Discussion on the application design of white light and color light LED lighting

High-brightness LEDs have brought continuous changes to the lighting industry, adding more flexibility and intelligence to various lighting systems, including white light and color 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 can 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 produced using basic components such as mixed-signal controllers , constant current drivers , and high-brightness LEDs. Multiple LED channels are usually used in white light and color light designs, so all LED design solutions need to address issues such as device sorting, temperature effects, aging, and overall color accuracy. The use of mixed-signal controllers is indeed a powerful and effective method that can intelligently handle the above issues while ensuring high-precision white or color light. For many designers who have switched from traditional lighting (incandescent, fluorescent) designs to LED lighting , how to make good use of mixed-signal controllers has become a huge challenge.

This article will explore the similarities and differences between designing for white and colored light applications, the challenges of LED system design, and some powerful, off-the-shelf solutions that can help designers address these issues (some without requiring any coding).

Smart lighting

High-brightness LEDs (HB-LEDs) represent the future of lighting technology, and interest in HB-LED technology has increased in recent years. It is no surprise that people do this, considering the significant improvement in HB-LED performance ( lumens /watt) and the sharp decline in cost (lumens/dollars). In addition, the world is actively participating in the "green movement", and in this environment, HB-LEDs even pose a strong challenge to the currently popular, cost-effective but less eco-friendly mercury-containing fluorescent lamps. Although the high efficiency and environmental advantages of HB-LEDs are the focus of publicity, the "smart lighting" function will become an important force in promoting the further development of HB-LED technology.

The application range of smart lighting technology is quite wide, and the only limit is our imagination. This article will focus on an important application area in smart lighting - dimming function. In the past, dimming mainly refers to adjusting the brightness of light or manipulating the scattering pattern of light through optical devices. In the case of HB-LED, dimming means manipulating different characteristics of light. First, designers must consider what type of light to generate: white light, color light, or both. In the case of white light, designers can adjust the color temperature and color rendering index (CRI). In the case of color light, designers can mix colors from the entire spectrum of the same fixed LED channel group based on the number of LED color channels used in the system. By mixing color light, white and color light can also be generated on the same lighting fixture. This flexibility does lead to increased complexity and trade-offs between each system. Fortunately, although white light systems and color light systems look very different, in fact, their design methods are basically the same.

HB-LED System Design

Every smart lighting system consists of the following basic building blocks (Figure 1): HB-LEDs, some type of power topology (this article discusses only switch -mode regulators), and a mixed-signal controller. The first challenge facing designers is choosing LEDs. Major suppliers of LEDs, including Lumileds, Cree , Nico , and Osram, vary in power and current ratings, scatter patterns, colors, efficiencies, form factors, thermal characteristics, bins, and the number of LEDs per package. These parameters are the same for white and colored light, but white light also has to consider color temperature and color rendering index (CRI).

Figure 1: Smart lighting system block diagram

Advanced industrial design constraints and market demands usually help narrow the selection of most LED characteristic parameters. In most cases, designers should focus on the thermal characteristics of the LED, especially for miniaturized devices or applications with limited space and cannot use large heat sinks. Similarly, optical technology can help alleviate the problem of poor scattering patterns, while mixed-signal controllers can greatly reduce the limitations of temperature and device binning.

Deciding whether to use discrete components or integrated circuits is the first step in narrowing down the types of power topologies that can be used in smart lighting systems. Discrete implementations are lower cost and more flexible because they can be tuned to a specific system, but they take up more board space and require specialized design skills. Power management ICs offer a compact solution that, while more expensive, takes up less board space and is easier to design.

Secondly, depending on the efficiency requirements of the lighting system, designers need to choose between linear or switching topologies. Efficiency is important in two ways. First, the more efficient the power conversion, the less power is wasted. Second, less power waste means less heat is generated in the system. Linear regulators are simpler and less expensive, but they are generally less efficient.

Switching regulators are more complex and usually more expensive due to the need for an inductor, but they are more efficient, regardless of the regulator's input and output voltages. Linear and switching regulators can be designed with either monolithic ICs or discrete components. Depending on the lighting system's supply voltage, designers should choose to use a buck, boost, or buck-boost switching topology. Another drawback of linear topologies is that they cannot boost voltage.

Third, designers must select a mixed-signal controller for the smart lighting system. This device provides much of the intelligence and flexibility of the HB-LED system, and it even solves some of the technical challenges of HB-LED dimming. Therefore, it is important to select a mixed-signal controller that has as much flexibility as possible 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 the digital and analog peripherals on the MCU device. For digital peripherals, the number of dedicated hardware dimming channels, their resolution, and the ability to implement different communication interfaces are important. The dimming channels are used to drive the buck regulator, and although software counters can also be used to achieve this function, software dimming channels will consume valuable processing power and make it difficult for the device to perform other functions.

Smart lighting systems typically use at least 8-bit resolution to achieve high color accuracy. If the system quality requirements are extremely high, resolutions up to 16 bits can be used. However, for most applications, 8-bit resolution is sufficient to achieve the required accuracy, and designers usually use higher resolution to achieve better dimming linearity at low output levels. Some designers turn to smarter interpolation methods to solve the problem of output variations at low levels.

Common communication interfaces include SPI, UART, and I2C, but it is also important that mixed-signal controllers support important lighting interfaces such as DALI, DMX512, RF communication, and even power line communication. In terms of analog peripherals, designers should pay attention to ADCs, PGAs, and comparators. ADCs can support temperature feedback by reading temperature sensor values, and can also enable intelligent interaction between the lighting system and various physical (analog) aspects of the surrounding environment. Comparators and PGAs can simplify the implementation of power topologies.

