Better Chips for Better LED Bulbs
Serial connections simplify LED power supplies
Softer White: Epistar's use of red LEDs to its chips for white LED light bulbs gives them a softer glow.
19 March 2012—The LED bulb is far from perfect. Converting high-voltage AC output from the mains into very-low-voltage DC to power the chips requires a bulky transformer, which takes up valuable space. And the bulb’s efficiency diminishes when a designer adjusts its color from a blue-tinted cold white to a more-pleasant warmer hue.
Although some of the leading LED chipmakers have worked hard on these issues, none of them have simultaneously tackled both problems head-on in a commercial product—until now.
That claim to fame goes to Epistar of Taiwan, one of the world’s biggest LED manufacturers. This firm has recently released a pair of warm-white high-voltage chip sets with efficiencies of 120 and 150 lumens per watt—suitable for making bulbs equivalent to 60- and 75-watt incandescents. The new chips combine warm color with high-voltage operation—eliminating the need for the transformer—and they do it without sacrificing efficiency. “I see the new Epistar product groups as a significant step in the right direction to exploiting the full potential of solid-state lighting,” says Fred Schubert, an LED expert at Rensselaer Polytechnic Institute, in Troy, N.Y.
To produce these high-voltage, high-efficiency, warm-white sources, Epistar connects several LEDs in series on a single chip, a modification that boosts the driving voltage to tens of volts. Then it bolts a handful of these chips together to create a product with a DC drive voltage equal in value to that of the mains outlet. Bulb manufacturers still need to convert the AC input to DC form, but there is now no need to step down the voltage, making the power supply smaller, cheaper to manufacture, and easier to build. According to Shao-You Deng, special assistant to Epistar’s general manager, most customers are using tiny silicon-based, bridge-type rectifiers for high-voltage AC-to-DC conversion. “You just need ripple-removing and current-sinking circuits to make the current more stable,” he says.
Epistar also uses an innovative light-generating process. Traditionally, a blue-emitting chip is coated with a yellow phosphor, and color mixing creates a white emission. Warmer hues result from stretching the phosphor to longer wavelengths, but this reduces the energy of the emitted photons and ultimately diminishes the efficiency of the LED bulb.
To get around this, Epistar, as do a handful of other manufacturers of warm-white chips, combines emission from red LEDs, blue LEDs, and a phosphor. In Epistar’s case, it puts red and blue chips right next to each other before coating them all with a phosphor that acts as a diffuser and ensures a uniform white emission in every direction.
In-house tests indicate that the addition of the red LED boosts overall efficiency by more than 40 percent. Gains in efficiency are welcome, because they allow a reduction in the number of chips required to deliver a given output, which in turn can help to bring down the cost of LED bulbs. Chips currently account for half of the cost of making LED bulbs, which are on sale today for tens of dollars, a high price that hampers sales.
Epistar is now focusing on increasing production of 120 lm/W chip sets to 1 million per month. At this stage it’s difficult to predict how big an impact this will have on the LED bulb market. Jamie Fox, a market analyst from U.K.-based IMS Research, believes that the new Epistar chip sets could appeal to some LED bulb makers. He points out, however, that other LED manufacturers are working on at least one of the issues that Epistar has addressed.
If these rivals succeed, they could blunt Epistar’s competitive edge. But if the Taiwanese chipmaker ramps up sales quickly, it will stand a good chance of winning a loyal customer base and ultimately playing a big role in the LED-lighting revolution.
About the Author
Richard Stevenson is a freelance journalist in Wales covering compound semiconductors. In the November 2011 issue, he updated us on the debate surrounding what causes efficiency-sapping LED droop.