Core Concept: Under-driving

February 24, 2015 3:42 am, Published by     Leave your thoughts

Simply defined, under-driving an LED is powering the LED at a current below its nominal rated forward current. The reason Green Ray chooses to under-drive our LEDs is because we are trying to push efficiency, not just light output, as well as ensure longer lifetimes for our products, while ensuring the best value possible.

Efficiency, defined as lumens produced per Watt, does not increase linearly with current, but rather increases progressively more slowly as current increases. This means that, for a given LED, it may take three times the current, and therefore power, to achieve two times the light output.

Each LED has a current at which it has the highest efficacy. Generally, the lower the current, the higher the efficacy. Therefore, achieving the highest efficacies means under-driving LEDs below the nominal rated forward current.

Efficiency, however, is not enough—we need to achieve the required light output for the application as well. If the LEDs are under-driven to the point that they are very efficient, but barely producing any light, then we will need a much greater number of LEDs to achieve the necessary total lumen output. This may be impossible due to spatial constraints in the luminaire, or may simply not be optimal from a cost-standpoint, even over the entire lifetime of the lamp. In this way, the “sweet spot” may not actually be simply the most efficient current.

Under-driving also helps with thermal management. The temperature of LEDs increase with increasing current, so under-driving will keep the temperature down. Temperature is the biggest determinant of an LEDs life, so lowering LED temperatures, coupled with proper thermal management, will increase the lifespan of the LED (as measured by TM-21 projected L70) and reduce color shift. This means better-quality light that lasts longer, and greater savings.

Additionally, lower temperatures and good thermal management helps prevent voltage drop across the LED, a side-effect of increased temperatures. If voltage were to drop, then, depending on the power supply used, current may increase to maintain power, leading to still higher temperatures and a further drop in voltage. This quickly becomes a negative self-reinforcing cycle, and causes premature failure in the LED.

Ultimately, under-driving helps us balance cost with efficiency and ensure proper thermal management in all of our product designs. This in part allows us to achieve the best value and quality possible over the lifetime of the LED.

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This post was written by gradmin

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