Application of electronic technology in general lighting and intelligent control in LED lighting

Compared with traditional lighting, LED lighting has many unique features, mainly in the following aspects: from a structural point of view, the light source and lamps of traditional lighting are separated, while LED lighting can realize the integrated design of light distribution, heat dissipation and lamps; from a functional point of view, only a small part of traditional lighting can realize dimming, while LED lighting can realize not only brightness adjustment, but also color and color temperature adjustment. In addition, LED lighting is more suitable for intelligent and digital control than traditional lighting, forming a complex lighting system.

Based on these characteristics of LED lighting, three models can be roughly given, targeting general LED lighting, intelligent control of LED lighting, and the combination of LED lighting with solar energy and wind energy, explaining some applications of electronic information technology in LED lighting.

Model 1: Power supply + light source

The essence of modern electric light sources is to convert electrical energy into light energy, accompanied by a certain amount of heat dissipation. Therefore, the general lighting model of LEDs only includes two parts: power supply and light source. Under this model, only the on-off control of lighting can be realized, and functions such as dimming, communication, and feedback of lighting cannot be realized.

The power supply specifically refers to the AC-DC constant current switching power supply. AC-DC refers to the conversion from AC to DC, because the LED is actually a semiconductor diode, which works under DC voltage. Another characteristic of LED is that its operating current is approximately an exponential function of the operating voltage. A small fluctuation in voltage will cause a huge change in current, and in severe cases, it will directly burn the LED. Therefore, to ensure the safe operation of the LED, its operating current must be constant. Usually, the power supply of household appliances is constant voltage, and the characteristics of LED determine that it must use a constant current power supply. In order to improve the reliability of LED operation, we generally use an isolated switching power supply to achieve effective isolation between the LED and the mains power grid. Corresponding to the switching power supply, there is also a type called linear power supply, which is rarely used now because the switching power supply has more advantages in efficiency and volume. The current switching power supply technology can achieve a conversion efficiency of 85% or even higher, a power factor of more than 0.95, and a total harmonic within 15%. These indicators are also the basic requirements for us to choose a switching power supply.

The light source is the LED chip, which is the core component of LED lighting. In addition to electrical parameters such as working current and tube voltage drop, we are more concerned about its optical indicators, such as luminous flux, luminous efficiency, color temperature, color rendering index, light decay, etc. The optical performance of LED is closely related to its temperature. The increase in temperature will cause problems such as reduced luminous efficiency, color temperature drift, and reduced life. Therefore, heat dissipation technology is particularly important in LED lighting. Thermodynamics knowledge tells us that there are only three ways for objects to dissipate heat: conduction, convection, and radiation. Since LED chips only emit visible light of a certain wavelength, there is no infrared radiation heat dissipation. In LEDs, heat dissipation is mainly conducted, accompanied by a certain amount of convection heat dissipation. Reducing the heat conduction links of LEDs and reducing the thermal resistance of each link, and using forced air cooling when necessary, are all considerations in heat dissipation design.

Model 2: Power supply + control + light source

Model 2 has an additional control module compared to Model 1, which greatly enriches the functions of LED lighting. If Model 1 is just a lighting product, then Model 2 can be said to be a lighting system. With the addition of the control module, the power supply becomes a constant voltage switching power supply, but the LED light source still requires a constant current power supply, which we implement together in the control module (constant current drive). Next, we focus on analyzing the control system of LED lighting.

LED lighting control systems can be large or small, with rich functions. For example: wireless remote control dimming system, color temperature adjustable lighting system, RGB dimming and color adjustment system, simulated daylight lighting system, intelligent lighting system with DALI protocol, wireless street light control system based on ZigBee and GPRS, etc. Looking through the phenomenon to see the essence, all these lighting control systems are developed and established by using embedded technology, communication technology, sensor technology, computer technology and power electronics technology in the field of electronic technology. Although it seems complicated, it actually has a basis to follow.

In order to clarify the context, we can divide the LED lighting control system into 8 levels and explain them layer by layer. If we can do a good job in the technical solutions of each level, then any complex control system can be easily realized like "building blocks".

The eight levels of the LED lighting control system are as follows from bottom to top: power supply layer, drive layer, optical layer, sensor layer, protocol layer, information layer, operation layer, and system implementation layer.

