LED driver and power chip development status and future trends

He Yirui, deputy director of the technology marketing department of Taiwan Accumulation Technology Co., Ltd., introduced that with the improvement of the luminous efficiency of LED chips, products of the same brightness can be driven with less power, and the problems of heat and cost are further reduced, and the market will Further expansion. At that time, various lighting applications will be introduced into LEDs as a light source for related applications.

The drive itself faces a smaller load power, and the efficiency of the power conversion must be further improved to avoid dragging the efficiency of the overall luminaire. On the other hand, it is also necessary to provide a stable and reliable power supply for the LED chip, to ensure the working life of the LED chip and to provide a protection function to avoid damage to the lamp in an unexpected situation.

Maxim's Piero Bianco emphasizes that one of the hot issues of LED technology is cooling, especially for retrofit lamps and small lamps such as the MR16. In addition to the heat sink, the cooling problem is also related to the drive, because operating the lamp at high temperatures means that the drive will become hot, which will reduce the life of the drive. Electrolytic capacitors are particularly sensitive to high temperatures. Fortunately, Maxim's existing drive solution eliminates the need for electrolytic capacitors, or although electrolytic capacitors are used in the drive, drive performance is hardly affected when performance due to high temperature electrolytic capacitors is degraded.

Key factors affecting the development of LED lighting

Then, the world is currently focusing on LED lights, chips and applications. But now the average life expectancy of LED is still relatively limited, some people think that the key is still insufficient driving force. So what are the main factors affecting the development of the LED industry?

Fairchild's Donghye Cho believes that if LEDs are to enter the general lighting market, it is important to establish industry-recognized standards and reduce costs. Other issues to be solved are still yield, luminous efficiency (lm/W) and heat dissipation. Yield directly affects LED manufacturing costs and system BOM (Bill of Materials) costs, which account for 70% of the overall BOM cost of LED lighting applications.

Xi’an Pucheng Technology Green Energy Division Xiao Enzhen pointed out that with the development of LED technology, the lumen of unit power has been continuously improved, and most of the LED bulbs currently in mass production in the market can reach more than 60 lm/W. In the next few years, LED cooling technology will continue to be optimized, and the lumen conversion efficiency of LED will continue to increase, truly achieving the goal of energy saving.

LED drivers are still in the "Warring States era". Diversification and flexibility are the embodiment of LED drivers in the market. There will be no fundamental changes in the next two to three years. However, with the constant standardization and convergence of market demand, LED drivers will gradually become a standard. Taking the lighting LED bulbs that are currently developing rapidly in the market as an example, at the beginning of the market, the isolation scheme of the driver chip is almost necessary for each solution designer, but with the development of the bulb housing technology, the volume of the bulb is continuously shrinking, the price The reduction of the LED bulb drive scheme has also undergone major changes, and the advantages of the non-isolation scheme will gradually emerge and be accepted by many customers. At present, LED bulbs are mostly in the range of 4~6W, and the power factor is not high. However, the requirements for power factor are imperative in the future. The LED driver integrated power factor correction function will become the mainstream in the LED bulb market.

He Yirui, who is accumulating in Taiwan, believes that in terms of LED luminaires, the most relevant to life is nothing more than the circuit part of the luminaire. Usually, the luminaire manufacturer needs to pay attention to two places when designing: one is the relationship between the temperature and the life of the LED chip during operation, and the other is the life limit of the driving power body component.

Factors affecting life

From a technical point of view, the life of LED itself is often tens of thousands of hours, but sometimes 8,000 hours is good [1]. What is the cause of this phenomenon?

Zheng Zongqian, senior application engineering manager of ON Semiconductor China, believes that this is because lighting system design engineers use lower-level external components in order to reduce costs in LED driver circuit design, such as lower rated operating temperature or lower precision. Resistor, etc., will reduce the reliability of the drive circuit, greatly shorten the service life, and cannot match the life of the LED itself. Therefore, a major challenge for engineers is to reduce costs while maintaining high reliability.

From the market point of view, as LEDs are improving in terms of brightness and energy efficiency, the pace of development in the commercial and consumer markets is inconsistent for the LED lighting industry. Commercial lighting such as building lighting and retail lighting is the first to apply LED general lighting. The overall cost of ownership, reliability and convenience are driving LED lighting in this field. In consumer markets such as residential applications, cost is still a key factor affecting the popularity of LED lighting.

