Detailed explanation of AC LED knowledge

As the fourth generation of green, energy-saving, power-saving and long-life lighting fixtures, LED light sources have emerged, attracted widespread attention 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 sources with DC has become more and more mature. Since the power supply used for our daily lighting is high-voltage AC (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 AC into 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 sources 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 ) and established a comprehensive patent portfolio to protect and improve the technology, firmly establishing its proprietary position. It 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 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 problem 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. ITRI has developed the process technology for white light, blue light and green light AC LEDs, which is not only in sync with the international level, but also one of the world's 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, with a high cost-effectiveness. 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, while using the diode characteristics of LED PN junction as rectification, and semiconductor process plays a very important role in it. AC LED is integrated into a pile of tiny crystal grains through semiconductor process, using a staggered matrix arrangement process, and adding a bridge circuit to the 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 the staggered matrix arrangement of AC LED crystal grains, and the right small picture is a photo of the actual arrangement of AC LED crystal grains. After the AC LED crystal grains are connected to AC, they emit light throughout the body, 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 and AX3211 for AC110V and AX3221 and AX3231 for 220V. The power for AC110V is 3.3W-4W, and the operating current is 40mA; the power for AC220V 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: Seoul Semiconductor's AC LED

Figure 5: AC LED pinout

Typical application technologies of AC LED

The typical application schematic diagram of AC LED is shown in Figure 6. It is very simple. A positive temperature coefficient thermistor PTC and current limiting resistors R1, R2, and R3 are connected in series at both ends of the AC LED. When connected to 110V or 220V AC, the lighting can be started.

Figure 6: Typical application schematic of AC LED

When LEDs are mass-produced, their impedance is somewhat discrete, and so is AC LED. To facilitate mass applications by downstream manufacturers, LED light source manufacturers classify mass-produced products by impedance before leaving the factory. Customers can use the VF classification table provided by the LED light source manufacturer to look up the corresponding current-limiting resistor. Table 2 shows the VF classification and current-limiting resistor table for AX3221/AX3231.

Table 2: VF classification and current limiting resistor table of AX3221/AX3231

Development of AC LED

AC LED is convenient for household electricity. It does not need to install an AC to DC converter for the lamp like DC LED. It not only saves the cost of the converter, but also avoids the dilemma that the LED light source itself is not broken, but the converter is broken first. The AC to DC converter can be said to be an electronic component that will age and break down over time. Its life is shorter than the LED light source itself. Therefore, many LED lamps are broken at present, not because the LED light source has reached the end of its life, but because the AC to DC converter used in the LED lamp is broken first. Another feature of AC LED is that because its process adopts a staggered matrix arrangement, it is lit in turn. In the 60Hz AC, it will light up at a frequency of 60 times per second, which also makes the service life of AC LED longer than that of DC LED.

However, AC LED has two disadvantages at this stage. One is that the luminous efficiency is not as high as that of DC LED. This is because the development of DC LED is currently the mainstream, and AC LED has just started. The luminous efficiency of AC LED can catch up with or even exceed that of DC LED. The second is that AC LED has the risk of electric shock. Therefore, if AC LED is to be used in LED lighting fixtures, the exposure of metal fins should be avoided, and the heat should be taken away indirectly. This is the core design concept for developing new liquid-filled LED solid-state lighting fixtures.

AC LED has just entered its growth stage, and is not ideal in terms of luminous brightness and power. However, AC LED has shown strong vitality due to its simple application, no need for transformers and constant current sources, and low cost and high efficiency. AC LED technology is developing by leaps and bounds. In a few years, high-brightness, high-power, low-cost products will be available in large quantities.

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