How to choose large and small power LED lighting solutions
Whether it is high-power or low-power LED lighting applications, it is generally composed of power supply, LED driver, LED, lens, and substrate. The key component is the LED driver, which must provide a constant current output for the light emitted by the LED. It will not flicker and dim, and will not cause LED color shift. It generally accepts 24V-48V DC voltage input, but there are also some advanced LED drivers that can directly accept 220V AC input.
The main design challenges for the application of LED lighting solutions include the following aspects: heat dissipation, high efficiency, low cost, dimming without flicker, wide-range dimming, reliability, safety, and color cast elimination. These challenges need to be solved through the comprehensive application of appropriate power system topology, drive electrical topology and mechanical design.
Liang Houquan, Director of Technology Marketing of Diodes Asia Pacific, pointed out: "For designers, the biggest technical challenge will be high efficiency requirements, optical design, thermal management and improving the reliability of certain applications, such as high-brightness LED street lights. We now provide A series of LED lighting solutions cover applications from 0.1W to 250W."
LED fluorescent lamp program "In the application of LED lighting program system, in addition to selecting suitable LED products, a complete LED lighting program design also includes optical design, thermal design, product design and electrical drive design. Since LEDs are low-voltage devices, Converting high-voltage AC power supply to low-voltage constant-current drivers for LEDs will face many challenges." Zheng Qian, senior application manager of ON Semiconductor China, pointed out: "Furthermore, for the advantages of LED lighting, LED electrical drivers must be reliable and high Efficient, safe and low-cost. Therefore, for different LED lighting applications, you must first select the correct driving electrical 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 solution market, National Semiconductor Corporation (NSC) recently aimed at a very large direct replacement market for incandescent lamps, that is, directly replacing incandescent lamps in existing homes or other application markets with LED lamps, and launched A direct mains input LED driver chip LM3445 for this market has been developed.
However, NSC Asia Pacific Power Management Product Marketing Manager Wu Zhimin said: "Incandescent lamps have been available for many years. For many years of unchanged technical standards, our home lighting systems have been following many years of unchanged technical standards. The situation cannot be changed overnight. For example, due to issues such as heat dissipation and lighting angles, the original old-style light sockets or devices are not suitable for installing LED bulbs. However, in addition to technical problems, cost-effectiveness also makes LED lights impossible The biggest reason for the popularity. Tungsten filament bulbs and neon tubes are about US$0.6-0.7/Klm, but the current price of LED lights is still as high as US$40-50/Klm."
As mentioned above, since the LED lamp must be able to be installed in the original old socket, heat dissipation is a big problem that must be overcome. But strictly speaking, this can be solved by mechanical engineering technology. The responsibility of LED system manufacturers is to work hard to develop new technologies to maximize the brightness of LEDs (that is, the amount of lumens produced per unit of power). Wu Zhimin confidently said: "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 that is still difficult for the LED lighting market to take off on a large scale. For example, Alexander Sommer, Product Marketing Director of the Power Management Business Unit of Infineon Technologies Co., Ltd. said: “Most typical LED lighting applications less than 25W are sign lamps, sign lamps, and alternative standard incandescent lamps and halogen lamps. Compared with the technology of fluorescent lamps and incandescent lamps, the initial cost of LED is still a major obstacle to entering the mass market."
Cytech product and design engineer Xu Ruibao also agrees that the main commercialization challenge is cost. He said: "Currently, LED lighting systems of various powers are electrically achievable. The technical challenge comes from the requirements of terminal applications, such as applications in automobiles, which must take into account optical design and overall heat dissipation design. Commercial deployment The challenge mainly comes from the cost of LEDs."
Heat dissipation considerations in LED lighting design
LED lighting scheme systems below 25W are generally designed for applications such as reading desk lamps, corridor lights, living room spotlights, home dining lights, night lights, etc. Customers generally hope that such applications are designed as small as possible, so the PCB can be placed The design space is relatively small, so the temperature in the packaging space may be very high when used for a long time. 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, even though the power consumption is not great. Adequate heat dissipation management measures must be provided by improved mechanical structures. In addition, high Electrical efficiency helps reduce power consumption.” AlexanderSommer pointed out, “If you need to further reduce thermal resistance, this can be done through electrical isolation, because it can achieve the highest efficiency of heat transfer. These methods also allow for optimized lumen output. "
Another way to prevent the LED from overheating for a long time is to use a dimming solution. Fairchild Semiconductor’s high-voltage IC product marketing manager SangCheolHer said: “Compared with fluorescent lamps and incandescent lamps, the use of dimming solutions is an important way to reduce LED power consumption. This solution is realized by dimming controllers. Especially for less than 25W Because of the small PCB size and limited packaging space, the heat dissipation problem is inevitable for the LED driver solution of the company, 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, in order to get rid of the heat dissipation problem, advanced technology must remove passive components such as electrolytic capacitors that are sensitive to temperature changes. However, most of the current LED driver solutions are derived from the power supply topology and based on this, so the temperature range should be considered, because general products are usually based on commercial standards, but they must be able to adapt to harsh industries such as industries.
