Commercialization process and solutions of LED general lighting

With the trend of energy conservation and environmental protection, LED lighting has become the target of many regulatory agencies. For example, the 1.1 version of the solid-state lighting standard of the US Department of Energy's "Energy Star" program came into effect in February 2009, and the China National Institute of Standardization is also reportedly preparing to release the Chinese version of the LED lighting energy efficiency standard in 2010. It is expected that by 2012, the market size of LED drivers and related discrete devices will grow from approximately US$688 million in 2008 to US$1.308 billion, with an annual compound growth rate of 17.4%.

LED general lighting is still in its infancy

Although the prospects of LED are now hot, relevant data show that in the general lighting market dominated by replacement of light bulbs and fluorescent tubes, recessed lights, street lights and parking lights, task lights (desk lamps, cabinet lighting), etc., the proportion of LED lighting applications is still less than 1%. Taking outdoor LED street lighting as an example, most of them are currently only used as government demonstration projects, and it seems that it will take some time for them to be truly commercially popularized.

Zheng Zongqian, senior application manager of Hong Kong and China at ON Semiconductor, believes that the reason why LED street lights can be used as demonstration projects is, of course, first of all, due to their high luminous efficiency, low reflection loss of lamps, no need for high voltage, no mercury, and higher safety. Secondly, compared with ordinary lighting, LED lamps are easier to add digital or analog dimming functions, which is convenient for remote control, and can provide ultra-high brightness and color rendering lighting in special conditions such as foggy and rainy days. In addition, in terms of laying costs, power consumption costs and lifespan, LED street lights are much better than high-pressure sodium lamps.

However, the relatively high cost, low power reliability, short range, and inconvenience in working at high altitudes are still important factors hindering the rapid development of LED street lamps. On the other hand, if the government can issue LED road lighting standards in a timely manner, it will also be a very powerful means to popularize and promote LED street lamps.

Fairchild PLM lighting technology marketing senior manager Lee Ying-woo said that the use of LED street lighting is part of the government's energy efficiency project. The upgrade includes not only replacing bulbs, but also using long-life rectifiers. "The first step of the upgrade is to install LED street lights in newly developed areas, and then gradually replace the existing old street lighting. It is expected that in the next five years, most of the current street lights will be replaced by LED lighting."

Zhang Jing, sales manager of ADI in Central China, believes that compared with outdoor lighting, LED indoor lighting is in its infancy and currently has a low cost-performance ratio. This is partly because the lumen cost is too high, and partly because there are too few types of LED indoor lighting products, which makes it difficult for lighting designers to choose from, thus creating obstacles to the promotion of indoor lighting products. His suggestion is that the development and promotion of LED indoor lighting products should focus on product familyization, integration, intelligence and humanization, improve luminous efficiency and luminous flux utilization, and minimize the overall comprehensive lighting cost.

Key words for the challenge: "heat", "drive" and "cost"

Piero Bianco, business manager at Maxim, believes that "form factor" and "electrical compatibility" are key factors in the success of LED lights. "The reason is simple: the replacement lamp must be able to be installed in the framework of the previous lamp source, and of course, it must be able to work properly on the existing electrical architecture to ensure that there is no light flicker."

He further explained that the existing framework imposes physical limitations (i.e. the driver board must be small enough) and thermal restrictions on replacement lamps. These factors restrict the conditions for designers to replace lighting solutions (such as PAR, R and A-type specifications), and the smaller the application, the greater the difficulty, such as MR16 and GU10. Compared with the size factor, thermal restrictions are often more critical because, unlike other technologies, LEDs only emit visible light and do not generate infrared wave radiation energy. Therefore, LEDs are more energy-efficient than incandescent or halogen lamps, and most of the heat is dissipated through the conductors inside the lamp.

