Switch Mode Power Supply Design Overview

　　Driving multiple high-power LEDs in switch mode and achieving uniform brightness, dimming function and power factor compensation is not a simple task, but for professional power designers, it only takes one day to meet all the design requirements. What we mean by "driving LEDs" is "designing a suitable power supply." But if you are not a professional power design engineer, then this design solution can be a big challenge.

　　Don’t be intimidated

　　In recent years, the demand for LED lighting solutions has been strong. Regardless of your ability level, you have access to a large number of power supply design solutions to choose from. If your goal is to design an entire lighting device, you can also refer to more other design solutions, including thermal management and optical systems. This article will focus on power supply design.

　　Before designing a power supply, you must first solve the problem of power supply load. To do this, you need to consider the following factors:

　　1. Power requirements - How many LEDs do you want to drive? Are they 1W, 3W or 5W LEDs? Is there a fixed number of LEDs in each series line, or does the power supply need to be adjusted based on the output voltage range? How bright do you expect the power supply to be? Cree provides an excellent online tool to help engineers determine the number of LEDs required for their application design.

　　2. Connection scheme - Do you want to drive the LED lamp in series, parallel or mixed mode? If you need to keep the brightness of all LEDs in each group consistent, it is generally recommended to use a series configuration. However, if there is a problem with the output voltage, it is recommended to use a parallel configuration.

　　3. Forward Voltage (VF) - Not only does the VF value vary from LED to LED in the circuit, but typical LEDs of different colors and chip technologies also have different forward voltages.

　　4. Features - Implementing features such as dimming and power factor compensation will increase the complexity of the design. Dimming can be achieved by changing the continuous forward current of the LED or performing pulse width modulation (PWM); and through PWM output, a low-cost microcontroller can achieve maximum control and flexibility, but it must be accurately integrated with the power supply circuit. As for power factor compensation (PFC), if traditional PFC cannot be implemented due to size or budget constraints, then you have other options. For example, ON Semiconductor provides a single-stage high power factor LED driver solution based on the NCL3000 critical conduction mode controller.

　　5. Topology - No one wants to be biased, including myself, but since I grew up in the semiconductor field, I inevitably tend to use switch mode for all designs (some of my best friends are linear designers). One of the biggest selling points of LED lighting and switch mode power supplies is high energy efficiency, so why not design the power supply for this? If you decide to use LED lighting and switch mode, you need to consider the following questions: Is your lighting application DC or offline? If it is offline, do you need isolation or universal input? Do you want the LED light groups to be synchronous or asynchronous? Each topology has its own advantages and disadvantages in achieving your application requirements.

　　The last important question to consider before starting power supply design is to determine your ability level. Professional power supply designers will find it easier to design a power supply even if they start from scratch, but there are only a few of us who can be considered as power supply designers. You can refer to the following three options according to your design goals:

　　Buy finished power supply

　　Although adopting a power supply solution designed by someone else is usually not a rewarding experience, it is the easiest way for engineers to build LED lighting. Even in this case, engineers have some excellent options. Taiwan's Mean Well Company provides an offline LED power supply with beautiful packaging, suitable for indoor and outdoor applications, and has functions such as dimming, comprehensive protection, PFC and universal AC input. In the words of the industry, it is a "full-featured" little guy. If you have other functional requirements, the company can also provide more basic options.

　　Modifying a proven reference design

　　If you have no experience in power supply design, this may be the best approach for you, especially if you are limited to a proprietary design. Semiconductor giants such as Texas Instruments (TI) and STMicroelectronics have published complete reference designs on their websites, including original schematics, bills of materials, application notes, and sometimes even Gerber files. You can get a solid resource to start your product design by looking at reference designs from suppliers such as TI, STMicroelectronics, and Linear Technology. In addition, try to maintain a gentleman's demeanor, that is, when you modify a vendor's reference design, you should purchase the vendor's products throughout the design, prototype, and manufacturing design process.

　　put all one's eggs in one basket

　　If you want to design your own power supply from scratch, nothing will stand in your way, except perhaps the occasional irritation of burnt fingertips or cigarette-smelling hair. It is important to understand that by going all in, I do not mean "alone". Life is too short to design in a vacuum. You need to find a community of peers or experts, such as the element11 community, to ask questions and exchange ideas.

　　If you use reference designs for product development, you'd better research the suppliers that provide the reference designs. As mentioned above, Cree can provide some useful design guides; Texas Instruments provides block diagrams and an online PowerLab™ reference design database; and Linear Technology provides designers with various application and design notes and reference circuits on its official website. In addition, STMicroelectronics, NXP and ON Semiconductor also provide similar resources to help designers design LED power supplies.

　　Even more fortunately, you can also get some excellent and affordable pattern capture and layout software for power supply design. And when you are ready and start building your solution, you can also easily get affordable PCB design services with fast quotes and response times.

　　For professional engineers and makers alike, going all in is the most fun option. With access to a wealth of resources around the world, affordable circuit design software, and top-notch prototyping equipment, this is the best time ever to make yourself an ODM. So, never let the LED go out!