Architecture selection of LED lighting scheme design
The choice of LED lighting solution 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 use the more efficient LLC topology and dual-stage PFC.
"Under 25W power LED lighting solutions can use PSR or Buck topology, because this power range is mainly for small designs, emphasizing the simplicity of the design. The medium power solution (25W-100W) is suitable for single-stage PFC, quasi-resonant (QR) PWM, flyback topology." SangCheolHer said, "High-power solutions (greater than 100W) are suitable for LLC, QRPWM, flyback topology design. From an efficiency point of view, LLC and QR performance is better; and The PSR scheme does not require secondary feedback, is simple in design, and has a smaller size than other schemes."
Zheng Qian also said: “LED lamps less than 25W 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. It can effectively reduce the PCB area and the volume of the lamp, and provide a maximum power output of 25W (230VAC input)."
"For non-isolated LED lighting applications with less than 25W, if the input to output conversion ratio is low, then a simple step-down converter can be a low-cost and small-volume choice. In the isolated topology where efficiency is important, use a The quasi-resonant flyback topology of Ling CoolSETICE2QS series devices is a good choice." AlexanderSommer said. Infineon is the first supplier to provide digital quasi-resonant flyback control ICs.
Typical LED lighting applications in the power range of 25W-100W are street lighting (community roads) and public places like parking lots. Power conversion efficiency, cost-effective realization 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 lifespans and the resulting lower costs are helping to overcome barriers to entry of higher initial costs. LED lighting applications from 25W to 100W have power factor requirements, so power factor correction power needs to be increased.
"This kind of electricity can adopt the traditional two-stage structure, that is, the active discontinuous mode power factor correction (PFC) electricity plus the DC-DCPWM conversion electricity, such as ON Semiconductor's power factor correction controller NCP1607, the peripheral electricity of the NCP1607 is very simple And can provide good performance." Zheng Qian said, "For high-efficiency, low-cost and small-volume LED solutions, it is worth recommending a single-stage PFC circuit, which can simultaneously achieve power factor and isolated low-voltage DC Output, and has a significant cost advantage, will certainly become the mainstream solution for medium-power LED lighting. ON Semiconductor’s NCP1652 provides an optimal control solution for the realization of single-stage PFC power."
Shenzhen Shiqiang Telecom uses SiliconLabs' C8051F3 series 8-bit MCU to implement PFC in software. The company’s assistant marketing manager Huang Sunfeng 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 be controlled by software to achieve a PFC value of up to 0.95. With hardware Compared with PFC, this software solution has higher flexibility, adaptability and upgradeability under the premise of the same performance index." The LED driver MIC3230 used in this solution has a maximum output current of 350mA and can drive up to 12 A 1W LED can well meet the needs of indoor lighting.
Alexander Sommer said: "For 25W-100W power LED lighting applications that require efficiency and performance over a wide input and/or load range (such as dimming), a quasi-resonant flyback topology with a PFC stage is used. Typically efficiencies of up to 90% can be achieved."
Applications above 100W include main road and high-speed public lighting (where up to 20K lumens or more brightness and 250W power input are required) and professional applications, such as stage lighting and architectural 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 is comparable to metal halide and low-pressure sodium lamps. The initial cost comparison may continue to be the barrier to entry in this market in the short term.
Zheng Qian 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 launched a new type of 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 require feedback control on the output side. This makes the half-bridge resonant DC-DC converter 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 emphasized: “For higher power LED lighting applications above 100W, efficiency becomes more important. Using LLC resonant topology, it can achieve efficiency of more than 90%. We use Infineon’s new 8-pin device ICE1HS01 ."
Regardless of the output power of the LED lighting solution system, the choice of LED driver power will largely depend on the input voltage range, the cumulative voltage drop of the LED string itself, and the current required to drive the LED. This has led to many 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 step-down converter is the simplest and easiest solution. -Boost converters are second, and SEPIC converters are the most difficult to implement. This is 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 terminal product determines the topological structure of the LED, and then reasonably select the Buck, Boost, SEPIC, or Buck-Boost structure according to the topological structure of the LED and the input power supply. "Generally speaking, more than 25W Bucks are used. The more powerful ones tend to choose the Boost structure. In terms of efficiency, both can generally achieve more than 85%, and the LT3755 can achieve up to 97% efficiency. Consider the driving part. The overall system cost should be considered when BOM cost.” Xu Ruibao said, “As competition intensifies, there will always be lower BOM cost solutions, but they may not be the most suitable. We do not design products according to this standard. The PCB area is mainly controlled by the main components. The low-power LED lights should adopt a highly integrated solution as far as possible. The high-power solution should choose products with high technical integration and simple external power. The discussion here refers to DC-DC solution."
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 step-down LED drivers because the overall efficiency is higher. If you consider the lowest BOM cost, the switch LED converters are not the cheapest. Such customers may try to use linear constant current LED drivers. This can provide the lowest BOM cost, but the efficiency may not be as high as the switch mode LED drivers. For example, from the smallest PCB board area Considering from the perspective, switch-mode converters are usually 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 constantly changing, and have not been fixed until now, so now there are PWM, analog and thyristor (TRA
