Expert interpretation: Design guide for high and low power LED lighting solutions

Whether it is a high- power or low-power LED lighting application, it is generally composed of a power supply , LED driver , LED , lens and substrate. The key component is the LED driver, which must provide a constant current output to ensure that the light emitted by the LED will not flicker and there will be no LED color deviation. It generally accepts 24V-48V DC voltage input, but some advanced LED drivers can directly accept 220V 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 heat dissipation, high efficiency, low cost, dimming without flicker, wide dimming range, reliability, safety and elimination of color shift. These challenges require the combined use of appropriate power system topology architecture, drive circuit topology and mechanical design to solve.

"The biggest technical challenges for designers will be high efficiency requirements, optical design, thermal management and improving reliability for certain applications, such as high-brightness LED street lights," said Liang Houquan, technical marketing director for Asia Pacific at Diodes. "We now offer a range of LED lighting solutions covering applications from 0.1W to 250W."

"In the application of LED lighting systems, in addition to selecting suitable LED products, a complete LED lighting design also includes optical design, thermal design, product design and electrical drive design. Since LEDs are low-voltage devices, there are many challenges in converting high-voltage AC power supply to low-voltage constant current drive for LEDs." Zheng Zongqian, senior application manager of ON Semiconductor China, pointed out: "Furthermore, 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 necessary to first select the correct drive circuit topology." ON Semiconductor can now provide all LED lighting solutions in the power range from 1W to 500W.

In order to quickly promote the take-off of the LED lighting market, National Semiconductor Corporation (NSC) of the United States has recently targeted a very large direct replacement market for incandescent lamps, that is, using LED lamps to directly replace incandescent lamps in existing households or other application markets, and launched a direct mains input LED driver chip LM3445 for this market.

However, Wu Zhimin, marketing manager of NSC's Asia Pacific Power Management Products, said: "Incandescent lamps have been on the market for many years, and our home lighting systems have been using many technical standards that have not changed for many years. This situation cannot be changed overnight. For example, due to problems such as heat dissipation and lighting angle, the original old-style lamp sockets or devices are not suitable for installing LED bulbs. But in addition to technical issues, cost-effectiveness is also the biggest reason why LED lamps cannot be popularized. The price of tungsten filament bulbs and neon tubes is about US$0.6-0.7/Klm, but the current price of LED lamps is still as high as US$40-50/Klm."

As mentioned above, since LED lamps must be installed in the original old-style sockets, heat dissipation is a big problem that must be overcome. But strictly speaking, this is a problem that can be solved using mechanical engineering technology. The responsibility of LED system manufacturers is to work hard to develop new technologies to maximize the brightness of LEDs (i.e. the amount of lumens produced per unit power ). Wu Zhimin said confidently: "We can provide the most efficient LED driver to ensure that the heat dissipation of the entire lighting system can be minimized."

The relatively high cost of LEDs is the main obstacle to the LED lighting market taking off on a large scale. For example, Alexander Sommer, product marketing director of the power management business unit of Infineon Technologies, said: "Most typical LED lighting applications less than 25W are sign lamps, logo lamps, and replacements for standard incandescent and halogen lamps. But compared with existing fluorescent and incandescent technologies, the initial cost of LEDs is still a major barrier to entering the mass market."

Xu Ruibao, an engineer in Cytech's product and design department, agrees that the main challenge to commercialization is cost. He said: "At present, LED lighting systems of various powers are achievable in circuits. The technical challenge comes from the requirements of terminal applications. For example, when applied to automobiles, optical design and overall heat dissipation design must be considered. The challenge of commercial deployment mainly comes from the cost of LEDs."

