DC-driven LED light source system application design scheme

As a fourth-generation lighting fixture with green, energy-saving, power-saving, and long-life LED light sources, LED light sources have sprung up, attracting wide attention, and are developing rapidly. The current LED light source is a low voltage (VF=2→3.6V), high current (IF=200→1500mA) semiconductor device, which must provide a suitable DC current to normally emit light.

The technology of direct current (DC) driving the LED light source to emit light has become more and more mature. Since the power supply we use for daily lighting is high-voltage alternating current (AC 100 ~ 220V), we must use step-down technology to obtain a lower voltage. The transformer or switching power supply step-down, then converts the alternating current (AC) into direct current (DC), and then into a direct current constant current source, in order to promote the LED light source to emit light. Therefore, the system application scheme of DC driving 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 install this module, but for the E27 standard screw lamp, the space is very limited and it is difficult to install it. Whether through transformer + rectification or switching power supply, the system will have a certain amount of loss. When the DC LED is converted between AC and DC, about 15%-30% of the power is lost, and the system efficiency is difficult to achieve more than 90%. . If AC can be used to directly drive the LED light source to emit light, the system application scheme will be greatly simplified, and the system efficiency will easily reach more than 90%.

Figure 1 System application scheme of DC drive LED light source

South Korean companies have invented AC LEDs that can be directly driven by AC to emit light as early as 2005, followed by US III-N Technology, and 3N technology developed a gallium nitride substrate based on MOCVD growth technology, which can enhance the application of lighting and sensors. , And reduce costs and improve production efficiency. Provide 6-inch production technology for large and small silicon light-emitting diodes. The single-chip AC LED (AC LED) invented by 3N has established a comprehensive patent portfolio to protect and improve technology and firmly establish its proprietary position. It is the premier large-scale commercial production of AC LED products.

In 2008, the "Industrial Technology Research Institute" of Taiwan Province of China also completed AC LED products that can be industrially produced and have practical application system solutions. They can be directly plugged into AC 110V AC voltage of 60Hz or higher to make AC LED products emit light. Indicator lights, neon lights, and low-wattage lighting can effectively solve the disadvantage that existing LEDs cannot be used directly under AC sources, resulting in higher product application costs. Taiwan Industrial Research Institute's On Chip AC LED (on-chip AC LED) therefore won the 2008 R&D 100 Award, which is known as the American Industrial Innovation Oscar. At present, only the United States, South Korea and Taiwan have this technology in the world. The Taiwan Industrial Technology Research Institute has developed white, blue and green AC LED process technology, which is not only synchronized with the world, but also one of the world's leaders.

Compared with incandescent lamps, halogen lamps, fluorescent tubes, fluorescent energy-saving lamps, and DC LED lamps, AC LED lamps are more energy-saving, electricity-saving, longer-lasting, and more energy-efficient. AC LED lighting saves the costly AC/DC converter and constant current source. The performance comparison between AC LEDs and existing lighting fixtures is shown in Table 1.

Table 1 Performance comparison between AC LEDs and existing lighting fixtures

The major technological breakthrough of AC LED light source is the special arrangement and combination technology of ultra-fine LED dies during packaging, and the diode characteristics of LED PN junction are also used as rectification, and the semiconductor manufacturing process plays a very important role in it. The AC LED is integrated into a pile of tiny dies through a semiconductor process, using a staggered matrix arrangement process, and adding a bridge circuit to the chip design, so that the AC current can be bidirectionally conducted to achieve light emission. The arrangement of the dies is shown in Figure 2. The left picture is a schematic diagram of the AC LED dies in a staggered matrix arrangement, and the right small picture is a photo of the actual AC LED dies. The AC LED dies will emit light after being connected to AC. It only needs two leads to lead into the AC source to be able to emit light.

