There’s no question that LED illumination represents the next technological step in home lighting. But how many consumers feel the same way? Opinions are divided as they weigh the tradeoffs between LED price, performance, lifecycles, and environmental impacts (Fig. 1).
The early attempts at home LED lighting failed. Its light output and color characteristics both were below standard. Today’s LEDs have improved, though, and their output and color performance probably are better than conventional light sources.
Despite those advantages, consumers still vacillate over LED prices. They also remain dubious about how long these products last and how much they’ll save in power costs after making that initial purchase. Environmental considerations are further down on their list of priorities.
LEDs will last between 50,000 and 100,000 hours. With 8760 hours in a year, an LED operating 24 hours a day would have a lifespan of 5.7 years. An LED operating just eight hours a day would last 17.1 years. Consumers often wonder how we know that these LEDs will last that long, though.
An LED’s true lifespan is related to lumen depreciation. According to the Illuminating Engineering Society, once an LED hits 30% depreciation, its lifespan is over. In other words, an LED specified with a 100,000-hour lifespan could be used effectively for about 70,000 hours. It would still work after that, but with reduced lumen output.
Lifespan and junction temperature are key elements in LED reliability. Junction temperature is the temperature at the point where an individual diode connects to its base. Maintaining a low junction temperature increases output and slows LED lumen depreciation.
The junction temperature typically should be maintained below 120°C, requiring a number of heat dissipation strategies. The three principal means of heat transfer are conduction, convection, and radiation. Generally, LEDs are encapsulated in a resin that happens to be a lousy thermal conductor
The heat issues that LEDs create stem from the P-N junction, which is formed in semiconductors by the doping process, in effect creating two semiconductors. The boundary between the two is the P-N junction, which forms a one-way street for the current to pass through.
As electrons move from one crystal to another within the structure of the semiconductor, they fill electron holes and emit photons (light). The heat is generated from the P-N junction by electrical energy that has not been converted into light. This heat must be conducted to the atmosphere via a heatsink. The junction temperature drops when the total thermal resistance from the junction point to atmospheric release is minimized.
Drive current and ambient temperature also can influence the junction temperature. The higher the drive current, the greater the heat generated at the junction. Heat must be moved away from the junction to maintain the specified light output, lifespan, and color. The amount of heat that can be removed depends on the ambient temperature and the design of the thermal path from the junction to the surroundings.
Each LED lighting design must efficiently transfer as much heat as possible away from the LED P-N junction. A severely heat-stressed LED will lose efficiency and light output will diminish, possibly resulting in product failure.