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 to ensure the LED is emitted. Light will not be flickering and LED color shifting will not occur. It generally accepts 24V-48V DC voltage input, but there are also some advanced LED drivers that can directly accept 220V mains AC input. Most customers require high-performance LED drivers to meet upcoming LED lighting specifications such as "PF 0.9" and "+85 Efficiency."
Liang Houquan, director of technology marketing for Diodes Asia Pacific, said: "The biggest technical challenge for designers will be high efficiency requirements, optical design, thermal management and improved reliability of certain applications, such as high-brightness LED street lights."
“In the application of LED lighting systems, in addition to selecting the right 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, it converts high voltage AC power. The low-voltage constant-current drive used for LEDs will face many challenges.†Zheng Zongqian, senior application manager of ON Semiconductor China, pointed out: “Further, in order to ensure the advantages of LED lighting, LED electrical drives must be reliable, efficient, and safe. And low cost. Therefore, for different LED lighting applications, the first choice is to choose the correct drive circuit topology.†ON Semiconductor now offers all LED lighting solutions from 1W to 500W.
In order to quickly push the LED lighting market off, National Semiconductor Corporation (NSC) recently targeted a very large incandescent lamp to directly replace the market, using LED lights to directly replace incandescent lamps in existing home or other application markets, and launched A direct mains input LED driver chip LM3445 for this market.
However, Wu Zhimin, marketing manager for power management products at NSC Asia Pacific, said: “The incandescent lamp has been around for many years. For many years of unresolved technical standards, our home lighting system has been in use. This situation can't be changed overnight. For example. Based on the problems of heat dissipation and illumination angle, the original old lamp socket or device is not suitable for installing LED bulbs. However, in addition to technical problems, cost-effectiveness is also the biggest reason why LED lamps cannot be popularized. Tungsten bulbs and neon The price of the light pipe is 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, heat dissipation is a big problem that must be overcome because the LED light must be able to fit within the original old socket. But strictly speaking, this can be solved by mechanical engineering techniques.
The relatively high cost of LEDs is a major obstacle to the large-scale take-off of the LED lighting market. For example, Alexander Sommer, Director of Product Marketing, Power Management Business Unit, Infineon Technologies, Inc. said: "The typical LED lighting applications that are less than 25W are sign lights, sign lights, and alternative standard incandescent and halogen lamps. But now The initial cost of LEDs is still a major obstacle to entry into the mass market compared to fluorescent and incandescent technology."
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 of various powers are achievable on the circuit. The technical challenges come from the requirements of terminal applications, such as automotive applications, taking into account optical design and overall thermal design. Commercial deployment The challenge comes mainly from LED costs."
Thermal considerations for LED lighting design
LED lighting 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 expect such applications to be designed as small as possible, so their PCBs can be placed in a design. The space is relatively small, so the temperature inside the package space may be high when used for a long time. Since the designer is unlikely to install a cooling fan inside it, its thermal design becomes critical and important.
“Most of the low-power LED lighting applications that are less than 25W require a certain amount of miniaturization. This often results in higher power density, although the power consumption is not very large. Sufficient thermal management measures must be provided by an improved mechanical structure. Electrical efficiency helps reduce power consumption,†Alexander Sommer points out. “If you need to further reduce thermal resistance, this can be done by electrical isolation because it enables the most efficient heat transfer. These methods also allow for optimized lumen output. ."
Another idea to prevent the LED from working overheating for a long time is to use a dimming solution. Sang Cheol Her, marketing manager for high-voltage IC products at Fairchild Semiconductor, said: "Comparing fluorescent and incandescent lamps, dimming solutions are an important way to reduce LED power consumption. This solution is implemented using dimming controllers. Especially for smaller than The 25W LED driver solution is more important because of the small PCB size and limited package space, and the heat dissipation problem is inevitable."
In fact, in this power range, LED lights will replace halogen lamps and compact fluorescent lamps (CFLs). In addition, in order to get rid of heat dissipation problems, advanced technologies must remove passive components such as electrolytic capacitors that are sensitive to temperature changes. However, most current LED driver solutions are based on and based on the power supply topology, so temperature limits should be considered, as general products are usually based on commercial standards, but the lights must be able to adapt to harsh environments such as Industrial environment.
