Whether high-power or low-power LED lighting applications, they are generally composed of power supply, LED driver, led, lens and substrate. The key element is led driver. It must provide a constant current output to ensure that the light emitted by LED will not flicker and lead color deviation. It generally accepts 24v-48v DC voltage input, However, some advanced LED drivers can directly accept 220V mains AC input. Most customers require high-performance LED drivers to meet the upcoming LED lighting specifications such as “pf value > 0.9” and “+ 85 efficiency”.

The main design challenges of LED lighting applications include the following aspects: heat dissipation, high efficiency, low cost, dimming without flicker, large-scale dimming, reliability, safety and eliminating color deviation. These challenges need to be solved by comprehensive use of appropriate power system topology, driving circuit topology and mechanical design.

Liang houquan, Asia Pacific Technical Marketing Director of diodes, pointed out: “for designers, the biggest technical challenges will be high efficiency requirements, optical design, thermal management and improving the reliability of some applications, such as high brightness LED street lamps. We now provide a series of LED lighting solutions covering applications from 0.1W to 250W.” “In the application of LED lighting system, in addition to selecting appropriate LED products, a complete LED lighting design also includes optical design, thermal design, product design and electrical drive design. Since LED is a low-voltage device, converting high-voltage AC power supply to low-voltage constant current drive for LED will face many challenges.” Zheng Zongqian, senior application manager of ansenmey semiconductor China, pointed out: “further, in order to ensure the advantages of LED lighting, led electrical drive must be reliable, efficient, safe and low-cost. Therefore, for different LED lighting applications, it is first necessary to select the correct driving circuit topology.” Ansenmey semiconductor can provide all LED lighting solutions in the power range from 1W to 500W. In order to quickly promote the take-off of LED lighting market, National Semiconductor Corporation (NSC) Recently, it aimed at a very large direct replacement market for incandescent lamps, that is, using LED lamps to directly replace incandescent lamps in existing families or other application markets, and launched a direct mains input LED driver chip lm3445 for this market.

However, Wu Zhimin, marketing manager of NSC Asia Pacific power management products, said: “Incandescent lamps and electric lamps have been available for many years. For many technical standards that have not been changed for many years, our home lighting system has been using many technical standards that have not been changed for many years. This situation can not be changed overnight. For example, based on the problems of heat dissipation and lighting angle, the original old electric lamp socket or device is not suitable for installing LED bulbs. But in addition to technology In addition to the problem, cost-effectiveness is also the biggest reason why LED lamps cannot be popularized. The selling price of tungsten bulb and neon tube is about US $0.6-0.7/klm, but at present, the selling price of LED lamps is still as high as US $40-50 / KLM. ” As mentioned above, since the LED lamp must be installed in the original old socket, heat dissipation is a big problem that must be overcome. But strictly speaking, this problem can be solved by mechanical engineering technology. The responsibility of the LED system manufacturer is to strive to develop new technologies to improve the brightness of the LED as much as possible (i.e. the amount of lumens per unit power). Wu Zhimin said confidently: “We can provide the most efficient LED drivers to ensure that the heat dissipation of the whole lighting system can be minimized.”

The relatively high cost of LED is the main obstacle to the large-scale take-off of LED lighting market. For example, Alexander sommer, product marketing director of power management business department of Infineon Technology Co., Ltd., said: “most typical LED lighting applications less than 25W are sign lamps, identification lamps, and alternative standard incandescent and halogen lamps. However, compared with the existing fluorescent and incandescent technologies, the initial cost of LED is still a major obstacle to entering the mass market.” Xu Ruibao, engineer of cytech’s product and design department, also agrees that the main commercialization challenge is cost. He said: “at present, LED lighting systems with various powers can be realized in the circuit. The technical challenges come from the requirements of terminal applications, such as optical design and overall heat dissipation design for automotive applications. The challenges of commercial deployment mainly come from LED costs.”
 

Heat dissipation considerations in LED lighting design

LED lighting systems below 25W are generally designed for applications such as reading table lamp, corridor lamp, living room spotlight, household meal lamp, small night lamp, etc. customers generally hope that such applications can be designed as small as possible. Therefore, the design space for PCB placement is relatively small, so the temperature in the packaging space may be very high during long-term use. Since the designer is unlikely to install a cooling fan in it, its cooling design becomes very critical and important.