Most MCU vendors offer some or all of these peripherals in their controllers, but designers may soon find that as system requirements change, the variety of peripherals needed will also change accordingly. It is indeed a huge challenge to design systems to take care of future innovations, especially considering that HB-LED lighting systems themselves are still a new thing. If the system requires ultra-high performance, then FPGAs will be a good value-for-money solution. Controllers with configurable peripherals and routable I/Os provide the greatest flexibility.

While each intelligent white and colored light system has the above components, there are differences in configuration and design between white and colored light-based systems. Lighting systems that generate white light (even if it has colored light mixed in) need to consider color temperature and color rendering index.

Color temperature refers to the color of white light (contrary to intuition, warm white light has a lower color temperature, while cool white light has a higher color temperature), and is usually related to the Planckian locus on the 1931 CIE colorimetric chart. Color temperature describes the color of white light produced when a standard black body radiator is heated to different temperatures (Figure 2). For example, a standard black body radiator heated to 2500K is considered warm white light; if it is heated to 7000K, it is considered cool white light. HB-LED systems cannot actually directly achieve colors that conform to the Planckian locus, but instead their color temperature is measured using the correlated color temperature (CCT) .

Figure 2: Planckian locus and color temperature (click on the image to enlarge)

The color rendering index is a parameter that describes the quality of white light by comparing the appearance of different colors between the primary light source and the reference light source. In layman's terms, the color rendering index describes the color fidelity of the surface of an object illuminated by the primary light source at an intensity 1 to 100 times that of the reference light source. 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 light LEDs have limited flexibility in color temperature, but they have excellent color rendering index (CRI) performance at the native color temperature of the system's white light LEDs. Because CRI is largely dependent on the color spectrum of LEDs in the system, as a rule of thumb, the more LEDs (especially LEDs of different colors), the higher the CRI.

For color light systems, designers are most concerned about color accuracy, color resolution, and the spectrum of mixable colors. As mentioned earlier, one factor that plays an important role is dimming resolution. Maximizing the spectrum of mixable colors 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 dimming resolution also affect color resolution. Most color light systems have a minimum of three LEDs, usually the primary colors of red, green, and blue. If the intelligent lighting system needs to generate a specific target color, the designer can determine whether the selected LED can mix that color by plotting the LEDs on the 1931 CIE colorimetric chart and simply connecting the plotted 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 mixable color by expanding the color gamut (Figure 3).

Figure 3: Example of extending the color gamut using 4 LEDs (click on the image to enlarge)

Design Challenges

As mentioned earlier, white and colored smart lighting systems can benefit from using three or more LEDs, but in addition to facing many challenges in optical technology and thermal performance, the algorithms will also be more complex. An obvious challenge is how to provide the required number of hardware dimming channels with flexible dimming resolution. Systems using four or more LEDs also require more creative algorithms to adjust color temperature, mix colors, or improve the color rendering index CRI.

Clearly, smart lighting systems need to manage heat dissipation and device binning in some way. LEDs do not dissipate heat by radiation, but rather by conducting heat through the junction of the diode. In fact, as the temperature of the LED increases, the lumen output of some LEDs will decrease (red light is severely affected, for example), and even the wavelength of the light output will shift. Therefore, it is very important to conduct as much heat as possible away from the LED base.

Good thermal design, plenty of air movement, and active cooling are a good start to solving thermal problems. However, these methods do not always ensure predictable and measurable results. Heat is always present in the system, and color accuracy is affected by temperature. Introducing a temperature sensor can help maintain color accuracy at a level of . This rating is a general requirement for systems that need to achieve high color accuracy. One of the input parameters of the algorithm used to calculate the dimming value of colored light is the luminous flux output. By maintaining a piecewise linear approximation of the temperature in the lighting system and the luminous flux curve of the LEDs, the mixed-signal controller can maintain color accuracy by appropriately varying the output level of each LED.

The reason for device binning is that HB-LEDs are solid-state devices that vary in luminous flux output, wavelength, and forward voltage using current manufacturing processes. Since luminous flux output is very important in calculating mixed colors, variations in this value must be taken into account. However, if the system does not require high color quality, this does not have to be considered.

For designers who are concerned about color quality, they can either buy some more expensive specialty LEDs (which cost 15% to 20% more), or they can compensate through the programmability of the mixed-signal controller. Designers can enter a device binning table that stores the possible binning characteristics of the LEDs in the system. In this way, when the actual LEDs are obtained during the manufacturing stage, the mixed-signal controller can be updated with the actual binning code and compensated accordingly.

Many people have found that solid-state lighting technology design requires comprehensive experience in optical, mechanical and electrical design, and few people have such skills, so new and complex technical problems continue to emerge. Especially now that designers must use mixed-signal controllers, they must also master embedded design technology. Fortunately, current tools can provide a visual design environment that can meet the design needs of HB-LED intelligent lighting systems without writing code, and designers can also use traditional languages ​​​​such as C language to program. In any case, excellent development tools , reference designs, and project examples are very important.

Therefore, designers face many challenges while leveraging the intelligence, flexibility and environmental benefits of HB-LEDs. Through intelligent lighting design methods, designers can cost-effectively reduce or eliminate most of these problems.