LED lighting intelligent control solution

The development of new LED solid-state lighting (SSL) products with higher energy efficiency and greater functionality is rapid and is considered a major revolutionary advance in the lighting market. In many vertical applications, such as signal lights, automobiles, and LCD TV backlights, LEDs have become an undisputed alternative to traditional light sources. However, for the wider range of general lighting, LEDs have not yet been widely accepted. The cost and efficiency of solid-state lighting products will undoubtedly continue to improve rapidly in order to be accepted by the market.

Solid-state light-emitting products in general lighting

The reasons that hinder the transfer of LED to general lighting are the large capacity of the traditional market and the inertia of the market. Meeting the compatibility of the latter is probably the biggest challenge. Thermal management, voltage conversion and color management are basic issues that need to be solved. As more and more industry insiders begin to recognize these issues, intelligent digital control methods are needed to cost-effectively solve various challenges before the benefits of new technologies can be enjoyed.

LED lighting Significant energy saving

If LED lighting is used in buildings, 48% of lighting electricity can be saved. The luminous efficiency of an incandescent lamp is generally 10-15lm/W, the efficiency of a fluorescent lamp can reach 70-100lm/W, and the efficiency of a xenon lamp is 80-120lm/W. The current commercial LED efficiency is 80lm/W, and some experimental products have reached 131lm/W, while the maximum theoretical luminous efficiency of LED is 200lm/W.

White is not a color

The biggest problem with LEDs is that they emit light in a narrow frequency band, so that the color of the light is very single, which can achieve high efficiency and will not generate heat. It would be best if the color of the light was exactly what we wanted, but in general lighting, we need white light. In other words, we need a variety of colors mixed in a certain proportion to imitate the spectrum of sunlight filtered through the earth's atmosphere.

White light can be obtained by coating the blue or ultraviolet light source of an LED with a layer of phosphorous material. All LED manufacturers are conducting extensive research on the composition, thickness, and location of phosphorous materials.

LED manufacturers continue to announce new research results, claiming that the efficiency of new devices is higher than that of any previous products. In addition, the quality of light sources is constantly improving. The quality of light that our eyes see can be measured by the correlated color temperature (CCT), which is the temperature of the black body that best matches the perceived color of the light.

Another way to

get white LEDs is to mix the red, green, and blue (RGB) colors in the right proportions to get not only white light but also the desired color temperature . Figure 1 shows an application circuit for this approach, using an 8-pin 8-bit MCU to control the three-color RGB LED. A simple algorithm is needed to control the relative intensity of the three light-emitting diodes, achieving 6 bits of resolution (64 light intensity levels), which is sufficient to control the color output and the associated color temperature.

The circuit uses a PIC 12HV615 flash MCU, voltage divider resistors, reset circuit, A/D converter, and an oscillator to provide an 8MHz clock , forming a simple single-chip solution. The in-circuit programmable nature of this flash device can also be used for color calibration during production.

LED Life

The circuit shown in Figure 1 is suitable for many applications, but has a significant disadvantage: low efficiency. This is a linear solution, and the power is consumed by the ballast resistor. In addition, more problems may occur throughout the product life cycle.

Figure 1: A white light LED system that can be color-calibrated

One of the main advantages of LEDs is their extremely long life, which also brings a serious problem, namely color cast. LEDs can be used for more than 50,000 hours before their luminous intensity gradually decreases to about 70% of the nominal value (incandescent lamps fail suddenly after about 1,500 hours of use).

Unfortunately, during these 50,000 hours, the correlated color temperature (CCT) of a white LED will change, drifting to higher temperatures, that is, to blue, as the phosphor ages. Similar problems will occur with RGB LEDs as the three color emitters age according to different curves. Several techniques have been developed to compensate for the effects of device aging

by using microcontrollers , coupled with predictive algorithms or closed-loop control systems. Some manufacturers have already produced color photosensors that , when combined with simple PID algorithms, can completely solve the color drift problem once and for all, of course, using such components will increase some cost.

Since the color drift process is very slow, high computing performance is not required, and even low-cost 8-bit MCUs can meet the requirements.

LEDs are not cold

Another big challenge in LED general lighting is heat control. As mentioned earlier, high- power LEDs do not waste energy when they generate electromagnetic radiation within a very narrow frequency band, but they still generate heat, which is dissipated by conduction rather than radiation, similar to incandescent lamps.

The heat problem of LEDs puts great restrictions on the design of general lighting systems. A lighting system designed for an incandescent lamp of a given power is difficult to adapt to LEDs of the same power because the heat conduction path is very limited.

Power conversion and control

When the entire LED industry focuses on how to achieve the maximum LED luminous efficiency, the efficiency of the drive /control circuit must also receive equal attention. LEDs are low-voltage devices (Vf is 3~4V) and the operating voltage is completely mismatched with the mains. To achieve the highest efficiency and maintain continuous light output, LEDs require precise current control and switch- mode power conversion.

To solve this problem, some constant current drive technology must be used. Not only isolation and power factor correction are required, but in some cases two-stage processing is also required. The input voltage is first reduced to an intermediate voltage, and power factor correction and high voltage isolation are used. The second stage solves the LED's needs for current and thermal control.

Figure 2 shows a constant current configuration using a boost converter (MCP1630). The 8-bit MCU provides a flexible clock signal, and the current set point can be programmed to accommodate different LED modules , dimming functions, and use external sensors to provide closed-loop temperature control.
Figure 2 Smart LED solution using MCU and constant current driver.

Figure 2 Smart LED solution using MCU and constant current driver

MCU-based solutions offer great flexibility. When the temperature of an LED approaches a critical threshold, the power output to the LED is gradually reduced rather than abruptly shutting down the system or simply sounding an alarm. This capability is especially critical when the LEDs are designed by different companies and proper thermal design cannot be guaranteed.

Smart driver design also means that the MCU's built-in serial peripherals can support simple communication protocols such as DMX-512 or DALI. For more advanced system integration, Ethernet or ZigBee connectivity is also required , allowing the design of new energy management systems.