As the lighting industry continues to mature, vendors continue to pursue inexpensive lighting solutions that offer universal compatibility and are easy to install. For OEMs and manufacturers, LEDs have proven to be a game-changer for lighting, offering better quality illumination with greater control and lower power requirements. The LEDs themselves, however, are only one part of the luminaire. The other pieces of the lighting fixture — the connectors, housing, optics, printed circuit board (PCB), and the power supply are all contributing factors to lighting energy consumption and performance. That's why more vendors are looking to use direct AC Engines to reduce manufacturing costs and improve performance.
Perhaps the biggest cost in LED luminaires, and the component that has the most control over power consumption, is the power supply. As with any semiconductor, the cost of LEDs continues to drop year after year, so to reduce costs and differentiate functionality, more vendors are looking at ways to improve the power supply to reduce costs and improve lighting controls. For luminaires such as linear fixtures, low-end down-lights, and bulkhead lights, cost is often the most important consideration. To eliminate the need for bulky, expensive LED drivers, vendors are adopting direct AC LED Engines that don't require a conventional AC-DC switch-mode power supply (SMPS).
Direct AC LED Engines make it possible to integrate self-contained LED chips and driver directly on the circuit board, converting 120V AC current to usable DC voltage right on the board. Using onboard direct AC LED Engines reduces manufacturing costs, making LED luminaires competitively priced with fluorescent lighting. Direct AC LED luminaires also offer greater diversity, great light quality, and they are dimmable.
The Need for Direct AC Voltage Conversion
To take advantage of the economies offered by LED lighting, global luminaire manufacturers need a universal approach to match voltage. Most high-brightness LEDs require fairly low DC voltage, typically about 3V of forward voltage, but the AC power is delivered at 120V or 240V, depending on geography. To step the AC voltage down to the DC forward voltage required for an LED string, which is typically below 60V, most manufacturers have been using an SMPS.
The SMPS has the advantage of delivering about 90 percent efficiency, since the switched mode architecture saves and discharges energy from inductors and capacitors at very high speed. The downside is that an SMPS requires bulky inductors, or transformers and capacitors, and it has a limited lifespan; the SMPS is usually the first component to fail in a luminaire. What's more, because of its size, the SMPS can't be mounted on the PCB along with the LEDs. The result is a bulky power supply that is expensive, with a shorter effective lifespan, that poses luminaire design challenges.
A direct AC engine provides a simpler approach to powering LEDs directly, without stepping down the voltage. The AC direct power regulators switch on the LEDs in sequence, following the main power input's sinusoidal voltage.
The advantages of direct AC are obvious: you eliminate the need for complex electronics between the power source and the light source, and you can handle the power more efficiently in a distributed environment. You also can get more light with less power.
The Drawbacks of Direct AC
Using an AC direct engine to power LED lighting does have some drawbacks. With direct AC, the AC mains voltage is rectified to convert it to DC, and the rectified output is a 120Hz sine wave that swings between 0V and 325V (with main voltage nominal at 230V/50Hz). Each LED in the circuit is activated in each half-cycle. With a linear sequence with four LEDs, for example, the first LED is switched in at about 70V, the second at 140V, the third at 210V, and so on, with all four LEDs illuminated as long as the total voltage is 20V or more.
The excess voltage generated by AC direct power regulation also has to be dissipated, which creates additional heat that needs to be dissipated as well. The result is an IC hot spot that requires additional cooling to avoid damage to the board from overheating. It also means that the board layout has to have wider gaps to go between the LED hot spot and other components.
Since some power has to be dissipated, it also limits the efficiency to 75-80 percent, as opposed to 90 percent with an SMPS. However, the overall efficiency of direct AC drivers can meet most lighting needs. For example, direct AC LEDs can deliver a total efficiency of 100lm/W as opposed to 130lm/W for an SMPS. The smaller, simpler design more than offsets the difference in efficiency.
Flicker also can be a drawback. LEDs will go dark every half-cycle, when voltage is at or near 0V (e.g. every 1/100th of a second in a 50Hz main power system). Since the flicker intervals are so short they are not perceptible, and even in low-power applications the degree of flicker is considered acceptable. At lower power levels, the light output from the LED is in direct relationship to the LED current, so a light output with a 50 percent duty cycle will have a flicker index of 0.5 while an LED running off DC power will have a flicker index of 0. AC direct engines solve the flicker by using capacitors, achieving a flicker percentage less than 30 percent.
Lower Cost to Operate and Programmable
Despite its shortcomings, the direct AC LED Engine is a cost-effective and efficient approach for luminaires used in wall sconces, ceiling fixtures, and other applications where you want a fixture that is easy to install or retrofit, and that will save power and money.
Direct AC lighting is best suited for lighting applications that don't require dimming or customization. However, direct AC engines are flexible enough to accommodate dimming using TRIAC/ELV drivers, which generally provide a better dimming profile. Using TRIAC/ELV you can dim to 10 percent.
These are just some of the benefits the AC direct LED lighting brings to the market. The steady-state, LED technology itself that opens new possibilities for lighting, and direct AC engines deliver better performance and lower power consumption in a versatile form factor. Since the direct AC driver is on the circuit board, luminaires can be designed to suit almost any space or environment. And LEDs provide more versatility in terms of light intensity and hue, with a longer effective lifespan. Integrating AC direct LED drivers into a single circuit board creates an infinite number of light configuration possibilities.