT of 4000K and above have a CRI of 70. Lumileds will offer the LEDs in 3- and 5-step MacAdam ellipse bins. Still, the primary focus of the new LED is more lumens per dollar through high efficacy.
The efficacy for a 3000K LED at 350 mA, at a hot 85°C, is a good but not extremely-high 115 lm/W. But a key goal of the Luxeon Q design was maintaining efficacy at higher drive currents. "Our LED is especially competitive when it's driven harder," said Kathleen Hartnett, product line director. "For instance, at 1 Ampere, a flux of more than 300 lm at an efficacy of 102 lm/W at 85°C is achieved in neutral white at 4000K."
Epi and conversion
Hartnett explained that the performance comes from a combination of an efficient die and light-conversion efficiency. When Lumileds announced the blue die, the company touted an industry-leading wall-plug efficiency of 56–61%. That die that the company has been selling as a bare-die product in a chip-scale package (CSP) is the basis for Luxeon Q.
While the company has been selling the die, Hartnett said it took longer to deliver a packaged white LED because Lumileds had to complete the phosphor targeting and optimize the package. Now the LED is available in an industry-standard form factor and beam distribution. Hartnett said the package with a three-pad base could be substituted directly in SSL designs that use the 3535 package.
Back to the conversion efficiency, Lumileds' prior-generation of die was based on thin-film flip-chip technology in which the sapphire layer was removed in the back-end of the manufacturing process. The new flip-chip die leaves the sapphire in place. One advantage of that decision was a robust enough device that the company could sell as CSP-based die.
Leaving the sapphire intact, however, is also part of the conversion efficiency story. Increasingly, patterning in the sapphire substrate is used to maximize light extraction, and Lumileds used some form of that technique with the flip-chip die.
When pressed on the efficacy angle, Harnett compared the Luxeon Q with other widely-used LEDs in a similar package. Specifically, she pointed us to the latest data sheet for Cree XT-E LEDs of similar size. Focusing on 4000K LEDs from both the Lumileds and Cree product lines, the Luxeon Q delivers 123 lm and the XT-E delivers 122 lm at 350 mA.
As Hartnett suggested, the advantage swings more significantly toward Lumileds at 1A of drive current. The Luxeon Q delivers 293 lm whereas the XT-E delivers around 281 lm. That equates to about a 10% efficacy advantage for the Luxeon Q at the higher drive current.