Architectural choice 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. In general, LED lighting systems smaller than 25W do not require power correction, so simple topology such as PSR or Buck topology can be used. 25W-100W LED lighting applications require power calibration, so single-stage PFC, quasi-resonant (QR) PWM or flyback topologies are typically used. LED lighting applications above 100W generally use more efficient LLC topology and dual-stage PFC.
“Low-to-25W power LED lighting solutions can be used in PSR or Buck topologies because this power range is primarily for small designs, emphasizing design simplicity. Medium power solutions (25W-100W) are suitable for single-stage PFC, quasi-resonant ( QR) PWM, flyback topology.†SangCheol Her said, “High-power solutions (greater than 100W) are suitable for LLC, QR PWM, and flyback topology designs. From an efficiency perspective, LLC and QR perform better; The PSR solution does not require secondary feedback, and the design is simple and the size is better than other programs."
“For non-isolated less than 25W LED lighting applications, a simple buck converter can be a low-cost and small-volume option if the input-to-output conversion ratio is low. In an isolated topology that emphasizes efficiency, use The quasi-resonant flyback topology of the Cooling ICE2QS family of devices is a good choice," said Alexander Sommer. Infineon is the first supplier to offer digital quasi-resonant flyback control ICs.
Typical LED lighting applications in the 25W-100W power range are street lighting (community roads) and public spaces like parking lots. Power conversion efficiency, cost-effective implementation of PFC functionality and high color quality are now the three most important technical challenges. For example, in commercial and street lighting applications, longer service life and resulting lower maintenance costs are helping to overcome barriers to entry at higher initial costs. 25W to 100W LED lighting applications have power factor requirements, so power factor correction circuits need to be added.
"This circuit can use the traditional two-stage structure, that is, active discontinuous mode power factor correction (PFC) circuit plus DC-DC PWM conversion circuit, such as the power factor correction controller NCP1607 of ON Semiconductor, the peripheral circuit of NCP1607 is very Simple and can provide good performance.†Zheng Zongqian said, “For high efficiency, low cost and small size LED solutions, it is recommended to have a single-stage PFC circuit that can simultaneously achieve power factor and isolated low voltage DC. The output, with significant cost advantages, is bound to become the mainstream solution for medium-power LED lighting. ON Semiconductor's NCP1652 provides an optimal control solution for single-stage PFC circuits."
Alexander Sommer said: "For 25W-100W power LED lighting applications that require efficiency and performance over a wide range of inputs and / or loads (such as dimming), it is recommended to use a quasi-resonant flyback with a separate PFC stage. Topology. Typically up to 90% efficiency can be achieved."
Applications above 100W include major road and highway lighting (where up to 20K lumens or more, and 250W power input) 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 can be compared to metal halides and low pressure sodium lamps. The initial cost comparison may continue to be the market entry threshold in the short term.
Zheng Zongqian pointed out: "For LED applications larger 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 type of integrated controller, which integrates Source discontinuous mode power factor controller and 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 converter 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 stressed: "For higher power LED lighting applications above 100W, efficiency becomes more important. It is recommended to use the LLC resonant topology, which can achieve more than 90% efficiency. We recommend that you use Infineon's new 8-lead Foot device ICE1HS01."
Regardless of the output power of the LED lighting system, the choice of LED driver circuitry will depend to a large extent 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 a number of different possible LED driver topologies such as buck, boost, buck-boost and SEPIC.
Tony Armstrong, director of product marketing at Linear Technology's Power Products Division, said: "Each topology has its advantages and disadvantages. Among them, the standard buck converter is the simplest and easiest to implement, boost and drop. The push-boost converter is second, and the SEPIC converter is the hardest to implement because it uses complex magnetic design principles and requires designers to have superior switch-mode power supply design expertise."
Analog, PWM and TRIAC dimming options
LED dimming solutions and specifications have been constantly changing, and have not been fixed until now, so there are three dimming schemes for PWM, analog and thyristor (TRAIC) on the market. The PWM and analog methods are simpler, but require a dimming infrastructure and a new dimming controller.
A disadvantage of the analog dimming scheme is that the adjustment range of the LED current is limited to a certain maximum to about 10% of the maximum (10:1 dimming range). Since the chromatogram of an LED is related to current, this method is not suitable for some applications.
The PWM dimming scheme switches between zero current and maximum LED current at a rate that is fast enough to mask visual flicker (typically above 100 MHz). This duty cycle changes the effective average current so that a dimming range of up to 3000:1 can be achieved (limited only by the minimum duty cycle). Since the LED current is either at its maximum or turned off, the method also has the advantage of avoiding LED color shift when the current changes, which is common in analog dimming.