“Most low-power LED lighting applications less than 25W require a certain degree of miniaturization. This often leads to higher power density, although the power consumption is not very large. Sufficient heat dissipation management measures must be provided through improved mechanical structure. In addition, high electrical efficiency helps to reduce power consumption,” Alexander Sommer pointed out, “If further reduction of thermal resistance is required, this can be achieved by electrical isolation because it can achieve the most efficient heat transfer. These methods also allow optimized lumen output.”

Another way to prevent led from overheating for a long time is to use dimming solution. Sangcheol her, high voltage IC Product Marketing Manager of Fairchild Semiconductor, said: “Compared with fluorescent lamps and incandescent lamps, dimming solution is an important way to reduce LED power consumption. This scheme is realized by dimming controller. Especially for LED driving solutions less than 25W, this scheme is more important because of the small PCB size, Limited packaging space and unavoidable heat dissipation problems.”

In fact, in this power range, LED lamps will replace halogen lamps and compact fluorescent lamps (CFL). In addition, in order to get rid of the problem of heat dissipation, advanced technology must remove passive components sensitive to temperature changes, such as electrolytic capacitors. However, at present, most LED driver solutions are derived from and based on the power supply topology, so the limitation of temperature range should be considered, because general products are usually based on commercial standards, but lighting lamps must ensure that they can adapt to harsh environments such as industrial environments.

Architecture selection of LED lighting design

The choice of LED lighting system architecture depends on whether your design goal is low cost, high efficiency or minimum PCB area. Generally speaking, LED lighting systems less than 25W do not require power correction, so a simpler topology, such as PSR or buck topology, can be adopted. 25w-100w LED lighting applications require power correction, so single-stage PFC, quasi resonant (QR) PWM or flyback topology are generally used. LED lighting applications above 100W generally adopt more efficient LLC topology and two-stage PFC.

“LED lighting solutions with power below 25W can adopt PSR or buck topology, because this power range is mainly for small design and emphasizes the simplicity of design. Medium power solutions (25w-100w) are suitable for single-stage PFC, quasi resonant (QR) PWM and flyback topology.” sangcheol her said, “high power solutions (greater than 100W) LLC, QR PWM and flyback topology design are suitable. From the perspective of efficiency, LLC and QR have better performance; while PSR scheme does not need secondary feedback, has simple design and smaller size than other schemes. “

Zheng Zongqian also said: “LED lamps less than 25 W are mainly used in indoor lighting. They mainly use low-cost flyback topology. The ncp1015 and ncp1027 monolithic conversion integrated circuits of ansenmey semiconductor integrate built-in high-voltage MOSFET and PWM controller, which can effectively reduce the area of PCB and the volume of lamps and provide the maximum 25W power output (230V AC input).”

“For non isolated LED lighting applications less than 25W, if the input-output conversion ratio is low, a simple buck converter can be a low-cost and small volume choice. In the isolated topology that attaches importance to efficiency, the quasi resonant flyback topology such as Infineon coolset ice2qs series devices is a good choice,” said Alexander Sommer. Infineon is the first supplier to provide digital quasi resonant flyback control IC.

Typical LED lighting applications in the 25w-100w power range are street lighting (community roads) and public places such as parking lots. Power conversion efficiency, cost-effective implementation of PFC function and high color quality are now the three most important technical challenges. For example, in commercial lighting and street lighting applications, longer service life and the resulting lower maintenance costs are helping to overcome entry barriers with higher initial costs. 25W to 100W LED lighting applications have power factor requirements, so power factor correction circuit needs to be added.

“This circuit can adopt the traditional two-stage structure, that is, active discontinuous mode power factor correction (PFC) circuit and DC-DC PWM conversion circuit, such as ansenmey’s power factor correction controller ncp1607. The peripheral circuit of ncp1607 is very simple and can provide good performance,” Zheng Zongqian said, “For the LED scheme with high efficiency, low cost and small volume, the single-stage PFC circuit is recommended. It can realize power factor and isolated low-voltage DC output at the same time, and has significant cost advantages. It will become the mainstream scheme of medium power LED lighting. The ncp1652 of ansenmey Semiconductor provides the best control scheme for realizing single-stage PFC circuit.”

Shenzhen Shiqiang Telecom adopts c8051f3xx series 8-bit MCU of silicon labs to realize PFC in software mode. Huang sunfeng, assistant marketing manager of the company, said, “we aim at household mains power (180v-260v) The fully digital LED lighting scheme developed by the application of input 10W – 30W low-power LED lighting can realize PFC value up to 0.95 by software control. Compared with hardware PFC, this software scheme also has higher flexibility, adaptability and scalability on the premise of ensuring the same performance index. “The maximum output current of LED driver mic3230 adopted in this scheme is 350mA, It can drive up to 12 1W LEDs, which can well meet the needs of indoor lighting.

