Research Paper on DIODE

INDUSTRIAL ENGINNERING ASSIGNMENT

TOPIC OF RESEARCH PAPER:

THERMAL MANAGEMENT OF LED’S (LIGHT EMMITTING DIODE)

BY ANITA LAFOND

Constructive Communication, Inc.

INTRODUCTION

In the continuing quest for energy-efficient lighting products, light-emitting diodes (LEDs) offer many advantages such as low-energy consumption, long service life, and compact size. These beneficial characteristics make LEDs very popular with fixture designers who want to expand into the lucrative and burgeoning energy-efficient lighting market. And with new federal energy standards coming into effect this year, LED light bulbs are becoming more prevalent. The United States Department of Energy (DoE) estimates that switching to LEDs lighting over the next 20 years could save $120 million in energy costs, reduce electricity consumption for lighting by one-fourth, and avoid 246 million metric tons of carbon. LEDs, therefore, have the promising capability to significantly reduce lighting energy use and impact climate change solutions in the U.S.

SWITCHING TO LED’S

In January 2012, the first phase of the Energy Independence and Security Act of 2007 (EISA) went into effect. The “Energy Bill” is an energy policy intended to make better use of our nation’s resources and help the U.S. become more energy independent. Part of the law sets energy efficiency standards for light bulbs.

Under the new law, screw-based light bulbs must use fewer watts for a similar lumen output.

Common household light bulbs that traditionally used between 40 and 100 watts must be redesigned to use at least 27% less energy by 2014. The second part of the law, which will go into effect in 2020, requires most light bulbs to be 60 to 70% more efficient than current standard incandescent bulbs. Many LEDs can meet this requirement today, and the industry is poised to provide viable, cost-efficient alternatives to standard lighting products. As a substitute for incandescent lighting, compact fluorescent lighting (CFL) has also become more popular. Compared to a typical incandescent bulb (which lasts approximately 1,000 hours), a CFL has a life expectancy of nearly 8,000 hours. These statistics, though, are “dim” in comparison to an LED bulb with its life span of 30,000 to 50,000 hours. CFLs, however, contain mercury, a toxic substance that makes disposal a potentially hazardous situation. LEDs contain no toxic materials, while offering high-quality illumination that is superior to incandescent and CFLs.

MANAGING EXCESS HEAT

Manufacturers are eager to meet the demand for energy-efficient LED designs and are looking for methods to make LED packages more cost efficient. Thermal management is one of the most important aspects of successful LED systems design. LEDs convert only 20 to 30% of their electric power into visible light. The remainder of the light is converted to heat that must be conducted away from the LED die. Excess heat is an unwanted by product of LED design because it reduces light output and shortens the life span of the LED bulb. Therefore, managing thermal output to dissipate heat is crucial to maximizing an LED’s performance potential. This is especially important for high-power/high-brightness LED applications such as streetlamps, traffic lights, and automotive head lights where long operating life is essential.

LESS HEAT, MORE LIGHT

According to Mitsuru Kondo, global LED project manager for Heraeus Precious Metals’ Thick Film Division, thermal management really is the key to good LED design. “The LED light is relatively cool; it is the LED die itself that generates excessive heat and it is this heat that must be dissipated into the substrate,” Kondo said. “The more heat you can dissipate into the substrate, the longer life will be generated from the LED light.” Long life expectancy is one of the LED advantages that is of particular interest to municipalities looking to save money and maintenance time on streetlight purchases. For example, in a typical streetlight outfitted with incandescent or fluorescent bulbs, the bulbs must be changed every six months to one year. Contrast this with an LED streetlamp fixture where the bulbs will last from three to four years. Less time spent maintaining streetlamps translates into immediate cost savings.

Although LEDS are currently more expensive to purchase, the long life expectancy makes them a worthwhile investment. An LED that costs $30 can last for up to three years, while a $5 incandescent bulb has to be changed every five to six months. The total cost of LED ownership is cheaper and electrical costs are less expensive, along with the time savings in not having to change bulbs as frequently. LED manufacturers are working to get their costs down, so that the price of LEDs can be lowered also.

SUCESSFUL DESIGN

Four areas of thermal management are important to LED designers: Increased LED life, higher output of lumens, reduced heat sink size, and less LED chips. Effective heat dissipation helps to keep the temperature of LEDs lower which in turn improves brightness and life span. The ability to use less expensive substrates, such as aluminium, can also help to lower the overall cost of LEDs.

“If you can reduce the temperature by just 10°C, you can double the life of an LED,” explained Kondo. “Lumen power can be increased and designers can reduce the number of LEDs needed without restricting performance. If you can reduce the amount of LEDs needed, while still producing the same output, you save money on the overall design cost.”

MCPCB’s require large heat spreading layers to dissipate heat. This increases the overall size of the substrate, and consequently the size of the heat sink, which is expensive. With the Celcion system, the circuit is constructed directly on the heat sink, reducing the thermal interfaces between the LED and the heat sink. This enables the designer to eliminate the large heat spreaders, and thereby reduce the size of the overall substrate. One of the materials in the Celcion material set is a silver paste that is used in place of a heat spreader, and allows printing directly on the aluminium substrate or the heat sink. Heat sink size can be reduced while lowering the temperature of the LED chip.

According to the EISA, LED bulbs have the potential to last up to 22 years and can save up to 75% or more in energy costs. However, the initial cost is currently more expensive than other alternatives. As manufacturing costs decrease through better thermal management techniques, LEDs are expected to increase in performance and become more affordable. Effective thermal management is the key to making a brighter future for LEDs.