Let the LED never go out: Overview of the switch mode power supply design

It is not a simple matter to drive multiple high-power LEDs in switch mode and achieve uniform brightness, dimming and power factor compensation. However, for professional power designers, it takes only one day to satisfy all designs. Claim. What we mean by "driving the LED" means "designing the right power supply." But if you are not a professional power supply design engineer, then this design can be said to be a big challenge.

Don't be afraid

In recent years, the demand for LED lighting solutions has been strong. No matter what your level of competence, you can get a large selection of power supply options. If your goal is to design the entire lighting fixture, you can also refer to more other design options, including thermal management and optical systems. This article will focus on power supply design.

The power supply must be resolved before power supply design. To do this, you need to consider the following factors:

Power Demand - How many LED lights do you want to drive? Is it 1W, 3W or 5W LED light? Is the LED light on each series line a fixed number, or does it need to adjust the power supply according to the output voltage range? What is the brightness of the power you expect? Cree offers an excellent online tool to help engineers determine the number of LED lights needed for their application design.

Connection scheme - Do you want to drive LEDs in series, parallel or hybrid mode? If it is necessary 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 the parallel method.

Forward Voltage (VF) — Not only does the VF value of each LED in the circuit vary, but the typical LED forward voltage with different colors and different chip technologies is also different. The following is the relevant data of Avago's ASMT-Ax00 1W series equipment:

LED light color chip type VF, minimum value VF, specification value VF, maximum current

Warm white InGaN2.83.23.5350mA

Red AlInGaP1.72.12.3350mA

Blue Green InGaN3.03.43.8350mA

Features - Features such as dimming and power factor compensation add complexity to the design. Dimming can be achieved by changing the LED's continuous forward current or by performing Pulse Width Modulation (PMW). With PWM output, maximum control and flexibility can be achieved with a low-cost microcontroller, but it must be connected to the power supply circuit. Accurate integration. As for power factor compensation (PFC), if you can't implement traditional PFC due to size or budget constraints, then you have other options, such as ON Semiconductor to provide a single-stage high power factor based on NCL3000 critical conduction mode controller. LED drive solution.

Topology - no one wants to be a biased person, including myself, but as I grow up in the semiconductor world, I inevitably tend to use switch mode for all designs (I have some good friends who are linear designers) . One of the biggest selling points of LED lighting and switch mode power supplies is energy efficiency, so why not design power supplies? If you decide to use LED lighting and switching mode, then you need to consider the following questions: Is your lighting application DC or offline? Is it necessary to perform isolation or universal input if offline? Do you want the LEDs to be synchronized or asynchronous? Each topology has its own strengths and weaknesses when it comes to implementing your application needs.

The last important issue to consider before starting power design is to determine your level of competence. Professional power supply designers feel that it is easier to design power supplies from scratch, but there are very few people among us who are really power supply designers. You can refer to the following three options according to your own design goals: