It's important to note that AC LED is really a misnomer: LEDs are diodes with current flowing in one direction (as in direct current). However, with a so-called AC-LED scheme, LEDs can be connected directly to the mains power supply (typically 110V/60 Hz or 230V/50 Hz) and create light without the use of a typical driver. For each half-cycle of the AC's sinusoidal waveform, half of the LEDs emit light and half are dark; this is reversed during the next half-cycle. In this configuration, sometimes referred to as "true AC" or an antiparallel approach, a large number of LEDs in series can operate directly from line voltage.
However, this approach has produced LED "strings" with limited efficacy. Therefore, several years ago AC-LED suppliers including Lynk Labs of Elgin, IL, Seoul Semiconductor of Seoul, Korea, and Epistar of Hsinchu, Taiwan, began producing LEDs that operate directly from low to high AC voltages with simple control circuitry. This includes low-voltage direct AC to high-voltage rectified LEDs. The LEDs typically accept between 12V on the low side on up to 240V today. One common design approach uses a series of LEDs to reach the 55V peak for the rising edge or trailing edge for the 110V line voltage, for instance. "This is really an AC implementation using a high-voltage architecture," explained Brian Wilcox, vice president of Seoul Semiconductor, North America, a maker of AC and DC LEDs, assemblies and packages.
FIG. 2A.
In comparison, DC LEDs require a driver to condition the AC power infrastructure to provide a DC low-voltage regulated power source for the LED light engine. This driver includes an AC-to-DC converter, typically a large electrolytic capacitor as well as other electronic components that can number up to 20 on the driver board for a 7W MR16 lamp. Even more components are used in high-power applications. However, Wilcox stated that despite the simpler electronics scheme, the challenge with AC LED is "you have to eliminate obstacles like total harmonic distortion, improve power-factor correction and provide zonal dimming. None of these three steps is trivial, especially when you're trying to do them all at once."
In fact, one could argue it was these limitations, and the lower efficacy relative to DC approaches, that have limited AC-LED adoption in the past. However, recent AC-LED and high-voltage products have largely addressed these concerns. Likewise, new designs have had to address flicker. "Many people say AC LEDs flicker. But flicker is really a function of the spatial separation of AC LEDs, which arises when the LEDs are spaced too far apart and the eye can pick up the rectified component at 50 or 60 Hz." said Mike Miskin, CEO of Lynk Labs, an AC-LED package, assembly and driver maker. Some of the company's latest designs use a high-frequency design scheme, stepping down the voltage using an electronic transformer or other method and creating a high-frequency signal (1000 Hz or higher) so that flicker is not a problem.
The fruits of the labor are the latest AC-LED light engines with better compatibility with the existing infrastructure, increased luminaire reliability due to fewer components and potentially faster time to market.