Powering up your LEDs
Lifetime, form factor, packaging and a range of colours - from the outside, it would appear as though LED lighting has everything going for it. But for design engineers, there’s a lot more to the story.
Driving multiple high-power LEDs in switch mode is not a trivial task assuming factors such as uniform brightness, dimming capability and power factor correction play an important role.
To the seasoned power supply designer, these requirements are all in a day’s work. But if you’re not an experienced power supply engineer, the design proposition can be more than a little intimidating. Don’t let that stop you.
Thanks to the increased demand for LED lighting solutions, there are plenty of options to help you get started building your power supply. If your goal is to design the entire luminaire, there will be other design considerations, including thermal management and optics. But for the purpose of this article, we’re focusing strictly on electrical power supply.
Before you begin any power supply design, consideration must be given to the load. Here are a few things to consider:
Power requirement: How many LEDs will you be driving? Are you driving 1, 3 or 5 W LEDs? Is the number of LEDs in each string fixed, or does your supply need to accommodate a range of output voltages? And how bright do you need your source to be?
Connection scheme: Are you driving your LEDs in series, parallel or a combination of both? A series configuration is often recommended when LED brightness must be consistent across the string. If output voltage is a concern, you may want to arrange them in parallel.
Forward voltages: While VF varies from LED to LED, there is also variance in typical forward voltage among different colours or dice technologies. For example, Avago indicates the following for their ASMT-Ax00 1 W devices:
|LED colour||Dice type||Vf,min||Vf,typ||Vf,max||Current|
|Warm white||InGan||2.8||3.2||3.5||350 mA|
While selecting supply rating, you should cover the maximum forward voltage of selected LED bin.
CV or CC mode: The next step is to choose between a constant voltage output (CV) or a constant current output (CC), depending on the applications. While for most applications the CC mode is preferred, for applications such as linear strip lighting, the CV mode is more suitable.
Performance ratings: Efficiency, power factor correction (PFC) and total harmonic distortion (THD) are key performance measurements of the power supply. The conversion efficiency is the ratio of output power to input power. Typically, the efficiency lies between 80 and 90% for AC power supplies and between 95 and 98% for DC power supplies. Newer designs are able to support higher efficiency at a higher system cost.
The purpose of the power factor correction circuit is to minimise the input current distortion and make the current in phase with the voltage thus achieving a near ideal PF value of 1. If traditional power factor correction isn’t in the cards due to size or budgetary constraints, you have other options.
THD of a signal is a measurement of the harmonic distortion present. It is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. Standard topologies give a THD performance of <20%; however, with much more complex designs we can get a THD performance of <10%.
Features: Additional features will mean higher hardware cost and increased design complexity. Dimming is a common feature that can be achieved by changing the continuous forward current of the LED or through altering the duty cycle of digital signal by pulse width modulation. A low-cost microcontroller with pulse width modulation (PWM) output offers maximum flexibility and control, but it must be integrated carefully with the power circuitry. Traditional dimming is achieved with a Triac control device present in the power supply. Additional safety features such as short circuit/overcurrent/overvoltage/overtemperature, EMI emission should also be considered. In many countries, the power supply may need to have approvals from laboratories like CE and UL. Depending on the environment of operation you may want to choose the correct IP rating for the power supply. The designer could also look at the MTBF (mean time between failures) to determine the reliability of the power supply.
Topology: I don’t want to seem biased, but having grown up in a semiconductor house I tend to favour switch mode anything. (Some of my best friends are linear designers.) Efficiency being the major selling point of both LED lighting and switching power supplies, why not design the supply accordingly? Within switch mode there will be multiple topologies such as Flyback, Quasi Resonant and LLC controllers and the like meeting various specification and performance requirements. With that in mind, consider the following: Is your application DC or offline? If it’s offline, will you require isolation or universal input? Do you like your bucks/boost synchronous or asynchronous? Each topology has its merits and pitfalls relative to your application requirements.
An important final step in preparing for your design is to decide on your level of engagement. Again, seasoned power supply designers may have an easier time starting from scratch, but few of us can truly call ourselves power supply designers.
Here are three ways to proceed depending on your ambition:
Buy an off-the-shelf power supply: The easiest way to light up your LEDs is to let someone else do it, although for us engineers this is usually the least exciting option. If you decide to go this way, however, you do have a number of choices, for example, Mean Well’s offline LED power supply.
Modify a proven reference design: If you’re new to power supply design, this may be the best way to go, especially if you are not confined to a proprietary design. Some semiconductor companies, such as TI and STMicroelectronics, publish complete reference designs on their websites that include original schematics, bills of materials, application notes and, in some cases, even Gerber files. And, please, keep it classy: if you’re modifying a supplier’s reference design, keep that supplier on your bill of materials (BOM) throughout design, prototype and production.
The same reference design can be modified if the design requirement does not match. In this case, the first thing that needs to be checked is the availability of components in the BOM and the exact equivalent of the components that you’d like to replace. In most cases, the magnetic components like the transformer may have to be custom built as per the design provided. If you are able to source all the components you may fabricate the PCB using the reference Gerber. In case you have to change the footprints of certain components, you may have to redesign the Gerber into the mechanical dimensions you wish to get the final product. You may also procure an evaluation board which could be used as a reference while you test your own board. Three reference designs are provided up to 15 W and are intended to drive from 4-15 LEDs at 350 mA. They can, however, be easily modified to support power levels ranging from 5 to 30 W.
Go all-in: If you want to design your own supply from scratch, there really is nothing stopping you, except maybe a disinclination for singed fingertips and smoking hair. Importantly, going all-in doesn’t mean ‘going alone’. Life is too short to design in a vacuum; find a community of peers and experts, such as element14, to ask questions and share ideas. As with reference designs, when it comes to research your best bet is to start with the suppliers. Cree offers some useful design guidance. TI offers design assistance with block diagrams and their online PowerLab Reference Design Library. Linear Technology offers designers an assortment of application and design notes and reference circuits on their website, and similar resources from STMicroelectronics, NXP and ON Semiconductor help shed light on LED power supply design.
Fortunately, there are good options for economical schematic capture and layout software for the design phase of your project. For professional engineers and makers alike, going all-in is the most fun option - and with access to a global knowledge base, affordable circuit design software and world-class prototyping facilities, there has never been a better time to be your own ODM.
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