"Considering that heat dissipation is a key factor restricting the light intensity of the lamp, the LED technology currently used in lighting upgrade products is difficult to achieve the brightness level acceptable to the mainstream market. Therefore, in order to break through the brightness limit, the heat dissipation problem must be solved, which is also an indispensable condition for the successful commercialization of the product." Piero emphasized.
Heat dissipation affects not only the light intensity, but also directly affects the drive

The life of the driver board. Piero said that in order to emit higher light intensity, the lighting must work at a very high temperature (+80℃ to +100℃). At this temperature, the life of the driver board will limit the performance of the entire lighting, especially the electrolytic capacitor, which becomes a difficult problem for the design. Because the electrolytic capacitor will dry out quickly at high temperatures, its working life will not exceed thousands of hours, which also becomes a factor restricting the service life of the entire lighting.

Tony Armstrong, product marketing director of Linear Technology's power products, analyzed that the choice of LED driver circuit depends largely on the input voltage range, the cumulative voltage drop of the LED string itself, and the current required to drive the LED, which leads to a variety of different feasible LED driver topologies. "But no matter which topology, an optimized LED driver is very important for the reliability and life of LED products."

In Zheng Zongqian's view, in addition to the fact that the current light quality of LEDs is still inferior to that of traditional lighting, another issue that cannot be ignored is that since LED lighting involves complex designs in terms of electricity, light, heat, mechanics, etc., there is currently a lack of unified design standards, which directly leads to the uneven quality of LED lighting products on the market. This is also one of the reasons why it is difficult for the public to accept it in the short term.

In addition to the difficulties and challenges mentioned in the article, cost pressure is also unavoidable. In this regard, Zheng Zongqian and Li Yingyou both said: "Compared with CFL lamps or incandescent lamps used in traditional lighting, the price of LED lamps is still expensive, which has also greatly hindered the development of LED lighting. While high reliability has become a key factor in LED lighting, it has also increased the cost pressure of the system."

LED Lighting Solutions

Piero focused on Maxim's LED driver solutions for 120VAC/230VAC input and 12VAC input lighting. It is reported that this type of solution uses single-stage conversion, and by shaping the input current, the lighting will not flicker in the dimming state, and is compatible with bidirectional triacs, trailing edge dimmers and electronic transformer designs. On the other hand, since no electrolytic capacitors are required, it not only helps to reduce the size of the solution, allowing the driver board to be installed in a small-sized replacement lamp frame, but also extends the life of the LED lamp from the usual less than 10,000 hours to 90,000 hours.

Li Yingyou suggested choosing four topology solutions based on the power rating of the application:

*Since small PCB area and low BOM cost are important factors for using globe-type embedded bulbs, PSR flyback solutions are mainly targeted at low-power applications (<15W). Its unique selling point is Fairchild's proprietary TRUECURRENT technology, which can reduce CC/CV variation to +-5%, and does not require secondary-side feedback circuits and can select built-in MOSFET devices, which can reduce PCB size.

* The single-stage PFC flyback solution with CRM PFC controller is mainly targeted at mid-range power applications (<40W) and is said to support high efficiency, high power factor, low THD and low bill of materials cost.

* For high power applications (40W~), PFC+flyback solutions for <100W applications and PFC+LLC half-bridge solutions for >100W applications are good choices. The latter can increase efficiency to more than 90% due to the use of zero voltage switching (ZVS) technology.

ON Semiconductor also has LED general lighting solutions suitable for AC-DC power supply of different powers. Take its NCL3000 as an example. This is a single-stage power factor correction TRIAC dimmable LED driver for residential and commercial AC-DC powered LED lighting applications with a power of 8W-25W. It uses a CrM flyback architecture and a single-stage topology to provide a high power factor greater than 0.95, thus eliminating the DC-DC conversion stage. In addition, the single-stage PFC LED driver NCL30001, CrMPFC+QRPWM driver NCP1607+NCP1377, and CrMPFC+LLCPWM driver NCP1607+NCP1397 or NCP1392/3 are mainly for street lighting and high-power area lighting applications with a power of ＞50W, and the typical energy efficiency can exceed 92%.

ADI focuses on AD converters, and its various 12-16-bit ADCs are suitable for use in LED street lights and indoor lighting. For example, AD7457, AD7466, and AD7153 and AD7745 that implement CDC (capacitance to digital conversion) functions can realize automatic switching of LEDs to achieve power saving.