High-brightness LEDs have revolutionized the lighting industry, adding more flexibility and intelligence to a variety of lighting systems, including white and colored light designs. These lighting systems allow designers to dynamically control color temperature while maintaining a high color rendering index (CRI) in white light applications. In addition, these systems produce a wide range of high precision color spectra. Although white light and color light look very different, most LED smart lighting applications are designed and manufactured using basic components such as mixed-signal controllers, constant-current drivers, and high-brightness LEDs. Multiple LED channels are typically used in white and color light designs, so all LED designs need to address issues such as device sorting, temperature effects, aging, and overall color accuracy. The use of a mixed-signal controller is a powerful and effective way to intelligently handle these problems while ensuring high-precision white or colored light. For designers who Switch from traditional lighting (incandescent, fluorescent) design to LED lighting, how to use mixed signal controllers has become a huge challenge.
This article explores the similarities and differences between white light applications and color light application designs, the challenges faced by LED system design, and some powerful off-the-shelf solutions that help designers solve these problems (some without even coding).
Intelligent lighting
High-brightness LEDs (HB-LEDs) represent the future of lighting technology, and in recent years, attention has been paid to HB- LED technology . It is not surprising that people are doing this in view of the significant increase in HB-LED performance (Lumens Watts) and the sharp decline in costs (lumens). In addition, the world is actively participating in the “Green Actionâ€. Under this environment, HB-LED has even presented a strong challenge to the currently popular and cost-effective but less ecologically friendly mercury-containing fluorescent lamps. Although the high efficiency and environmental protection advantages of HB-LED are the focus of publicity, the "smart lighting" function will become an important force to promote the further development of HB-LED technology.
The application of intelligent lighting technology is quite extensive, and the only limitation is our imagination. This article will focus on one important application area in smart lighting - dimming. In the past, dimming mainly refers to adjusting the light and darkness of light, or manipulating the scattering pattern of light through optics. In the case of HB-LEDs, dimming means manipulating different characteristics of light. First, designers must consider what type of light to generate: white light, colored light, or both. For white light, the designer can adjust the color temperature and color rendering index (CRI). In the case of colored light, the designer can mix the entire spectrum of colors from the same fixed LED channel group based on the number of LED color channels used in the system. By mixing the colored lights, white light and colored light can also be generated on the same illumination device. This flexibility does lead to an increase in complexity and a trade-off between each system. Fortunately, although the white light system and the color light system look very different, in fact they are basically the same design method.
HB-LED system design
Each intelligent lighting system includes the following basic building blocks (Figure 1): HB-LED, some type of power topology (only the switch mode regulator is discussed in this article) and mixed-signal controllers. The first challenge for designers is to choose LEDs. Major LED suppliers include Lumileds, Cree, Nichia and Osram, whose products are rated for power and current, scattering patterns, color, efficiency, form factor, heat dissipation characteristics, bins, and number of LEDs per package. There are different. These parameters are the same for both white and colored light, but white light also takes into account color temperature and color rendering index CRI.
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The limitations of advanced industrial design and market demand often help to narrow the selection of most LED characteristic parameters. In most cases, designers should focus on the thermal characteristics of LEDs, especially for small form factor devices or applications where space is limited and large heat sinks cannot be used. Similarly, optical technology helps to alleviate the problem of poor scattering patterns, while mixed-signal controllers can significantly reduce the limitations of temperature and device sorting.
The first step is to determine whether to choose discrete components or integrated circuits. This is the first step in reducing the type of power topology used in intelligent lighting systems. Discrete implementations can be tuned to a particular system, so they are less expensive and more flexible, but take up more board space and require specialized design techniques. Power management ICs offer a compact solution that, while costly, occupies less board space and is easier to design.
Second, depending on the efficiency requirements of the lighting system, designers need to choose between linear or switching topologies. The importance of efficiency is reflected in two aspects. First, the higher the power conversion efficiency, the less power is wasted. Second, reducing power waste means that the system generates less heat. Linear regulators are simpler and less expensive, but are generally less efficient.
Switching regulators are more complex and often more expensive due to the need for inductors, but they are more efficient and can achieve higher efficiency regardless of the regulator's input and output voltage. Linear regulators and switching regulators are available in either a single IC design or a discrete component design. Depending on the power supply voltage of the lighting system, the designer should choose to use a buck, boost or buck-boost switch topology. Another disadvantage of linear topology is that it cannot be boosted.
Again, the designer must choose a mixed-signal controller for the intelligent lighting system. Most of the intelligence and flexibility of HB-LED systems is achieved by this device, which even solves some of the technical challenges of HB-LED dimming. Therefore, it is important to choose a mixed-signal controller with the highest possible flexibility and as many useful peripherals as possible. Typically, an 8-bit MCU core is sufficient to provide enough processing power for most lighting applications, as well as enough RAM or flash memory.
Designers should pay special attention to digital and analog peripherals on MCU devices. For digital peripherals, the number of dedicated hardware dimming channels, their resolution, and the ability to implement different communication interfaces are all important. The dimming channel is used to drive the buck regulator, and the software counter can be used to do this, but the software dimming channel consumes valuable processing power, making it difficult to perform other functions.
Intelligent lighting systems typically use at least 8-bit resolution for higher color accuracy. If the system quality is extremely high, a resolution of up to 16 bits can be used. But for most applications, 8-bit resolution is enough to achieve the required accuracy, and designers typically achieve better dimming linearity with higher resolution at low output levels. Some designers turned to smarter interpolation to solve output variations at low levels.
