Engineers and designers in the LED lighting industry, often refer to two types of concepts for electrical efficiency LED lighting. It is important to recognize and understand the difference between both types. Energy efficiency pertains to the ratio of light to heat produced by the LED lamp. Cost efficiency pertains to the ratio of light to cost, as determined by pricing set by the light emitting diode manufacture, distributor, or reseller. Both types of LED efficiency can become very important during custom LED lighting design.
Corresponding component datasheets contain the necessary information to calculate for LED efficiency. Start by locating the typical luminous output stated in lumens (lm). Some datasheets may state the candela rating (mcd), in which case a conversion is required. Continue by noting the test condition stated in milli-amperes (mA), for the corresponding typical luminous output. For example, a datasheet may state the typical luminous output is 45 lm at 350 mA. In this case, the test condition is equal to 350 mA. Move on to determine the maximum forward voltage drop at test condition. This information is normally located in tabular format within the first several datasheet pages. Note that the test condition stated for voltage drop must correspond with the test condition stated for typical luminous output. Moving on, multiplying the maximum forward voltage drop by the test condition. The result is power dissipation expressed in watts. Finally, multiply the typical luminous output by the reciprocal of calculated power dissipation, to determine LED efficiency expressed as lumens per watt. Higher numbers correspond with LEDs that are more efficient.
The graph to the left depicts two different LEDs, and displays the cost and energy efficiency of each. The lateral bars correspond with cost efficiency, given in lumens per watt. Longitudinal bars correspond with LED energy efficiency, given in lumens per dollar. The data point located near the upper right corner of the graph represents a modern high efficiency LED lamp. As you can see, this LED offers a very high efficiency, in terms of both cost and energy. Rated at 15 lumens per dollar and 45 lumens per watt, a designed would find this optoelectronic device very attractive. The data point located near the lower center area of this graph represents a traditional 5mm LED lamp. At only 8 lumens per dollar and 20 lumens per watt, this LED is obviously less efficiency overall. It is also important to understand how drive current can affect efficiency. Typically, an increase in forward drive current will result in an increase of cost efficiency, but decreased energy efficiency. At the higher drive current, the LED produces a higher luminous output. Therefore, cost efficiency increases. However, energy efficiency will decrease due to significantly increased thermal dissipation.
Examining the relationship between luminous output and forward current, it is apparent that as current increases, efficiency decreases. From this, we may conclude that LEDs tend to be more efficient while operating at reduced current ratings. Increasing current can generate more wasted energy in the form of heat dissipation. An array of LEDs operating at less than half power may produce as much light as a single LED operating at full power. Although both examples produce an equivalent amount of light, the LED array is more efficient than the single LED because it operates within the range of maximum efficiency. However, the disadvantages include increased physical dimensions as well as overall cost. The following example corresponds with graph 1.0 below. At 350 mA, the normalized luminous flux rating is 1.0. Since the LED is less efficient while operating at higher current values, the normalized luminous flux is only 1.7 at 700 mA. This example demonstrates that doubling drive current does not double luminous output. Achieving a clear understanding of this concept is critical when considering drive currents for new design applications. A common mistake is driving the LED at its absolute ratings in order to achieve the maximum luminous output. Although the luminous output is increased, efficiently suffers and more energy is wasted as heat. In addition, thermal complications may sacrifice lifespan or lead to catastrophic failure.