Heat dissipation considerations for LED lighting designLED

lighting systems below 25W are generally designed for applications such as reading lamps, corridor lights, living room spotlights, home dining lamps, and night lights. Customers generally hope that such applications are designed to be as small as possible, so the design space where the PCB can be placed is relatively small, so the temperature in the package space may be very high when used for a long time. Since designers are unlikely to install a cooling fan inside it, its heat dissipation 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 by improved mechanical structures. In addition, high electrical efficiency helps reduce power consumption." Alexander Sommer pointed out, "If thermal resistance needs to be further reduced, this can be done through electrical isolation, because it can achieve the most efficient heat transfer. These methods also allow for optimized lumen output."

Another idea to prevent LEDs from overheating when working for a long time is to use dimming solutions. SangCheol Her, marketing manager of high-voltage IC products at Fairchild Semiconductor , said: "Compared with fluorescent and incandescent lamps, dimming solutions are an important way to reduce LED power consumption. This solution uses dimming controllers to achieve this. Especially for LED driver solutions less than 25W, due to the small PCB size and limited packaging space, heat dissipation issues are inevitable, so this solution is even more important."

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

Architecture selection for 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 simpler topologies 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 use more efficient LLC topology and double-stage PFC.

"LED lighting solutions with power below 25W can adopt PSR or Buck topology, because this power range is mainly aimed at small designs and emphasizes the simplicity of design. Medium-power solutions (25W-100W) are suitable for single-stage PFC, quasi-resonant (QR) PWM, and flyback topologies." SangCheol Her said, "High-power solutions (greater than 100W) are suitable for LLC, QR PWM, and flyback topology designs. From the perspective of efficiency, LLC and QR have better performance; while the PSR solution does not require secondary feedback, is simple in design, and is smaller in size than other solutions."

Zheng Zongqian also said: " LED lamps with power less than 25 W are mainly used in indoor lighting, and they mainly use low-cost flyback topology. ON Semiconductor's NCP1015 and NCP1027 monolithic conversion integrated circuits integrate built-in high-voltage MOSFETs and PWM controllers, which can effectively reduce the area of ​​PCB and the volume of lamps, and provide a maximum power output of 25W (230V AC input)."

"For non-isolated LED lighting applications less than 25W, a simple buck converter can be a low-cost and small-footprint option if the input-to-output conversion ratio is low. In isolated topologies where efficiency is important, a quasi-resonant flyback topology using devices like Infineon's CoolSET ICE2QS family is a good choice," said Alexander Sommer. Infineon is the first supplier to offer a digital quasi-resonant flyback control IC.

Typical LED lighting applications in the 25W-100W power range are street lighting (community roads) and public places like parking lots. Power conversion efficiency, cost-effective implementation of PFC functions, 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 the entry barriers of higher initial costs. LED lighting applications from 25W to 100W have power factor requirements, so a 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 plus DC-DC PWM conversion circuit, such as ON Semiconductor's power factor correction controller NCP1607. The peripheral circuit of NCP1607 is very simple and can provide good performance." Zheng Zongqian said, "For high-efficiency, low-cost and small-volume LED solutions, a single-stage PFC circuit is recommended. It can achieve power factor and isolated low-voltage DC output at the same time, and has significant cost advantages. It will definitely become the mainstream solution for medium-power LED lighting. ON Semiconductor's NCP1652 provides the best control solution for realizing a single-stage PFC circuit."

Shenzhen Shiqiang Telecom uses Silicon Labs' C8051F3XX series 8-bit MCU to implement PFC in software. Huang Sunfeng, the company's assistant marketing manager, said: "We have developed a fully digital LED lighting solution for household mains (180V-260V) input 10W-30W low-power LED lighting applications, which can achieve a PFC value of up to 0.95 through software control. Compared with hardware PFC, this software solution has higher flexibility, adaptability and scalability while ensuring the same performance indicators." The maximum output current of the LED driver MIC3230 used in this solution is 350mA, which can drive up to 12 1W LEDs, which can well meet the needs of indoor lighting.

“For 25W-100W power LED lighting applications requiring efficiency and performance over a wide input and/or load range (e.g. dimming), a quasi-resonant flyback topology with a separate PFC stage is recommended. Typically efficiencies of up to 90% can be achieved,” said Alexander Sommer.

