Using High-Power LEDs: Some Considerations

My very first introduction to high-power LEDs, usually surface mount LEDs with a power consumption of a half-watt or more, was quite illuminating (pardon the pun). I was shopping for Christmas presents for the family in one of the larger retailers and stumbled into their flashlight aisle. All the familiar types were there, huge 6-volt lanterns, aluminum billy club like tactical flashlights that take like 12 D-batteries, and then I saw something unusual. It was very small compared to the rest, only about as long a typical pen and about a half-inch round. The packaging indicated it was a super-bright white LED flashlight. Even though I worked for a company that sells technical lamps and LEDs I had never seen an LED flashlight before.

I guess they had been out for awhile at that time but I hadn't run across any yet. I had no idea at the time that super-bright was an actual LED classification and figured it was marketing buzz. The brand name was from a famous outdoorsman who also shares his name with a certain line of SUVs and the sticker price was an astronomically high $25! I was really intrigued to find out just how bright this thing really could be but the packaging was that blisterpack stuff that you need the jaws of life to open. I was about to walk away when I noticed someone had ripped open one of these flashlights (must have been a gorilla) and had left it sitting at the bottom of the shelf. Cool! I picked it up and gave it a quick once over. It was surprisingly hefty since it was made of solid aluminum. The head was twice the size of the body and was solid as well. Three mirror-coated holes were bored into the head and I could see three "clearish/yellowish" LEDs. As I continued to stare at right into the head of the flashlight I pressed the power button. Wham! I immediately felt like someone had just let off a flash grenade in the room and I stumbled back into the shelf behind me and tripped and nearly hit the deck as I tried to steady myself. I dropped the flashlight in the process. It took several minutes for the flashing color patterns in my eyes to stop to the point where I could function again. The man at the end of the aisle was looking at me funny, chuckled to himself and left me alone. Fortunately, the flashlight survived the drop. I picked it up, turned it off, and put it back on the shelf. I walked away from this incident highly impressed with the advances made in the brightness of LEDs. My prior experiences with them proved them to be quite dim, good for use as indicators of "power on" and sticking into pictures of Corvettes parked in front of garages but not for much else. It was clear that much ground had been gained in the industry.

As technology in the field continues to advance, the light output and power consumption of LEDs continues to rise. Efficiency, that magical ratio between the power output of the LED in light versus the power consumption to produce that light, continues to improve. Now LEDs are poised to overtake the compact fluorescent as the defacto standard in energy efficient home lighting. Lets hope the LED industry has learned a thing or two from the miserable product launch of the CFL industry. Customers I encounter seem to be ever more concerned with the power of the LED. "I need a 2 watt LED" or "do you have anything brighter than a 3 watt LED" are common questions. There is the age-old habit of confusing the power output with the power consumption but in general, the higher the power consumption the higher the output. Of course, some LEDs are more efficient then others. 3-watts...3-watts! Holy cow! The LEDs that blinded me so efficiently a few years back were only half-watts. At a half-watt, thermal issues come into play. At lower power consumption, the heat produced by the die does not require additional cooling consideration but beyond that things change rapidly. At these powers, the heat is tremendous. Also of note is that at these power levels, the LEDs are only available as surface mount to facilitate channeling heat straight out the back of the LED package to a heat conductive substrate by way of a thermal slug (a small metal block that is thermally connected to the die inside the package). These LEDs must be mounted to a base or substrate that can conduct heat efficiently enough to prevent the die from overheating. A dielectric layer between the the thermal slug in the LED packaging and the substrate is used just like thermal grease is used between a computer processor and it's heatsink in a modern PC. As the power levels continue to rise, additional methods for transferring the heat away fast enough from the substrate must be employed. This could mean using a different substrate material altogether with better thermally conductive properties or using active cooling of the existing substrate with larger heatsinks with cooling fins or even the use of fans. The ability or inability to transfer heat from the LED will have a definite effect on the life of the LED. Where I am seeing these new 3-watt LEDs getting there most use is in my favorite product: the flashlight. The average consumer seems to be looking for brighter flashlights, thats whey we often see spin on the packaging sort of like, "1,000,000 candlepower" or "brighter than the surface of the sun" or whatever. Even though people want brighter flashlights, especially the military or law enforcement crowd, they don't expect these things to last very long. We've all had traditional halogen flashlights either as a kid when they came with your Snoopy radio or into your adulthood and the one thing that remains, at least in my memory, is that these things last something like three months at best. That imprint on our minds has now made us accept the shortened lifespan of any flashlight we buy, including these new highpower LED versions. Since little additional cooling is employed beyond the aluminum substrate which may or may not be conducted to the aluminum housing of the flashlight, the LEDs are operating beyond their ideal parameters and as a result, they will stop working after about 1000 hours, way short of their factory spec life of 5,000 hours or so. This, as it turns out, doesn't matter much since consumers have come to expect flashlights that die off way before a 1000 hours anyway so would actually feel like they got a "super" flashlight anyway. This is smart business for the flashlight manufacturers. Brightness is their major concern and heat management takes a backseat. They need brighter and brighter LEDs since the average consumer is now infatuated with almost anything that says "LED" on it but in terms of flashlights, their used to getting some pretty bright stuff from the gas-filled bulbs of yore. They can only drive the power and heat output up and the life down by so much, or so I would think. Is there a theoretical ceiling these manufacturers will hit? Time will tell.

Actually, all this talk about heat management from LEDs has reminded me of an application where the lack of heat and not dealing with too much heat was a problem for one OEM. Since all of the heat from an LED is produced out the back of the die, the light output itself is thermally cool (don't confuse this with color temperature). So cool, in fact, that a manufacturer of aviation runway beacons who recently went with all high-powered LEDs instead of the usual halogen bulbs found that once the beacons were covered with snow, the lack of heat in the light output, didn't melt the snow from the surrounding glass globe and the runway lights were no longer visible from the air. Makes you nervous to fly, don't it? All their initial testing had been in warmer climes. The LEDS have it all over bulbs in terms of lifetime. A typical half-watt or 1 watt high-power LED has an expected lifetime of about 10 to 20 thousand hours compared with 1 to 2 thousand for a bulb. Thats a huge savings on maintenance. The brightness of LEDS is now to the point where it meets federal safety guidelines for such lighting although I have heard talk of rewriting these guidelines to deal specifically with LED lighting. While the brightness and longevity of the LEDs made the switch from halogens seem like the way to go, the fact that the heat in the light was insufficient to keep the lens clean in the winter has left them in a real bind. Now they are scrambling to somehow channel the heat from the back of the LED die, to the globe to prevent snow buildup while still effectively cooling the LED. What a pickle to be in. Their solution will be interesting to say the least. I'll follow up with another article as soon as I hear back on it.

So back to power output and consumption. So my flashlight and runway beacon stories illustrate two of the problems faced in high power LED product development and both center on heat management and brightness. How high can these high-power LEDs go? I have no idea. I have heard snippets from engineers overseas about 5-watt LEDs. The heat from these things must be good enough to cook with. Three months from now, we'll probably be talking about LEDs with twice that power consumption. How will manufacturers deal with the thermal issues? What products or markets will these things turn up in? It's a safe bet that as consumers continue to adopt LEDs in their everyday lives and look for ever brighter output, we'll find out soon enough.