As the power of high-brightness LEDs increases, they have spawned many new applications, from architectural lighting to medical products. ENERGY STAR lighting standards are gradually focusing on overall system efficiency.
Key points
Overall lighting system efficiency includes power conversion, current regulation, and the efficiency of the HBLED (High Brightness Light Emitting Diode) device itself, not just the efficiency of the HBLED core.
HBLED lighting systems are now less efficient than the most efficient fluorescent lamps. But HBLED will catch up in three years.
Other advantages of HBLEDs are pure light source, tightness and dimming control capabilities that will help them enter new medical and lighting applications.
The power of HBLEDs (high-brightness LEDs) is increasing: for example, Cree now offers 88lm/W devices, and plans to offer 100lm/W devices by the end of this year, with 150lm/W devices in five years. As a stand-alone component, HBLEDs are more efficient than incandescent or even fluorescent lamps. However, at the system level, their advantages are diminished because of the power loss issues they must consider (including AC/DC and DC/DC conversion and current regulation). In addition, LED lighting devices (or light sources) can cause losses, and the LED components themselves also have heat loss.
The US DOE (Energy Department) recently completed a new Energy Star specification for SSL (Solid State Lighting) sources, so that system designers can use the same comparison numbers for light sources and devices. The new specification does not focus on the luminous efficiency of HBLEDs at the component level, but the overall source efficiency.
The current-driven LED intensity is directly proportional to the forward current. These devices have the same steep voltage/current curve as the diode. A small change in voltage causes a relatively large change in current and a change in brightness, so control current is more important than control voltage. Many IC manufacturers (usually engaged in the power controller business) have entered the LED driver current regulation market. These suppliers include: Texas Instruments, National Semiconductor, Intersil, Cypress, Maxim, and Linear Technology.
In addition to the current regulator, the lighting system may also have to have an AC/DC converter, and a battery-powered system may also require a DC/DC boost converter. In short, in terms of conversion alone, it is possible to lose 10% to 15% of system power. In addition, due to reflections and lens loss, the brightness loss of SSL in the device can be as high as half.
The ENERGY STAR SSL specification will take effect on September 30, 2008. The specification has two parts. CategoryA covers today's existing devices. It states that a CategoryA-compliant embedded illuminator (or "downlight spotlight") must be 35 lm/W. Category B will cover efficient SSL devices that appear within three years. At that time, SSL was able to compete with today's most efficient lighting systems using traditional light sources. For example, the most common high-performance T8 fluorescent lamps and electronic ballast systems now produce about 100 lm/W. These high-quality products of fluorescent ballast systems can achieve efficiencies of about 70%, achieving a light source efficiency of 70 lm/W. With today's commercial SSL technology, HBLED sources cannot achieve the minimum luminous efficiency of Category B. However, LED technology is rapidly evolving and will meet the requirements of the Energy Star CategoryB in the future. But LEDs have other advantages besides efficiency and long life, and it is worthwhile to continue to track its development before achieving higher efficiency. For example, in fluorescent lighting, dimming is very difficult, but the LED is simply a direct reduction of current. In addition, you can dynamically change the color of a room with multiple cool white LEDs and a warm white LED array. SSL is expected to become an important technology for home and industrial lighting in the next five years.
Most HBLEDs available today require DC voltage and current, so most SSL systems include a conversion circuit that converts AC power to a regulated DC power supply. However, Seoul Semiconductor has recently introduced the Acriche HBLED, which can be operated directly from an AC power source (Figure 1). The input voltage is set by a surface mount resistor ranging from 100V to 110VAC and 220V to 230VAC. In the core, the LED contains Multilayer LED semiconductor junction. The diode junction is established when the total forward voltage is close to an AC voltage of 110V or 220V. These devices have two strings of back-to-back LEDs. The first string is turned on and conducts during a positive half voltage cycle, while the second turns on during another series of cycles, so the LED can illuminate throughout the AC voltage cycle. Direct AC operation simplifies power conversion circuitry, increases system reliability, and reduces design time. The AcricheHBLED is available in two models: the AW3200 for 100V/110V and the AW3220 for 220V/230V. Both models have an efficiency of 59lm/W, which is lower than that of DC-powered HBLEDs, but it is also comparable.
As energy costs continue to rise, the importance of lighting efficiency is increasing. The US DOE estimates that lighting consumes 20% of a building's electricity. However, in developing countries, there is often a lack of reliable power networks, and people can use solar SSL to power the battery for reliable nighttime illumination. The only other option is the kerosene lamp, which is both dangerous and expensive.