Applications above 100W include major road and highway lighting (where brightness of up to 20K lumens or more and 250W power input are required) and professional applications such as stage lighting and architectural floodlighting. A key driver 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 is comparable to that of metal halide and low-pressure sodium lamps. Initial cost comparisons are likely to continue to be a barrier to entry in 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 that integrates an active discontinuous mode power factor controller and a half-bridge resonant controller with high-voltage drive."

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

Alexander Sommer emphasized: "For higher power level LED lighting applications above 100W, efficiency becomes more important, and 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 choice of LED driver circuit will be largely determined by the input voltage range, the cumulative voltage drop of the LED string itself, and the current required to drive the LEDs sufficiently. This leads to a variety of different possible LED driver topologies, such as buck, boost, buck-boost, and SEPIC.

Tony Armstrong , Product Marketing Director of Linear Technology's Power Products Division , pointed out: "Each topology has its advantages and disadvantages. Among them, the standard buck converter is the simplest and easiest to implement, followed by the boost and buck-boost converters, and the SEPIC converter is the most difficult to implement because it uses complex magnetic design principles and requires designers to have superb switch-mode power supply design expertise."

In short, the application of the end product determines the topology of the LED, and then the Buck, Boost, SEPIC, or Buck-Boost structure is reasonably selected based on the LED topology and input power. "Generally speaking, Buck is more commonly used for lamps below 25W. Boost structure is preferred for lamps with higher power. Both can generally achieve efficiency of more than 85%, and LT3755 can achieve efficiency of up to 97%. When considering the BOM cost of the driver part, the overall system cost should be considered." Xu Ruibao said, "With the intensification of competition, there will always be solutions with lower BOM costs, but they are not necessarily the most suitable. We do not recommend designing products according to this standard. The PCB area is mainly controlled by the main components. Low-power LED lamps should use highly integrated solutions as much as possible. High-power solutions should use products with high technical integration and simple peripheral circuits. What is discussed here are Refers to DC-DC solutions. "

Liang Houquan also pointed out: "In order to achieve high efficiency requirements, switch-mode LED drivers should be considered. Most of these customers prefer to choose buck LED drivers because the overall efficiency is higher. If you consider it from the perspective of the lowest BOM cost, the switch-type LED converter is not the cheapest. Such customers may try to use a linear constant current LED driver. This can provide the lowest BOM cost, but the efficiency may not be as high as that of a switch-mode LED driver. If you consider it from the perspective of the smallest PCB board area, switch-mode converters will usually be selected because they generate less heat and even the related components will be smaller. "

Analog, PWM and TRIAC Dimming Solutions

LED dimming solutions and specifications have been changing and have not been fixed until now, so there are three types of dimming solutions on the market today: PWM, analog and TRIAC . PWM and analog methods are the simpler of the three, but require building a dimming infrastructure and a new dimming controller.

The disadvantage of analog dimming is that the adjustment range of the LED current is limited to a maximum value to about 10% of the maximum value (10:1 dimming range). Since the color spectrum of LEDs is current-dependent, this method is not suitable for some applications.

PWM dimming schemes switch between zero current and maximum LED current at a rate that is fast enough to mask visual flicker (usually above 100MHz). This duty cycle changes the effective average current, allowing dimming ranges of up to 3000:1 (limited only by the minimum duty cycle). Since the LED current is either at maximum or off, this method also has the advantage of avoiding LED color shifts when the current changes, which is common with analog dimming.

SangCheol Her is optimistic about the market prospects of TRIAC dimming solutions. He said: "TRIAC (TRIAC, 2-wire dimming) will become a very popular solution because this technology can fully use the traditional system without any changes. Moreover, it can also be expanded to 3-wire dimming to avoid defects associated with low power factor values."

