Meeting LED Expectations

The past year has seen significant advances in LED packaging and manufacturing technique as well as LED price reduction. Thanks to these achievements, the expectations based on using LED as an alternative light source for general illumination are now much closer to realization. Among such expectations significant energy savings as well as longer life expectancy will drive long-term cost savings for consumers while other features like creative color management and optimized dimming will provide a more comfortable experience. However, the full potential of LED used as light sources for general illumination will only be realized if intelligent LED driver systems are used. These drivers are fully optimized to support those LED characteristics that are dramatically different than those of cold cathode fluorescent lighting (CCFL) and other traditional light sources. If a non-optimized LED driver system is selected, lighting-fixture designers and users are likely to be disappointed by the performance and by the actual product life of LED lighting fixtures. They will experience flickering and cold, un-tunable light, as well as poor energy efficiency and premature fixture failures. They will also not be able to extract full value from the currently much more expensive LED fixture as dimming and other intelligent integration into networks will not be possible. The first reason why optimized power supplies are necessary is due to the fact LEDs behave very differently from traditional light sources, and therefore have unique driving requirements in terms of power supply. LEDs are complex, sophisticated semiconductor devices whose tightly interdependent photometric (luminous flux and efficacy), electrical (current, voltage, power) and thermal (junction temperature) characteristics often behave in a highly nonlinear manner. LED behavior can be particularly hard to predict as temperature changes. For instance, as temperature rises, the LED spectrum shifts to longer wavelength, but the absolute value of this change varies for red, green and blue LEDs. Meanwhile, there is an opposite shift, as LED current rises, to shorter wavelengths. The first step toward optimizing LED performance and life expectancy is to understand that existing, off-the-shelf non-optimized power supplies are not good choices for driving LED fixtures. They are usually designed to provide constant voltage to the load, but LEDs are current-driven devices whose brightness is proportional to their forward current. So, while a constant DC current source can be used to power the light fixture, it will also not ensure the maximum efficiency or brightness control and will for sure not protect the fixture in case of an over temperature event (all features that can be built into an optimized driver). An optimized LED driver system can significantly improve fixture performance and life expectancy by directly affecting light efficacy, life expectancy and light quality. Light efficacy, or lm/W, is directly dependent on the electrical efficiency, or power output versus power input, that the LED driver can deliver. Life expectancy is covered by Energy Star standards and defined as time in operation before the light output decreases typically to 70 percent of the initial value, which varies by application and operating conditions. Most LED manufacturers promise at least 50,000 hours of operation before failure (the value keeps improving), but many LEDs that use sub-optimized power supplies are failing. The two main failure causes are either the use of electrolytic capacitors in the power supply or the lack of control over the maximum temperature the system can see. It is important for LED driver suppliers to include estimates of mean time between failure (MTBF) and specific system operating conditions in their life-expectancy claims. The LED driver system must also be designed to provide the best possible power factor (PF), or the ratio of real power in Watts to complex power in Volt-Amperes (VA) PF is particularly important for maximizing energy efficiency. When PF is low, the electric utility must supply more current for a given amount of real power use. Governments generally set minimum national PF specifications that are reflected in international standards for energy efficiency. PF is the product of two components - displacement power factor (DPF), and distortion factor, which was referenced earlier in this article and is also known as THD. The first component, DPF, is defined as the cosine of the phase shift angle phi (?), and can otherwise be described as the phase shift induced between the sinusoidal input voltage and current, due to either the inductive or capacitive nature of the load. The second factor, THD, results from the non-linear characteristics of the load. Optimized intelligent LED driving systems are focused primarily on controlling THD in order to meet PF requirements. The second reason for selecting optimized LED drivers is the opportunity to add to the most basic driving capabilities other functions that can enhance even further the value extracted from the LED fixture both in terms of energy savings, user experience Dimming capability (automatic or user-controlled) is critical for realizing the full potential of LED fixture. Dimming adapts light to various applications. The human eye is adept at detecting flickering and jittering, so proper LED dimming is imperative and is directly controlled by the LED driver. Dimming an LED light source poses specific challenges that are very different as compared to dimming traditional light sources. In fact, TRIAC dimmers and other traditional approaches simply do not work well with LED light sources, because they don't consider the unique features of LED semiconductors. The wrong dimming method can cause radio frequency (RF) interference, power harmonics, audible noises, flickering and other interference. An intelligent LED driver should minimize these problems without having to re-wire the existing infrastructure. Another feature that can be added with an intelligent LED driver is color control or color management. Color management, is what enables LED lighting to be truly differentiated from earlier technologies, by enabling new applications in which the user has full control over color mixing, selection and sequencing. An intelligent LED driver system with integrated color-management control system plays an enormous role in ensuring high color accuracy while meeting color maintenance requirements. When choosing a color-management system, it is important to understand that LED-based white light can be created either by using phosphor-based white LEDs or by using RGB LEDs. The RGB-based white light can have variable color point, which is particularly beneficial for high-end luminaries and various types of architectural lighting. Microsemi has developed a very high-performance, extremely accurate color management system for RGB luminaries that includes an RGB color sensor, a color manager and LED drivers (see Figure 1).

With this type of RGB-based system, the targeted white color point can be represented by color temperature (CT) and luminance level (Yref), or by tri-stimulus values (Xref, Yref, Zref). The accuracy of color management depends on the performance the RGB color sensor. The sensor should be capable of the response times that are required for CIE color-matching functions, as illustrated in Figure 2.

Unfortunately, commercially available RGB sensors do not provide these response times. For this reason, Microsemi has developed a special procedure which compensates for the mismatch between RGB color sensor response times and those required by color-matching functions. This is done during the calibration process. The output of color sensor [R, G, B] is sent through low pass filters (LPF), which are connected to the input of the color manager. The calibration block of the color manager then converts [R, G, B] value of the color sensor to the tri-stimulus value. Next, a digital controller compares reference data to measured data and, using a digital proportional integral (PI) algorithm, develops RGB pulse width modulation (PWM) signals for the LED drivers. For higher accuracy, a 12-bit PWM algorithm is used. LED drivers control the luminary RGB LEDs, which the mixed light will represent as a targeted white point temperature. In a sophisticated LED fixture, the color temperature can be changed under the user's control through a communication protocol such as PLC, DXM412 or DALI. To improve system accuracy, the temperature sensor can be connected to a color manager, which can adjust the influence of the temperature on LED spectral characteristics. Despite tremendous advances in manufacturing and packaging, LED fixtures demand high-performance intelligent drivers in order to deliver the performance and life expectancy that users demand. The latest driver solutions take a system approach to optimizing light efficacy, life expectancy and light quality by solving key challenges related to PF, dimming, and color management. Users expect warm, non-flickering, tunable light plus superior energy efficiency, and optimized LED drivers that are designed and specified over realistic operating ranges will contribute to significantly better, "greener" overall performance. These intelligent drivers also will enable the longer life that is critical if LED technology is to deliver the necessary return, over time, on its higher initial fixture investment, so that it can support the necessary technology-deployment payback models. www.microsemi.com