Figure 2 Photo and schematic diagram of AC LED die arrangement

The working principle of the AC LED light source is shown in Figure 3. A pile of small LED dies are equally divided into five strings using a staggered matrix arrangement process. The string of AC LED dies is similar to a rectifier bridge, and the two ends of the rectifier bridge are respectively connected to the AC source , The other two ends are connected to a string of LED dies, the positive half of the AC flows along the blue path, 3 strings of LED dies emit light, and the negative half of the cycle flows along the green path, and there are 3 strings of LED dies that emit light. The LED dies emit light in turns, and the LED dies on the opposite bridge arms emit light at the same time, and the middle string of LED dies keep emitting light due to sharing.

In 60Hz communication, it will turn on at a frequency of 60 times per second. The DC obtained by the rectifier bridge is a pulsating DC, and the light of the LED is also flashing. The LED has the characteristics of continuous afterglow after power failure. The afterglow can be maintained for tens of microseconds, because the human eye is inert to the memory of the flowing light point. The eye's interpretation of the working mode of LED light source luminescence + afterglow is continuous luminescence. The LED is working half of the time and resting half of the time, so the heat generation can be reduced by 40% to 20%. Therefore, the service life of AC LEDs is longer than that of DC LEDs.

Figure 3 Working principle of AC LED light source

Mature AC LED products such as Seoul's AX3201, AX3211 for AC110V and AX3221, AX3231 for 220V. For AC110V power at 3.3W-4W, working current 40mA; for AC220V power at 3.3W-4W, working current 20mA (Figure 4). The LED die is directly bonded on the copper-aluminum substrate. The pin is shown in Figure 5.

Figure 4 AC LED of Seoul Semiconductor

Figure 5 AC LED pin diagram

The typical application electrical schematic diagram of AC LED is very simple as shown in Figure 6. Connect the positive temperature coefficient thermistor PTC and current-limiting resistors R1, R2, R3 in series at both ends of the AC LED, and then connect to 110V or 220V AC. Enter lighting work.

When LEDs are mass-produced, their impedance has a certain degree of discreteness, and the same is true for AC LEDs. In order to facilitate the mass application of downstream manufacturers, LED light source manufacturers divide the mass-produced products according to impedance at the factory. When customers use According to the VF classification table provided by the LED light source manufacturer, the current-limiting resistance of the corresponding resistance can be checked. Table 2 shows the VF classification and current-limiting resistance table of AX3221/AX3231.

Figure 6 Typical application electrical schematic diagram of AC LED

Table 2 VF classification and current limiting resistance meter of AX3221/AX3231

The convenience of AC LED in household electricity does not need to install an AC to DC converter for the lamp like DC LED, which not only saves the cost of this converter, but also avoids that the LED light source itself is not broken, but The dilemma that the converter breaks first. AC-DC converter can be said to be an electronic component that will age and break over time. Its life is shorter than the LED light source itself. Therefore, many LED lamps are broken at present. It is not that the life of the LED light source has expired, but the LED The AC/DC converter used by the lamp broke first. Another feature of AC LED is that it uses a staggered matrix arrangement, which is turned on in turn. In 60Hz AC, it will turn on at a frequency of 60 times per second, which also makes the life of AC LED longer. The DC LED is long.

However, AC LEDs have two disadvantages at this stage. One is that the luminous efficiency is not as high as that of DC LEDs. This is because the development of DC LEDs is currently the mainstream. AC LEDs have just started. The luminous efficiency of AC LEDs can catch up or even exceed DC. LED's. The second is that AC LEDs have the risk of electric shock. Therefore, if AC LEDs are to be applied to LED lighting fixtures, the metal fins should be avoided from being exposed, and the heat should be removed indirectly. This is the core concept of the design of developing new liquid-filled LED solid-state lighting fixtures.

AC LEDs have just entered the growth stage and are currently not ideal in terms of brightness and power. However, the application of AC LEDs is simple, without transformer converters and constant current sources, and low cost and high efficiency have shown strong vitality. The technology of AC LED is developing by leaps and bounds. In a few years, high-brightness, high-power, low-cost products will be available in large numbers.