TRIAC dimming is a very hot topic in the industry today. Initially, TRIAC dimmers were designed for incandescent lamps, but most users hope that the same TRIAC dimmer can also dim the replacement LED lamps. Liang Houquan said: "Diodes Zetex currently provides customers with a full range of dimming solutions (including PWM, analog and TRIAC). For example, the ZXLD1362 LED driver uses an ADJ pin to achieve analog and PWM dimming, which brings great design flexibility to customers."

However, Zheng Zongqian believes that the application of TRIAC dimmers on the market should only be transitional, and in the long run, PWM dimming should be used. He said: "The three main decisive factors are: 1) There will be no flickering when using PWM dimming from zero to the brightest. 2) The performance will be better. Because the dimming output power uses a power factor correction circuit, this is in line with the global mandatory requirements for the use of power factor in lighting. Although this requirement generally starts from 25 W, the United States requires mandatory power factor correction circuits for lighting from zero watts. If TRAIC dimming is used, the power factor will be sacrificed and the complexity of the circuit will be increased. Therefore, using PWM dimming can provide the best performance option and is also the trend of the future. 3) The cost will be better. Using PWM to adjust the duty cycle does not require too much additional control circuit cost."

Alexander Sommer is also optimistic about the prospects of PWM dimming solutions. He said: "Compared with analog dimming methods, LED PWM dimming methods have the following advantages: 1) higher efficiency; 2) no matter how large the dimming level is, it allows the LED to operate at an optimized and constant current; 3) the color tone of the LED remains consistent throughout the dimming range (the color tone changes with the LED operating current like the lumen output)."

Xu Ruibao also said frankly: "Personally, I think the choice of modulation method should not be determined by the power of the LED. Instead, it should be determined by the application requirements of the end product. For example, display Backlight or LED decorative lights may choose PWM dimming, which has good color consistency and high brightness level. But for general household lighting or commercial lighting, analog dimming or TRIAC can also be selected, but it will produce color deviation and the dimming level will be very low. "

Liang Houquan also said: "In order to achieve flicker-free 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, the flicker phenomenon can be minimized. Analog dimming is easier to implement because it only requires a DC voltage to dim the LED without flicker. But generally speaking, the dimming range is 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 the PWM method can easily solve the flicker problem during dimming. "If the duty cycle of the PWM modulator can be kept constant, there should be no problem of uneven light brightness." Wu Zhimin said, "National Semiconductor's LED driver can not only ensure uniform output current, but also ensure that the picture has a very high light-dark contrast . In these aspects, our LED driver is better than its peers on the market." Tony Armstrong pointed out that in short, the dimming method adopted by the end user will be largely determined by the final use of the LED itself. For example, in the automotive infotainment system where LEDs are used to provide backlighting for the display, the brightness of the ambient lighting varies widely, ranging from extremely bright in the sun to pitch black on a moonless night. Because 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 a PWM dimming method.

However, he added: "In LED streetlights, since the lamps are often either on or off, only a limited dimming range is required. In this case, a simple analog dimming method can meet the requirements."

As mentioned earlier, LED lighting applications less than 25W are primarily replacements for standard incandescent and halogen lamps. In this power range, the most likely application is to replace incandescent or CFL lamps controlled by TRIAC-based step-down wall dimmers. There are leading-edge and trailing-edge trim dimmers on the market, which presents a challenge for overall compatibility, as TRIAC dimming is poor from an EMI perspective.

"For non-dimming applications requiring the best price/performance ratio, a single-stage PFC flyback topology using a DCM PFC like the Infineon TDA486 3G is a suitable choice," said Alexander Sommer. "LED lighting applications in the 25W and above power range are targeted at more professional markets. The choice of dimming control method will depend on whether it is a replacement or new installation. Digital lighting controls (such as DALI or wireless solutions) allow for more precise control of dimming levels, as well as more features such as daylight dimming and duty cycle sensing. Replacement installations may require compatibility with older analog 1-10V dimming controllers."