A Brief Analysis of the Rapidly Developing AC Driven LED Light Source Technology

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 in our daily lighting is high-voltage AC (AC100~220V), it is necessary to use the technology of step-down to obtain a lower voltage. The commonly used technology is transformer or switching power supply to step down the voltage, and then convert AC into DC, and then convert it 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 space in the LED lamp to place this module, but for the lamp with E27 standard screw mouth, the space is very limited and it is 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 DCLED 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 of DC driven LED light source

As early as 2005, Seoul Semiconductor Co., Ltd. in South Korea, now known as Seoul Semiconductor, invented ACLED that can be directly driven by AC to make it emit light. The second is III-N Technology in 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 (ACLED), 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" of Taiwan Province, China also completed an ACLED product that can be industrialized and has a practical application system solution. It can be directly plugged into an AC110V AC voltage of 60Hz or higher frequency to make it emit AC light. It is 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. Taiwan Industrial Technology Research Institute's OnChipACLED won the 2008 R&D100 Award, which is known as the Oscar of American industrial innovation. Currently, only the United States, South Korea and Taiwan have this technology. Taiwan Industrial Technology Research Institute has developed the process technology of white, blue and green ACLED, which is not only in line with the international level, but also one of the world leaders.

Advantages of ACLED lamps

Compared with incandescent lamps, halogen lamps, fluorescent lamps, fluorescent energy-saving lamps, and DC LED lamps, ACLED lamps are more energy-efficient, longer-lasting, and more energy-efficient, with a high cost-effectiveness ratio. ACLED 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 Performance comparison between AC LED and existing lighting fixtures

The ultra-fine grains of ACLED light sources are arranged in a special staggered matrix

The major technological breakthrough of ACLED light source is the special arrangement and combination technology of ultra-fine LED grains in packaging , and the diode characteristics of LED PN junction are used for rectification. Semiconductor process plays a very important role in this. ACLED is integrated into a pile of tiny 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 grains is shown in Figure 2. The left picture is a schematic diagram of ACLED grains arranged in a staggered matrix, and the small picture on the lower right is a photo of the actual arrangement of ACLED grains. ACLED 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 AC LED die arrangement photo and schematic diagram

Working principle of ACLED light source

The working principle of ACLED light source is shown in Figure 3. A bunch of tiny LED crystals are divided into five strings by using staggered matrix arrangement process. The ACLED crystal string is 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 crystals. The positive half cycle of the AC flows along the blue path, and 3 strings of LED crystals emit light. The negative half cycle flows along the green path, and 3 strings of LED crystals emit light. The LED crystals on the four bridge arms emit light in turn, and the LED crystals on the opposite bridge arm emit light at the same time. The middle string of LED crystals is always emitting light because of sharing. In 60Hz AC, it will light up at a frequency of 60 times per second. The DC obtained by the rectifier bridge is pulsating DC, and the LED light is also flashing. The LED has the characteristics of power-off afterglow, 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 working mode of the LED light source + afterglow 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 ACLED is longer than that of DCLED.

Figure 3 Working principle of AC LED light source

[page] ACLED mature products include Seoul's AX3201, AX3211 for AC110V and AX3221, AX3231 for 220V. For AC110V, the power is 3.3W~4W, and the working current is 40mA; for AC220V, the power is 3.3W-4W, and the working 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 pin diagram

Typical application technology of ACLED

The typical application schematic diagram of ACLED 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 ACLED . When connected to 110V or 220V AC, the lighting can be started.

When LEDs are mass-produced, their impedance is somewhat discrete, and so is ACLED. 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.

Figure 6 Typical application schematic diagram of ACLED

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

Development of ACLED

ACLED is convenient for household electricity . It does not need to install an AC-to-DC converter on the lamp like DCLED. 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-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-DC converter used in the LED lamps is broken first. Another feature of ACLED is that because its process adopts a staggered matrix arrangement, it is lit in turn. In a 60Hz AC, it will light up in turn at a frequency of 60 times per second, which also makes the service life of ACLED longer than that of DCLED.

However, ACLED currently has two disadvantages. One is that the luminous efficiency is not as high as that of DCLED. This is because DCLED is currently the mainstream and ACLED is just starting out. The luminous efficiency of ACLED can catch up with or even exceed that of DCLED. The second is that ACLED has the risk of electric shock. Therefore, if ACLED 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 the development of new liquid-filled LED solid-state lighting fixtures.

ACLED has just entered its growth stage, and is not yet ideal in terms of brightness and power. However, ACLED is easy to use, does not require a transformer or constant current source, and is low-cost and highly efficient, and has shown strong vitality. ACLED technology is developing by leaps and bounds, and in a few years, high-brightness, high-power, low-cost products will be available in large quantities.