Alexander Sommer said, “for 25w-100w LED lighting applications requiring efficiency and performance over a wide input and / or load range (such as dimming), it is recommended to use a quasi resonant flyback topology with an independent PFC level. Typically, up to 90% efficiency can be achieved.”

Applications above 100W include major road and highway lighting (where brightness of up to 20K lumen or above and power input of 250W are required) and professional applications, such as stage lighting and building floodlighting. A key driving force for the use of LEDs in high-power applications is the low cost of ownership due to reliability and low power consumption. For example, its system efficiency can be compared with metal halide and low-pressure sodium lamps. The initial cost comparison may continue to be the entry threshold of the market in the short term.

Zheng Zongqian pointed out: “for LED applications greater than 100W, we will use the traditional active discontinuous mode power factor correction circuit and half bridge resonant DC-DC conversion circuit. We have introduced a new integrated controller, which integrates active discontinuous mode power factor controller and half bridge resonant controller with high voltage drive.”

The half bridge resonant controller works at a fixed switching frequency and a fixed duty cycle, and the circuit does not need a feedback control loop on the output side. This makes the half bridge resonant DC-DC conversion circuit work in the most efficient ZVS and ZCS states. The DC output voltage will follow the output of the power factor correction circuit.

Alexander Sommer stressed: “for higher power level LED lighting applications above 100W, efficiency becomes more important. It is recommended to use LLC resonant topology, which can achieve more than 90% efficiency. We recommend that you use Infineon’s new 8-pin device ice1hs01.”

Regardless of the output power of the LED lighting system, the selection of the LED driver circuit will largely depend on the input voltage range, the cumulative voltage drop of the LED string itself, and the current sufficient to drive the LED. This leads to a variety of possible LED driver topologies, such as Buck, boost, buck boost and SEPIC. Tony Armstrong, product marketing director of power products Department of linglilte company, pointed out: “Each topology has its advantages and disadvantages. Among them, the standard step-down converter is the simplest and easiest to implement, followed by the step-up and step-down boost converters, while the SEPIC converter is the most difficult to implement, because it adopts complex magnetic design principles and requires the designer to have superb switching mode power supply design expertise.” In a word, the application of terminal products determines the topology of LED, and then reasonably select buck, boost, SEPIC, or buck boost structure according to the topology of LED and input power supply. “Generally speaking, there are more bucks below 25W. Those with higher power tend to choose the boost structure. In terms of efficiency, both can achieve more than 85%, and the lt3755 can achieve up to 97%. When considering the BOM cost of the driving part, the overall system cost should be considered,” Xu Ruibao said, “With the intensification of competition, there will always be lower BOM cost solutions, but it is not necessarily the most appropriate. We do not recommend designing products according to this standard. PCB area is mainly controlled by main components, and low-power LED lamps should adopt high-integration solutions as far as possible. High-power solutions should choose products with high technical integration and simple peripheral circuits. All discussed here refer to DC-DC Solutions. “

Liang houquan also pointed out: “In order to achieve high efficiency, the switch mode LED driver should be considered. Most of these customers prefer the step-down LED driver because the overall efficiency is higher. If considering the lowest BOM cost, the switch LED converter is not the cheapest. Such customers may try to use linear constant current LED driver. This can provide the lowest BOM cost, However, the efficiency may not be as high as that of switch mode LED drivers. For example, from the perspective of minimum PCB area, switch mode converters will usually be selected because they generate less heat and even the volume of related components will be smaller. “

Analog, PWM and triac dimming schemes

LED dimming solutions and specifications are constantly changing, and have not been fixed until now. Therefore, there are three dimming schemes in the market: PWM, analog and thyristor (traic). PWM and simulation methods are relatively simple, but it is necessary to build a dimming infrastructure and a new dimming controller.

The disadvantage of the analog dimming scheme is that the adjustment range of LED current is limited to a maximum value to about 10% of the maximum value (10:1 dimming range). Because the chromatography of LED is related to current, this method is not suitable for some applications.

PWM dimming scheme switches between zero current and maximum LED current at a rate fast enough to mask visual flicker (usually higher than 100MHz). This duty cycle changes the effective average current to achieve a dimming range of up to 3000:1 (limited only to the minimum duty cycle). Because the LED current is either at the maximum value or turned off, this method also has the advantage of avoiding LED color deviation when the current changes, which is very common when analog dimming is used.

