Lighting drive technology threshold is quite high
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â—It takes time for LED lighting to enter the general civil field
â—The key technology is monopolized by foreign countries
LED lighting has shown great potential, but its level of development is not balanced in different application areas. Therefore, the performance of the driver circuit market in different fields is also uneven. Wu Jianxing, general manager of Hangzhou Silan Microelectronics Power and Power Drive Product Line, said in an interview with China Electronics News that the main obstacle to the development of LED lighting market is the high cost, so LED lighting will be relatively insensitive to cost first. The field has been developed, such as landscape lighting, street lights and headlights; Hangzhou Mooli Semiconductor Technology Co., Ltd. and the general manager of MPS Hangzhou R&D Center have also expressed similar views. He believes that due to the high price, LED lighting Applications are still concentrated on municipal projects, and large-scale access to the general civilian sector may take three to five years.
Zheng Zhaoxiong, director of global sales and marketing for ON Semiconductor's power and portable products, also expressed to the reporter the company's emphasis on the LED lighting market. He said: ON Semiconductor will give full play to the company's technical expertise and mass manufacturing aspects in power management. Its strength provides LED drive solutions for automotive lighting, commercial lighting, outdoor lighting and general lighting applications.
Technically, lighting applications have also placed higher demands on LED driver circuits. Li Zaipeng, director of Fairchild's PCIA lighting department, told reporters that LED lighting driver circuits are increasingly focusing on stable brightness and power conversion efficiency, pursuing higher efficiency, higher integration and smaller form factor. As far as the system is concerned, high-power LED lighting technology involves many disciplines such as electricity, optics, thermals, mechanics, etc. Dr. Huang Wei, Chairman of Wuxi Jingkai Technology Co., Ltd. told reporters: From the point of view of realization, LED lighting technology includes power devices. And its high-pressure process technology, power integrated circuit design, thermal management, advanced packaging and optoelectronic integration technology and many other advanced manufacturing technologies. Compared with developed countries, China started late in the field of lighting LED driving and its foundation is weak, lacking core technology with independent intellectual property rights. Huang Wei said that the current driver IC chip technology for high-power LEDs is mainly monopolized by several large European and American semiconductor companies, especially for residential road lighting with a power range of 25W to 100W and main road lighting with an output power of more than 100W. Isolated flyback converters are needed, so only a few companies have mastered the technology. According to Huang Wei, the resistive TRIAC dimmer used for incandescent lamps is difficult to dim the capacitive electric light source such as CFL and LED. This has always been the bottleneck of LED drive technology. Jingkai Technology has developed it for 110V～260V AC. The first domestic AC/DC high-voltage chip for lighting applications breaks through this bottleneck with a number of original precision dimming ballast technologies, which can directly replace incandescent lamps with high energy consumption and realize LED freely adaptable resistive TRIAC dimming. The purpose of the device.
Indoor and outdoor applications have their own characteristics
â— Street lighting needs to adapt to harsh environments
â— Indoor applications should consider compatibility
In LED lighting applications, there is a clear distinction between outdoor lighting and indoor lighting. According to Wu Jianxing, the street lamp adopts AC power supply, and it needs to convert the AC into DC, and then achieve constant current control. The output power of the street lamp is large, the working environment is the worst, and the extreme temperature conditions need to be considered. At the same time, considering the maintenance cost of the street lamp, the reliability requirement of the product is also high; compared with the street lamp, the indoor lighting power is small, and the working environment is relatively good. However, it is sensitive to cost, and must also consider the compatibility with the original lighting interface, and also has strict requirements on the volume of the product.
Ren remote explained the difference between outdoor and indoor lighting LED drive circuits from the perspective of product structure. He said: Street lighting uses a three-stage architecture, namely PFC (power factor correction) + isolated DC / DC converter + step-down constant Stream converter. Usually the first two stages are designed and produced by the power supply manufacturer. It is a relatively independent power supply that provides a stable DC voltage to the luminaire, while the third stage is designed by the LED system manufacturer and is integrated with the luminaire. The third-stage circuit requires extremely high conversion efficiency due to the combination of LEDs with severe heat generation. Indoor lighting usually adopts a single-stage architecture. Although there is no final conclusion on whether PFC is needed, the industry has generally believed that if all the traditional incandescent lamps are replaced by LEDs, the pollution of the harmonics can not be underestimated, so PFC is still necessary; Incandescent and fluorescent lamps are often used in conjunction with dimmers. The replacement LED fixtures must be compatible with the original dimmers. In addition, depending on the application, the driver circuit can be isolated or not. isolation.
Extending life expectancy
â—Solving heat dissipation is the key to extending LED life
â—Thermistor bears the responsibility of LED device protection
The service life of LED products is also a matter of great concern to consumers. There is even a view that the short circuit of the driver circuit makes LED products short-lived. Zheng Zhaoxiong told reporters: LED products have a very long service life. If the reliability of the drive circuit is low, it will affect its service life and affect the life of the entire lighting system. Providing a highly reliable drive circuit itself is not difficult, and it is difficult to maintain high reliability while reducing costs. Therefore, it is especially important for designers to choose the right LED driver and external components to ensure that the reliability and lifetime of the driver circuit match the reliability and lifetime of the LED. In Li Zaipeng's view, the solution to the life problem needs to be selected according to the power of the device. He said that the non-isolated buck, isolated PSR flyback, single-stage flyback can be used according to the power from low to high. , quasi-resonant and LLC half-bridge topology for higher efficiency and precise constant current control. In the field of lighting, the biggest difficulty in driving circuits is how to solve the problem of heat dissipation. Ren remote said that if the heat dissipation effect is not good, the life of the electrolytic capacitor in the circuit will be greatly reduced, which will seriously affect the life of the LED product. The solution includes increasing the efficiency of the driver circuit, improving the heat dissipation capability, and even removing the electrolytic capacitor through some unique design in the circuit. Wu Jianxing believes that LED lighting is a system consisting of LEDs, electronic systems, heat dissipation systems, etc. These three are inseparable; the life of LEDs needs to be stable input voltage, constant output current, and environment below 60 °C. It can be reflected at temperature.
In addition to improving system efficiency, the industry also considers the use of special devices to protect the LEDs, thereby increasing the life of the system. The senior application manager of Tyco Electronic Circuit Protection Division said in an interview with China Electronics News that the performance and life of LED are closely related to the operating temperature of LED PN junction. The effective control of LED junction temperature is to protect LED. Essential requirements. Although this technology is becoming more and more popular, LED lighting manufacturers continue to overcome the problem of strong thermal sensitivity of LED light sources. Excessive heat or improper application can compromise the performance of LED light sources. When the junction temperature of the LED chip rises by 10 degrees Celsius, the luminous flux is attenuated by 1% and the LED lifetime is shortened by 50%. Overcurrent, overvoltage, and overheating can significantly reduce the LED's luminescence performance and shorten its lifetime. Therefore, in addition to good thermal design, safe and reliable overheat protection must also be considered. According to the senior application manager of Tyco Electronics Circuit Protection Division, Tyco Electronics first used PPTC (Polymer Positive Temperature Coefficient) material as a thermistor for protection circuits in 1980. This single-device solution provides protection against damage caused by improper power usage, transient immunity, reverse bias protection, and protection against overcurrent events. Resettable PPTC devices prevent LEDs. Damage caused by overcurrent and overtemperature faults in lighting applications. 