Power supply layer

The power supply layer provides efficient and stable DC power for the entire control system. Unlike the constant current power supply required for the operation of LED light sources, the control system requires a constant voltage power supply. There are two ways to obtain power for the power supply layer: one is to obtain it from the constant voltage power supply on the LED lighting fixture, which is a DC-DC (direct current to direct current) conversion. Because the voltage of the constant voltage power supply on the lamp is usually very high, and the voltage of the control system is very low, a step-down conversion from DC to DC is required; the second is to obtain it from the AC mains, which is an AC-DC (alternating current to direct current) conversion. For safety reasons, it is usually also made into an isolated switching power supply. Because the power consumption of the control system is usually very low, the constant voltage power supply here does not have very high requirements for conversion efficiency.

Driver layer

As mentioned above, the power supply in Model 2 is a constant voltage source, while the LED light source requires a constant current source. Therefore, there must be a constant current drive module in the control system to provide a constant current source for the normal operation of the LED light source. In the design of constant current drive, a dedicated driver chip is generally used in conjunction with certain peripheral circuits.

The output end of the driver chip usually has a string of LEDs, and the output constant current is the working current of the LED. The higher the output voltage range, the more LEDs can be connected. There are exceptions, and the output end needs to connect multiple strings of LEDs in parallel. In this case, we need to connect a small resistor in series with each string of LEDs to achieve current shunting. The driver chip generally also has a PWM (pulse width modulation) port for dimming control, which is the basis for realizing intelligent lighting. The quality of the constant current drive solution is directly related to the reliability and service life of LED lighting. Therefore, the research on LED drive technology is particularly important.

Dimming layer

The reason why LED lighting is easy to achieve intelligent control is that it is easy to achieve dimming control. All intelligent functions are realized through the dimming function. The understanding of LED lighting dimming is not limited to adjusting the brightness of the light, but also includes the adjustment of the LED color temperature and color. This is also a major factor that LED lighting can subvert traditional lighting. There are two ways to implement dimming technology: analog and digital. Among them, digital dimming is mainly achieved through PWM (pulse width modulation) technology, which is the main means of current LED dimming solutions. PWM signals can be implemented by programming the microcontroller to generate different duty cycles, thereby realizing the dimming of LED lighting. Designing an integrated solution for LED "three adjustments" (brightness, color temperature, and color) functions is the focus of research at this level.

Sensor layer

The driver layer and dimming layer mentioned above are analyzed from the perspective of system output. Obviously, the sensor layer is the input of the intelligent system. It is like the eyes and ears of the intelligent system, which can sense changes in the external environment and convert these changes into electrical signals, which are transmitted to the brain of the intelligent system - the central processing unit for analysis and processing. In the field of lighting control, the sensors we are concerned about are mainly: photosensitive sensors, which collect brightness information of natural light; human body sensors, which detect whether there are people near the lamps; temperature sensors, which collect temperature information of the external environment; and the collection and analysis of current and voltage data when the system is working.

Protocol Layer

There is no order without rules. To achieve orderly coordination between the various modules in the intelligent lighting system, there must be a complete set of "rules and regulations" to convey instructions and execute operations. This set of rules is the protocol of the intelligent system. In the current lighting industry, there are two relatively mature protocols, one is the DALI (Digital Addressable Lighting Interface) protocol in lighting communication, and the other is the DMX512 protocol for RGB color control. Unfortunately, neither of these two protocols is tailor-made for LED lighting. The research on the relevant protocols for LED lighting control is also a hot topic now, and there is no unified standard internationally. This may be an opportunity for us!

Communication Layer

In the field of electronic information, the transmission of protocols is achieved by using communication technology as a carrier. There are three main ways of communication in LED lighting: bus mode, power line carrier mode and wireless radio frequency mode. The bus mode is currently the most widely used communication method due to its mature technology, high reliability, simple networking and moderate cost, such as RS485 bus. The power line carrier mode transmits the protocol through the AC power grid, eliminating the cost of repeated wiring, and should be said to be an excellent communication method. However, it has high requirements on the quality of the power grid. In view of the serious harmonic pollution of my country's power grid, whether this method is suitable for my country's national conditions remains to be verified. In the wireless radio frequency mode, the most popular technology now is ZigBee. It is an emerging short-distance, low-speed, low-power wireless network technology.

Operational Layer

This level realizes the information interaction between users and systems. For the design of human-computer interaction software, simple operation, friendly interface, easy maintenance and upgrade should be the primary considerations. According to different application occasions, interactive software can be divided into two categories: one runs on embedded systems; the other runs on PCs.

System Implementation Layer

At this level, we conduct a global analysis and design of the control system, and use the research results of the previous 7 levels to complete the construction of the entire system. In other words, the design of this level is based on the previous 7 levels and is a synthesis of the research results of the previous 7 levels.