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 require the comprehensive use of appropriate power system topology architecture, drive circuit topology and mechanical design to solve.

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 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 of 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 less than 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." SangCheolHer said, "High-power solutions (greater than 100W) are suitable for LLC, QRPWM, 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.