Sangcheol her is optimistic about the market prospect of triac dimming scheme. He said: “thyristor (triac, 2-wire dimming) will become a very popular solution because this technology can completely use the traditional system without any change. Moreover, it can also be extended to 3-wire dimming to avoid defects related to low power factor values.”

Triac dimming is a very hot topic in the industry today. Initially, triac dimmer was designed for incandescent lamps, but most users hope that the same triac dimmer can also dimming alternative LED lamps. Liang houquan said: “diodes Zetex can now provide customers with all dimming solutions (including PWM, analog and triac). For example, zxld1362 LED driver uses an adj pin to realize analog and PWM dimming, which brings great design flexibility to customers.”

However, Zheng Zongqian believes that the application scheme of triac dimmer in the market should be only transitional. In the long run, PWM dimming should be used. He said: “The main three decisive factors are: 1) there will be no flicker from zero to the lightest with PWM dimming. 2) the performance will be better. Because the dimming output power adopts the power factor correction circuit, which is in line with the global mandatory requirements for the power factor of lighting. Although this requirement is generally from 25 W, the United States requires the mandatory power of lighting from zero watt Factor correction circuit. If traic dimming is adopted, the power factor will be sacrificed and the complexity of the circuit will be increased. Therefore, PWM dimming can provide the choice of the best performance, which is also the trend in the future. 3) the cost will be better. Adjusting the duty cycle with PWM does not require too much additional control circuit cost. “

Alexander Sommer is also optimistic about the prospect of PWM dimming scheme. He said: “compared with the analog dimming method, the PWM dimming method of LED has the following advantages: 1) higher efficiency; 2) regardless of the dimming degree, the LED is allowed to work under optimized and constant current; 3) the color tone of LED is consistent in the whole dimming range (the color tone changes with the LED operating current like the lumen output). “

Xu Ruibao also said frankly: “Personally, the choice of modulation mode should not depend on the power of LED. It should depend on the application requirements of terminal products. For example, PWM dimming mode may be selected for display backlight or LED decorative lamp, with good color consistency and high brightness level. However, for general household lighting or commercial lighting, analog dimming or triac can also be selected, but color deviation will occur And the dimming level will be very low. “

Liang houquan also said: “In order to achieve no flicker during continuous dimming, most customers prefer PWM dimming because it provides a larger dimming range and better linearity. Depending on the dimming frequency you are using, flicker can be minimized. Analog dimming is easier to achieve because it requires only one DC voltage to dimming the LED without flicker. But generally speaking, dimming The light range should be narrower. “

For high-power lighting applications composed of multiple LEDs, ensuring that each LED has uniform brightness and does not produce any flicker has also become a major design obstacle, but PWM method can easily solve the flicker problem during dimming. “If the duty cycle of PWM modulator can be kept constant, there should be no problem of uneven light brightness,” said Wu Zhimin, “National Semiconductor’s LED drivers can not only ensure uniform output current, but also ensure high light dark contrast. In these aspects, our LED drivers are better than their peers in the market.”

Tony Armstrong pointed out that in short, the dimming method adopted by the end user will be largely determined by the end use of the LED itself. For example, in the automotive infotainment system where LED is used to provide back lighting for the display, the brightness variation range of ambient lighting is very wide, ranging from extremely bright when the sun is full to dark on a moonless night. Since the human eye is extremely sensitive to slight changes in ambient lighting conditions, a wide dimming range of 3000:1 is required. This will require the LED driver circuit to adopt PWM dimming method.

However, he added: “in LED street lamps, because they are often either on or off, only a limited dimming range is required. In this case, only a simple analog dimming method can meet the requirements.”

As mentioned earlier, LED lighting applications less than 25W are mainly to replace standard incandescent lamps and halogen lamps. In this power range, one of the most likely applications is to replace incandescent lamps or energy-saving lamps controlled by triac (bidirectional thyristor) step-by-step cut in wall dimmers. At present, there are front and back edge cut dimmers in the market, which brings challenges to the overall compatibility, because triac dimming is very poor from the perspective of EMI.

“For non dimming applications requiring the best cost performance, the single-stage PFC flyback topology using DCM PFC such as Infineon tda4863g is a suitable choice.” Alexander Sommer believes that “LED lighting applications with power range of 25W and above are facing more professional markets. The choice of dimming control method will depend on whether it is an alternative type or a newly installed type. Digital lighting control (e.g. Dali or wireless solution) allows more precise control of dimming level and more functions, such as daylight dimming and duty cycle sensing. Alternative installations may require compatibility with old analog 1-10V dimming controllers. “
Source; Electronic Engineering Network

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