Common communication interfaces include SPI, UART, and I2C, but it is also important that mixed-signal controllers also support important lighting interfaces such as DALI, DMX512, RF communications, and even power line communications. For analog peripherals, designers should pay attention to ADCs, PGAs, and comparators. The ADC can support temperature feedback by reading temperature sensor values, as well as intelligent interaction of the lighting system with various physical (analog) aspects of the surrounding environment. Comparators and PGAs simplify implementation of power supply topologies.
Most MCU vendors will provide these peripherals in part or in full in their controllers, but designers may soon discover that as the system requirements change, the variety of peripherals required will change accordingly. The forward-looking nature of system design to take care of future innovations is indeed a huge challenge, especially considering that the HB-LED lighting system itself is still a new thing. If the system requires ultra-high performance, then FPGA will be a better value for money solution. Controllers with configurable peripherals and routable I/O provide maximum flexibility.
Achieve high quality white light
While each of the smart white and color light systems has the above components, systems based on white and colored light differ in configuration and design. A lighting system that produces white light (even if it is mixed with colored light) needs to consider color temperature and color rendering index.
Color temperature refers to the color of white light (unlike intuition, the warm white light has a lower color temperature and the cool white light has a higher color temperature), which is usually related to the Planckian trajectory on the 1931 CIE color chart. Color temperature describes the color of white light produced by a standard blackbody source heated to different temperatures (Figure 2). For example, a standard blackbody source heated to 2500K is considered warmer white light; if heated to 7000K it is considered cool white. The HB-LED system cannot actually directly achieve a color that matches the Planckian trajectory, whereas the color temperature is measured by the correlated color temperature (CCT).
The color rendering index is a parameter that describes the quality of white light by comparing the presentation of different colors between the primary source and the reference source. In general, the color rendering index describes the color fidelity of the surface of the object illuminated by the primary light source at an intensity of 1 to 100 times the reference light source. The color temperature and color rendering index can be adjusted by selecting the appropriate LEDs, using the appropriate number of different LED channels, and intelligently controlling these channels with a mixed-signal processor. White light systems that only include white LEDs have limited flexibility in color temperature, but at the native color temperature of the system's white LEDs, the white light system's color rendering index CRI performance is excellent. Since CRI depends to a large extent on the LED spectrum in the system, the more LEDs (especially LEDs of different colors), the higher the CRI.
For color light systems, designers are most concerned with color accuracy, color resolution, and spectrum of blendable colors. As mentioned earlier, one factor that plays an important role in this is the dimming resolution. The spectrum that maximizes the color of the mix depends on the color gamut generated by the LEDs in the system, which is directly related to the number of different LED colors that make up the color gamut. The number of LEDs and the dimming resolution also affect the color resolution. Most color light systems have at least three LEDs, usually red, green, and blue. If the intelligent lighting system needs to generate a specific target color, the designer can judge whether the selected LED can mix the color by drawing the LED on the 1931 CIE color chart and simply connecting the drawing points to observe the color gamut. If the color gamut does not cover the target color, the designer can add a new LED color to include this blendable color by expanding the color gamut (Figure 3).
Design challenge
As mentioned earlier, white and color light intelligent lighting systems can benefit from the use of three or more LEDs, but in addition to the many challenges in optical technology and thermal performance, the algorithm is more complex. An obvious challenge is how to provide a hardware dimming channel that meets the required number and has a flexible dimming resolution. Systems that use four or more LEDs also require more creative algorithms to adjust color temperature, blend colors, or increase color rendering index CRI.
Clearly, intelligent lighting systems need to manage heat dissipation and device sorting in some way. Instead of radiating heat through the radiation, the LED conducts heat by means of the junction of the diode. In fact, as the LED temperature increases, the lumen output of some LEDs will decrease (for example, red light will be severely affected), and even the light output wavelength will shift. Therefore, it is very important to conduct as much heat as possible from the LED base.
Good thermal design, large air flow and active cooling are a good starting point for solving heat dissipation problems. However, the above methods do not always ensure predictable, measurable results. Heat is always present in the system, and color accuracy is affected by temperature. The introduction of a temperature sensor helps to maintain the color accuracy at a level. This rating is a general requirement for systems that require high color accuracy. One input parameter of the algorithm used to calculate the color light dimming value is the luminous flux output. By preserving the piecewise linear approximation of the temperature in the illumination system and the luminous flux curve of the LED, the mixed signal controller can maintain color accuracy by appropriately varying the output size of each LED.
The reason for device sorting is that HB-LEDs are solid-state devices that vary in luminous flux output, wavelength, and forward voltage using current manufacturing processes. Since the luminous flux output is very important in calculating the mixed color, it is necessary to take into account this change in value. However, if the system does not require high color quality, you do not have to consider it.
For designers who care about color quality, they can purchase some of the more expensive special LEDs (which cost 15% to 20% more), or they can be compensated for by the programmability of a mixed-signal controller. Designers can enter device sorting tables that store the possible sorting characteristics of the LEDs in the system. In this way, when the actual LED is obtained during the manufacturing phase, the mixed signal controller can be updated with the actual binning code and compensated accordingly.
Many people find that solid-state lighting technology design requires a combination of optical, mechanical, and electrical design experience, and few people have such a skill, so new complex technical problems continue to emerge. In particular, designers must now use mixed-signal controllers, so they must also master embedded design techniques. Fortunately, today's tools provide a visual design environment that requires programming code to meet the design needs of HB-LED intelligent lighting systems, and designers can use traditional languages ​​such as C to program. In any case, great development tools, reference designs, and project examples are all very important.
Therefore, designers face many challenges while playing the intelligence, flexibility and environmental advantages of HB-LED. With intelligent lighting design methods, designers can cost-effectively reduce or eliminate most of these problems.
Author: Shone Tran, product manager for Cypress Semiconductor Corporation
Ben Kropf, Applications Engineer, Cypress Semiconductor Corporation
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