When evaluating the working stresses of a semiconductor device, such as an LED, the output current from the power supply or the current delivered to the circuit board, LED string and LED package are of little consequence without associating these values to the device ratings. With respect to power, the electrical metric that is most closely associated with LED performance and lifetime is current density. Performance metrics such as efficacy, lumen maintenance and color point stability depend largely upon the designed current level (or current density) of the LED, which may be related to the physical size of the LED die or electrical arrangement of multiple die within an LED package.

Current density can be thought of in terms of the amount of current observed over a specified surface. For example, the LED that is used in Hubbell Industrial’s HBL luminaire has an effective surface area of 2mm². While the current applied directly to the LED package is measured as 700mA, the current density is actually 350mA/mm² (700mA ÷ 2mm²). This would be equivalent to operating a 1mm LED at 350mA. Where the HBL utilizes 72 high-power large-area ceramic LEDs to produce over 16,000 lumens, competitive systems utilizing smaller-die LEDs may require a greater quantity of LEDs and optics to provide a comparable lumen package.

With respect to lumen maintenance, the two areas that most commonly provide insight into the effectiveness of a luminaire’s electrical and thermal design are the operating temperature of the LEDs and system efficacy. In the case of the above referenced HBL luminaire, the system efficacy is measured at 95 lm/W, with an LED drive current of 700mA. In addition to the power supply and optical efficiency, the high LED efficacy can also be attributed to the LEDs being operated at a full 700mA below the recommended maximum drive current of 1400mA.

Additionally, in an elevated 45°C ambient condition the operating temperature of the LEDs within the HBL is less than 75°C, which is 30°C lower than the recommended maximum case temperature (105°C) specified by the LED manufacturer. Under these conditions, the reported lumen maintenance, as defined by IES TM-21-11 is 94.62% at 60,000 hours. The calculated L70 value (70% lumen maintenance) as defined by the same standard is 501,000 hours.

In summary, as LEDs continue to be made available in more diverse packages it is important to not only consider the drive current applied to the LED, but also the current density. While many low-power LEDs may operate at a low current on account of their die size, larger LEDs may be driven at several amps for similar reasons. Therefore, obtaining greater insight into the operational limitations of the LEDs in question, along with the measured system performance, will lead to a greater understanding and clarity when evaluating LED luminaires.