Analyze the principles and application technologies of AC LED light sources

LED light source As the fourth generation of green, energy-saving, power-saving and long-life lighting fixtures, it has emerged as a new force, widely concerned and developed rapidly. The current LED light source is a semiconductor device that works at low voltage (VF=2→3.6V) and high current (IF=200→1500mA). It must provide a suitable DC current to emit light normally. The technology of driving LED light source with DC (direct current) has become more and more mature. Since the power supply used in our daily lighting is high voltage alternating current (AC 100～220V), it is necessary to use step-down technology to obtain a lower voltage. Transformers or switching power supplies are commonly used to step down the voltage, and then convert alternating current (AC) into direct current (DC), and then into a DC constant current source to make the LED light source emit light. Therefore, the system application solution for DC-driven LED light source must be: transformer + rectifier (or switching power supply) + constant current source (Figure 1).

There must be a certain amount of space in the LED lamp to place this module, but the space is very limited for the E27 standard screw lamp, making it difficult to place it. Whether it is through transformer + rectification or switching power supply step-down, the system will have a certain amount of loss. When DC LED is converted between AC and DC, about 15% to 30% of the power is lost, and the system efficiency is difficult to achieve more than 90%. If the LED light source can be directly driven by AC (AC), the system application solution will be greatly simplified, and the system efficiency will easily reach more than 90%.

Figure 1: System application solution for DC-driven LED light source

As early as 2005, a Korean company invented AC LED that can be directly driven by AC to make it emit light. The second is III-N Technology of the United States. 3N Technology developed a gallium nitride substrate based on MOCVD growth technology, which can enhance the application of lighting and sensors , reduce costs and improve production efficiency. It provides 6-inch production technology for large and small silicon light-emitting diodes. 3N invented the single-chip AC light-emitting diode (AC LED), established a comprehensive patent portfolio to protect and improve the technology, firmly established its proprietary position, and is the leading large-scale commercial production of AC light-emitting diode products.

In 2008, the Industrial Technology Research Institute (ITRI) of Taiwan also completed an AC LED product that can be industrialized and has a practical application system solution. It can be directly plugged into an AC 110V voltage of 60Hz or higher frequency to make it emit AC light. It can be used in indicator lights, neon lights, and low-wattage lighting lamps. It can effectively solve the disadvantage that existing LEDs cannot be used directly under AC sources, resulting in higher product application costs. The On Chip AC LED of ITRI won the 2008 R&D 100 Award, which is known as the Oscar Award for American Industrial Innovation. Currently, only the United States, South Korea and Taiwan have this technology in the world. ITRI has developed the process technology of white light, blue light and green light AC LED, which is not only synchronized with the international level, but also one of the global leaders.

Advantages of AC LED lamps

Compared with incandescent lamps, halogen lamps, fluorescent lamps, fluorescent energy-saving lamps, and DC LED lamps , AC LED lamps are more energy-efficient, longer-lasting, and more energy-efficient. AC LED lighting eliminates the need for expensive AC/DC converters and constant current sources. The performance comparison of AC LEDs and existing lighting fixtures is shown in Table 1.

Table 1: Comparison of AC LED and existing lighting performance

AC LED light source ultra-fine grains are arranged in a special staggered matrix

The major technological breakthrough of AC LED light source is the special arrangement and combination technology of ultra-fine LED crystal grains during packaging, and the diode characteristics of LED PN junction are used for rectification. Semiconductor process plays a very important role in this. AC LED is integrated into a pile of tiny crystal grains through semiconductor process, using staggered matrix arrangement technology, and adding bridge circuit to chip design, so that AC current can be bidirectionally conducted to achieve light emission. The arrangement of crystal grains is shown in Figure 2. The left picture is a schematic diagram of AC LED crystal grains arranged in a staggered matrix, and the right small picture is a photo of the actual arrangement of AC LED crystal grains. AC LED crystal grains emit light all over after being connected to AC, so only two leads are needed to connect to the AC source to emit light.

Figure 2: Photo and schematic diagram of AC LED die arrangement

Working Principle of AC LED Light Source

The working principle of the AC LED light source is shown in Figure 3. A bunch of tiny LED grains are divided into five strings using a staggered matrix arrangement process. The AC LED grain strings are similar to a rectifier bridge. The two ends of the rectifier bridge are connected to the AC source respectively, and the other two ends are connected to a string of LED grains. The positive half cycle of the AC flows along the blue path, and 3 strings of LED grains emit light. The negative half cycle flows along the green path, and 3 strings of LED grains emit light. The LED grains on the four bridge arms emit light in turn, and the LED grains on the opposite bridge arms emit light at the same time. The middle string of LED grains is always glowing because they are shared.

Figure 3: Working principle of AC LED light source

In a 60Hz AC, it will light up 60 times per second. The DC obtained by the rectifier bridge is pulsating DC, and the LED light is also flickering. The LED has the characteristic of afterglow after power failure, and the afterglow can last for tens of microseconds. Because the human eye has an inert memory of the flowing light spot, the human eye interprets the LED light source's light + afterglow working mode as continuous light. The LED is working half of the time and resting half of the time, so the heat can be reduced by 40% to 20%. Therefore, the service life of AC LED is longer than that of DC LED.

Mature AC LED products include Seoul's AX3201, AX3211 for AC110V and AX3221, AX3231 for 220V. For AC110V, the power is 3.3W~4W, and the operating current is 40mA; for AC220V, the power is 3.3W-4W, and the operating current is 20mA (Figure 4). The LED die is directly bonded to the copper-aluminum substrate. The pins are shown in Figure 5.

Figure 4: AC LED from Seoul Semiconductor

Figure 5: AC LED pinout