Heat dissipation or light decay

Many LED companies believe that chips and heat sinks are the source of LED life, but they have not noticed the problem of light decay caused by LED power supply , which has caused consumers to buy and go back less than two years and become like candles. So how to better solve the problems of heat dissipation or light decay?

He Yirui, who is accumulating, believes that the calculation of the lifetime of the LED chip body is usually determined by the degree of light decay (for example, some estimation methods are 30% or 50% of light attenuation). The degree of light decay depends on the light decay coefficient and power-on time at a certain temperature, and there is an exponential relationship between the two. Therefore, when the temperature rises too high, the light decay speed increases sharply, which causes the LED chip life to be greatly attenuated.

In addition, when the output current ripple of the power supply terminal is too large, it may also have a significant impact on the life of the LED chip. It is important that the power driver design can output high stability and high precision current after satisfying high efficiency or even high PF (power factor) value. By selecting a suitable power supply scheme, it is possible to ensure that the life of the LED chip is within a controllable range and avoid abnormal LED light decay.

In a drive power body, most semiconductor components can withstand higher ambient temperatures. However, some components such as electrolytic capacitors have extremely high environmental conditions. In addition to the longer life specifications of the component selection specifications, the circuit design can also obtain a higher power conversion efficiency by selecting an appropriate power supply scheme, thereby reducing the thermal energy accumulation of the circuit inside the lamp.

Fairchild's Donghye Cho gives a better solution to the problem of heat dissipation or light decay, using a linear balance technique with margin control? A metal core PCB is a good alternative to reducing the heat generated by the drive circuit. Otherwise, the forward voltage may vary with temperature. In some cases, temperature compensation circuits can also be considered to reduce heat generation.

The definition of "lifetime" varies from industry to industry, and the useful life of a semiconductor device is defined as the time when the level of luminescence is attenuated by 50% from its original value. For the lighting industry, the average life of a particular bulb type is 50% of the life of such bulbs. The life of an LED depends on its package configuration, drive current, and operating environment. Higher ambient temperatures can significantly shorten the life of the LED.

In order to reduce light decay, power factor and precise continuous current control must be improved in the design of the application circuit.

According to Zheng Zongqian of ON Semiconductor, the key challenges in LED lighting design include managing temperature and heat or heat dissipation to keep the LED's operating temperature below their junction temperature. In fact, the LEDs illuminate from the substrate. In the process of generating light, the LED also generates heat, and this heat causes the junction temperature of the LED to rise. Since the LED is a semiconductor device, as with any semiconductor device, it is important that the LED operates at a temperature below its maximum junction temperature of 150 °C. More specifically, it is recommended that the LED operating temperature is always below 100 ° C; once above 100 ° C, the reliability of the LED may be affected. To solve the problem of heat dissipation, it is necessary to improve the conversion energy efficiency of the driving power supply and the LED with higher luminous efficiency, the LED driver chip that provides strong thermal fault protection performance, the driver chip package with better heat dissipation performance, and the LED substrate with better heat dissipation performance. Wait.

In order to extend the life of the LED, in addition to heat dissipation problems, it is necessary to deal with potential electrostatic discharge (ESD) damage. Integrating ESD protection into the LED module package is a suitable solution. ESD protection can be applied as an additional die next to the LED emitter die or as a submount for bonding the LED emitter die above in a more compact layout, see Figure 6.

Pu Cheng Xiao En said that the mismatch between the driving power and LED life is a factor in the current life reduction of LED lamps. At present, LED power supply on lighting still uses electrolytic capacitors, the circuit is more complicated, and there are many external components, which makes the life of the whole lamp greatly reduced. In the field of LED bulb lighting, future drive solutions will be highly integrated with electroless capacitors and driver chips. At the same time, the driver chip needs to solve the heating problem of itself and the controlled device, which is also a non-negligible part of the heat source of the lamp.

Full color LED display and signage

Last year, Shanghai World Expo shared one billion LEDs, most of which were used for RGB (red, green and blue) three-color LEDs, such as the Qingming Shanghe map of the China Pavilion.

Markus Luidolt, marketing manager for the flat panel light source business at the Austrian Microelectronics Consumer and Communications Division, said that LEDs with a lifespan of up to 100,000 hours have been developed for demanding applications such as outdoor display backlighting.

According to the analysis of CCID's Zhang Xiaokang, in the field of LED display driver chips, the constant current driver IC used in full-color LED display will become the main battlefield of market competition, which can meet the higher uniformity and reliability of LED display and integration. LED display driver ICs with higher functions, more functions and lower cost have become the focus of product development and market attention at present and in the future.