SangCheol Her is optimistic about the market prospects of the TRIAC dimming solution. He said: “TRIAC (2-wire dimming) will become a very popular solution because it can completely use traditional systems without any changes. Moreover, it can be extended to 3-wire dimming to avoid defects associated with low power factor values."
TRIAC dimming is a hot topic in the industry today. Initially, TRIAC dimmers were designed for incandescent lamps, but most users expect the same TRIAC dimmers to dim alternative LEDs. Liang Houquan said: “Diodes Zetex currently offers customers full dimming solutions (including PWM, analog and TRIAC). For example, the ZXLD1362 LED driver uses an ADJ pin for analog and PWM dimming, which brings customers Great design flexibility."
However, Zheng Zongqian believes that the application of TRIAC dimmers on the market should be only transitional. In the long run, PWM dimming should be used. He said: "The main three decisive factors are: 1) PWM dimming from zero to the most light, there will be no flickering phenomenon. 2) Performance will be better. Because the dimming output power uses a power factor correction circuit This is in line with the global requirements for the use of power factor for lighting. Although it is generally required from 25 W, the United States requires a mandatory power factor correction circuit from zero watts. If TRAIC dimming is used, it will be sacrificed. Power factor and circuit complexity increase. Therefore, PWM dimming can provide the best performance choice and is the future trend. 3) The cost will be better. Adjusting the duty cycle with PWM does not require much extra 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, allow LEDs all the time The group is operated under optimized and constant current; 3) the color tone of the LED remains the same throughout the dimming range (the color tone changes like the lumen output of the LED as the LED operating current).
Xu Ruibao also said frankly: "People think that the choice of modulation method should not be determined by the power of the LED. It should be determined by the application requirements of the terminal product. For example, the display backlight or LED decorative light may use PWM dimming mode. , color consistency, high brightness level. But for general household lighting or commercial lighting, analog dimming or TRIAC can also choose, but will produce color shift, and the level of dimming will be very low."
Liang Houquan also said: "In order to achieve flicker-free in continuous dimming, most customers prefer to choose PWM dimming because it provides a larger dimming range and better linearity. Depending on the dimming frequency you are using. The flicker can be reduced to the best analog dimming is easier to achieve, because it only needs a DC voltage to dim the LED without flicker. But in general, the dimming range is narrower."
For high-power lighting applications consisting of multiple LEDs, ensuring uniform brightness per LED and not producing any flicker has become a major design challenge, but the PWM method easily resolves flicker problems during dimming. "If the duty cycle of the PWM modulator can be kept constant, there should be no problem of uneven brightness of the light." Wu Zhimin said, "National Semiconductor's LED driver not only ensures that the output current is uniform, but also ensures that the picture has Extremely high light and dark contrast. From these aspects, our LED drivers are better than their peers on the market."
Tony Armstrong pointed out that, in short, the dimming method used by the end user will largely be determined by the end use of the LED itself. For example, in an automotive infotainment system where LEDs are used to provide backlighting to displays, the range of brightness of ambient lighting varies widely, from incredibly bright when there is plenty of sunshine, to darkness without moonlight nights, which can vary. 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 use a PWM dimming method.
However, he added: "In LED street lights, because such lights are often either in the on state or in the off state, only a limited dimming range is required. In this case, only one A simple analog dimming method will meet the requirements."
As mentioned earlier, LED lighting applications of less than 25W are primarily replacing standard incandescent and halogen lamps. The most likely application in this power range is to replace incandescent or energy-saving lamps controlled by TRIAC (two-way thyristor)-based step-down in-wall dimmers. There are cutting-edge and trailing edge dimmers on the market today, which poses a challenge for overall compatibility because TRIAC dimming is poor from an EMI perspective.
“For non-dimming applications that require the best price/performance ratio, a single-stage PFC flyback topology using a DCM PFC like the Infineon TDA4863G is a suitable choice.†Alexander Sommer believes that “25W and above power range LED lighting applications are geared toward The choice of more professional city dimming control methods will depend on whether it is an alternative or a new installation. Digital lighting control (such as DALI or wireless solutions) allows for more precise control of dimming levels, and more Features such as daylight dimming and duty cycle sensing. Alternative installations may require compatibility with older analog 1-10V dimming controllers."
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