Various factors to consider in LED lighting design

The main design challenges of LED lighting applications include heat dissipation, high efficiency, low cost, dimming without flicker, wide range dimming, reliability, safety and elimination of color deviation. These challenges can only be solved by combining the appropriate power system topology architecture, drive circuit topology and mechanical design.

Since LED lamps must be installed in the original old sockets, heat dissipation is a big problem that must be overcome. But strictly speaking, this problem 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 (that is, the amount of lumens produced per unit power). Wu Zhimin said confidently: "We can provide the most efficient LED drivers 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 design

LED lighting systems below 25W are generally designed for applications such as reading lamps, corridor lights, living room spotlights, dining lamps, night lights, etc. Customers generally hope that such applications are designed to be as compact as possible, so the design space where the PCB can be placed is relatively small. Therefore, the temperature in the package space may be very high when used for a long time. Since it is unlikely for designers to install a cooling fan inside, its heat dissipation design becomes very critical and important.

"Most low-power LED lighting applications below 25W require a certain degree of miniaturization. This often results in higher power density, although the power consumption is not very large. Adequate heat management measures must be provided by improved mechanical structure. In addition, high electrical efficiency helps to reduce power consumption." Alexander Sommer pointed out, "If thermal resistance needs to be further reduced, this can be done through electrical isolation, as it can achieve the most efficient heat transfer. These methods also allow for optimized lumen output."

Another way to prevent LEDs from overheating after long-term operation is to use a dimming solution. Sang Cheol Her, marketing manager of high-voltage IC products at Fairchild Semiconductor, said: "Compared with fluorescent lamps and incandescent lamps, the use of a dimming solution is an important way to reduce LED power consumption. This solution uses a dimming controller. This is especially important for LED driver solutions less than 25W, as the PCB size is small and the packaging space is limited, so heat dissipation is inevitable."

In fact, within 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 problems. However, most current LED driver solutions are derived from power topology and based on this, 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 use 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." Sang Cheol 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 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 25W are mainly used in indoor lighting, and they mainly adopt low-cost flyback topology. ON Semiconductor's NCP1015 and NCP1027 monolithic converter ICs integrate built-in high-voltage MOSFET and PWM controller, which 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 less than 25W, if the input-to-output conversion ratio is low, a simple buck converter can be a low-cost and small-volume option. In isolated topologies where efficiency is important, a quasi-resonant flyback topology using devices like Infineon's CoolSETICE2QS series 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-DCPWM 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, the single-stage PFC circuit is worth recommending. 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 upgradeability 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.

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 separate PFC stage is recommended. Typically, efficiencies of up to 90% can be achieved."

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 short term.

“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,” said Zheng Zongqian. “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 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 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, director of product marketing for 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 according to the topology of the LED and the input power. "Generally speaking, Buck is more commonly used for LEDs below 25W. Boost structure is preferred for LEDs with higher power. In terms of efficiency, both can generally achieve more than 85%, and LT3755 can achieve up to 97% efficiency. 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, and 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. The solutions discussed here refer 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 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 the switch-mode LED driver. If you consider 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 dimming solutions on the market: PWM, analog, and thyristor (TRAIC). PWM and analog methods are simpler, but they require the construction of dimming infrastructure and new dimming controllers.

The disadvantage of analog dimming is that the LED current can only be adjusted from a maximum value to about 10% of that maximum value (10:1 dimming range). Because the color spectrum of the LED is current dependent, this method is not suitable for some applications.

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

Sang Cheol 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 the 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 want the same TRIAC dimmer to dim alternative LED lamps. Liang Houquan said: "DiodesZetex 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 implement 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 25W, the United States requires mandatory power factor correction circuits for lighting from zero watts. If TRIAC 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 degree is, it allows the LED to always work at an optimized and constant current; 3) the color hue of the LED remains consistent throughout the dimming range (the color hue changes with the LED operating current like 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 use 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 chosen, 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 consisting of multiple LEDs, ensuring that each LED has uniform brightness and does not produce any flicker has 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 that the output current is uniform, but also ensure that the picture has a very high light-dark contrast. In these aspects, our LED driver is better than similar products 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 range of the ambient lighting is very wide, ranging from extremely bright when the sun is full 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 (bidirectional thyristor) based step-down wall dimmers. There are both leading edge and trailing edge 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 DCMPFC like the Infineon TDA4863G is a suitable choice." Alexander Sommer believes that "LED lighting applications in the 25W and above power range are aimed at more professional markets. The choice of dimming control method will depend on whether it is a replacement or a new installation. Digital lighting control (such as DALI or wireless solutions) allows more precise control of dimming levels and more functions, such as dimming under daylight and duty cycle sensing. Replacement installations may require compatibility with old analog 1-10V dimming controllers."