Recently, TI introduced a serial control 16-channel constant current LED driver with 4-channel group delay - TLC59282. Wu Tao, a power product business development engineer in TI China, explained in detail the various considerations for the launch of the product. Because the current 16 channels are the trend of the whole market, 8 or 24 channel chips are not excluded, but 8 and 24 are more for some special applications and some customers' chips. The TLC59282 chip released this time supports a current range of 2 to 45 mA, of which numbers 2 and 45 are important. 2 means it supports a minimum of 2 mA per channel. What are the benefits? The LED display now has normal and high-brightness LEDs. For ordinary LED displays, the drive current is generally 15~20mA. Although the high-brightness LED is more expensive, it can reduce the drive current. For example, the operating current of 8mA or 5mA can achieve the same brightness of the LED display. If the LED driver supports a minimum of 5 mA, its accuracy of current regulation and long-term service life will be flawed. Usually we have to leave some margin for the indicator. For a 5mA application, usually 3~4mA LED driver can ensure 5mA can be used well. So why is 2mA important? Because it can support high-brightness LEDs. In addition, when the ordinary LED is used during the daytime, because of the sunshine, the LED light can be seen very brightly; however, the LED lamp current can be significantly reduced at night. Therefore, when using 15 mA normally, it may only require 5 mA or 8 mA at night. Also, for some low-gray screens, lower currents may be required, such as displaying a person wearing a gray suit, LEDs for a short period of time, or making the LEDs low in brightness, and low currents to ensure LEDs at low gray levels. use.

45mA is mainly for LEDs that require large currents. If the LED shows a static screen, then one channel is connected to one lamp, and the LED current is 15~20mA. If it is a scanning screen, a channel has a total of 4 lights connected, and the MOS tube is used for switching. At this time, the current of the LED lamp needs to be pulled up, that is, in some applications, the LED lamp needs 40~50 mA driving current to be normal. jobs. Therefore, for static display, LED operating current of 15~20mA is enough, but the scanning screen needs 30~40mA current.

Therefore, the benefit of 2mA is that it can meet the display requirements of low-gradation, high-brightness, low-current LEDs, and 45mA can meet the needs of scanning screens.

Medium and large size LCD panel LED backlight

According to Zhang Xiaokang of CCID, with the gradual start-up of the large-size LCD panel LED backlight market, its driver IC will become a hot spot, which is the number of channels for the driver IC, the output voltage, the stability of the output current, the reliability of the driver IC, Circuit protection functions, cost and price control have raised higher requirements.

According to Austrian microelectronics Markus Luidolt, in general, the driving current required for LED backlighting is increasing. For example, LEDs used in large LCD backlights typically have a driving current of 50-100 mA last year, and in 2011, their driving currents Almost doubled, driving currents up to 300mA in 3D TVs.

Among the driving forces affecting the LED chip, the light output of the LED and the cost reduction are the dominant factors. The cost per lumen, that is, the cost paid by the user for lighting, is rapidly decreasing. Heat dissipation is also important, so optimizing the PCB (printed circuit board) layout is critical. More importantly, it is to avoid heat dissipation. Instead of making extra efforts in heat dissipation, it is better to use a more efficient drive architecture, such as intelligent adjustment of LED power supply.

In terms of research and development trends, in very thin LED TVs and ultra-thin mobile phones, LEDs have more and more light output, and packaging has become more and more slim. Adopting a better color gamut to make lighting more natural is also an important development trend in the industry. For LED driver manufacturers such as austriamicrosystems, there is a need to work more closely with LED manufacturers to develop drive solutions that are geared to the latest LED technology trends.

Pu Cheng Technology Xiao Enzhen introduced that the backlight technology for liquid crystal display TVs is mainly divided into two types: direct type and side type.

The direct-lit backlight technology can quickly fine-tune the brightness of the LED according to the brightness of different parts of the picture. Therefore, the current of each output channel of the LED driver needs to be able to be independently controlled. Since the direct type requires relatively more LEDs, the problem of heat dissipation is overcome to integrate more channels into the future of direct-drive drivers. The side-lit backlight technology uses relatively few LEDs, so its driver needs to output a large drive current to provide the same brightness.

At present, most of the side-illuminated functions only provide dimming function, and the control of the current output channel is relatively simple, so that it is easier to integrate the DC-to-DC converters in a relatively straight-down manner. For side-illuminated drivers, how to increase the output current and provide withstand voltage to increase the number of LED drivers will become the key to stand out.

Headlight

Tony Armstrong, Product Marketing Director, Linear Technology's Power Products Division, introduced the application status of the lights: Audi was the first car manufacturer to use LED headlights in a variety of models, and it has been in use since 2008. Audi's automotive lighting system consists of two main low-beam headlights. The system consists of two LED arrays, each with four active components. The other three LED arrays (each containing 2 LED ICs) are located behind the optical lens and their task is to control the light/dark boundary and headlamp illumination range. For high beam headlights, the four LED arrays are located adjacent to the low beam array. Near the lower edge of the lighting system, a row of 24 LEDs form the daytime running lights (DRL). However, it is these daytime running lights that make the Audi car look new and its appearance is very popular with car buyers. Soon after, many other manufacturers followed suit, and Mercedes-Benz, Jaguar, Lexus and Porsche also provided similar daytime running lights.

The LED headlight system is lagging behind in adoption, which is mainly due to cost. However, some early signs of 2011 seem to indicate that this situation will begin to change, because in fact, LEDs allow for more flexible design choices, while LED efficiency increases the cost/performance balance. Therefore, we now see that LED headlights are available as an optional accessory for all models of Cadillac, Toyota and Lexus. For example, Cadillac Escalade, Lexus LS600H and Toyota Prius can be equipped with LED headlights.

Maxim believes that LEDs have been widely used in automotive backlights and are now being used in automotive daylight running lights, position indicators, high beam lights and short focal length lights on the front. In this area, the main problem is to have a very reliable LED driver - should be able to work in an environment with a wide range of input voltage and temperature; in addition, the driver must be able to prevent short circuits or other faults. Maxim has begun to pay attention to this market, its driver IC can work in the above-mentioned severe environment, and at the same time, the IC integrates a variety of protective components, thereby reducing the number of components and reducing the solution. cost.

In order to be suitable for automotive-grade applications, LEDs and chips need to be modified. For example, Japanese Roma products are generally packaged in a ceramic package with high heat resistance, -40~130 °C. Resin and sulfur resistant packages are generally used.

Portable application

Linear Technology introduced the camera flash function on mobile phones. Many mobile phones today have built-in digital camera capabilities. The increased performance of the camera also results in the need for a high power white light source to use the camera indoors or in dim ambient light conditions. Among the cell phones equipped with cameras, white LEDs have become the main source of light, because white LEDs have all the features that modern cell phone designers want: small size, large light output, "flash" and continuous "video" Object illumination. Large output power LEDs have been developed specifically for use as integrated camera sources.

Austrian microelectronics Markus Luidolt said that overall, the driving current required for LED lighting is increasing. For example, in the mobile phone flash application two years ago, the LED driving current range was 300~800mA, but now it is 1000mA, 2000mA or higher. .

ROHM began producing LEDs in 1930 and is committed to miniaturization. Especially popular in the market is the 1006 size (1.0mm × 0.6mm), the thickness of only 0.2mm small LED chip products - PIC OLED series, called the thinnest product, display lights and backlights for small electronic devices Products for other purposes. One of the PICOLED-eco series is capable of achieving twice the brightness of previous products in the low current range of 1 mA (Figure 7). It is reported that many manufacturers have adopted PICOLED-eco on the mobile phone keyboard.

Some new products and research and development trends of some companies

Fairchild's latest achievement is a linear current balancing controller using a high voltage process.

From a system perspective, ON Semiconductor offers high-reliability products that take into account a variety of factors including electricity, heat and light, including a wide range of AC-DC and DC-DC power-efficient LED driver solutions.

Pucheng Technology has been engaged in the development of LED driver chips for many years and has recently entered the Japanese LED market. In addition, the PFC integrated series driver IC was developed for the future demand of high power factors.

ROHM is developing an LED driver IC with TRIAC dimming, featuring thyristor/PWM/linear dimming for a variety of dimming applications (dimming ratio 0.2% to 100%). A unique system that combines a variety of LED components, driver ICs, and illuminance sensors is also being developed.

On TI's linear AC/DC LED lighting solutions, only one power conversion chip is available. There are also a series of switch-type chips, including buck, buck-boost and boost, such as TPS62110 and TPS54160.

The Austrian microelectronics LED driver product line is optimized for high efficiency, extending the battery life of mobile devices and making LED TVs even thinner.