Designing a good LED lighting power supply

Your LED lighting system design usually comes down to a building switch-mode power supply, but don’t’ worry there are a number of design options available, writes Paul Ward.

LED lighting offers longevity and design variety and now component prices are falling.

But, as always, the decision to switch over to LED lighting becomes a lot trickier when it comes to the design.

When one factors in everything that is desirable in a good LED lighting setup, that is uniform brightness, dimming capabilities and power factor correction, it quickly becomes apparent why driving multiple high power LEDs in switch mode is no trivial task.

In my opinion driving LED lights is just another way of saying designing a suitable power supply.

But not everyone who wishes to use LEDs is an experienced power supply engineer. Luckily the demand for LED lighting solutions in recent years has meant there are plenty of options for the designer to get started building a suitable power supply, whatever their level of expertise.

If the goal is to design the entire luminaire, there will of course be other design considerations, such as thermal management and optics, but this article will concentrate strictly on the electrical power supply.

First of all, here is a checklist of considerations you should take in to account before anything else:

Power requirement

How many LEDs will you be driving? Are you driving 1W, 3W, or 5W 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?

Cree, for example, offers an online tool to help you determine how many LEDs you’ll need for your application.

Connection scheme

Avago

Avago ASMT-Ax00 1W devices

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. But if output voltage is a concern, you may want to arrange them in parallel.

Each additional feature, for example dimming or power factor correction, on offer will add another layer of complexity to the design. Dimming can be achieved through either pulse width modulation, or varying the continuous forward current of the LED.

In addition, though it needs to be carefully integrated in to the power circuitry, maximum flexibility and control can be afforded via a low-cost microcontroller with PWM output. In the case of PFC, traditional power factor correction would be the first option to consider.

Should that not be an option, whether because of size constraints or budget there are other options, such as the single-stage, high power factor LED drive solution available from On Semiconductor, which is based on their NCL3000 critical conduction mode controller.

Topology

A major selling point of both LED lighting and switching power supplies is efficiency; why not design the supply accordingly?

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 synchronous or asynchronous? Each topology has its merits and pitfalls relative to your application requirements.

Forward voltages

While forward voltage (VF) varies from LED to LED, there is also variance in typical forward voltage among different colors or dice technologies.

Avago, for example, indicates the following for their ASMT-Ax00 1W devices:

LED   Color Dice   Type VF,MIN VF,TYP VF,MAX CURRENT
Warm   White InGaN 2.8 3.2 3.5 350mA
Red AlInGaP 1.7 2.1 2.3 350mA
Cyan InGaN 3.0 3.4 3.8 350mA

 

 

 

 

Once you’ve considered what it is you require from a power supply, the next step is to decide on your level of engagement in its design. An experienced power supply designers will no doubt have an easier time starting from scratch, but what are the options for the rest of us? Here are three possible ways of proceeding.

Get one off the shelf

This might be the least exciting option for some engineers, but it’s also the easiest. Even better, nowadays there are a number of great options to choose from.

A personal favourite is Mean Well’s offline LED power supply which comes nicely packaged for indoor and outdoor applications, and it’s well equipped with features such as dimming, integrated protections, PFC, and universal AC input.

This is a “feature-rich” supply, the flexibility of which will be great for many people, but there are more basic alternatives out there if needed.

Tweak something that works

If you’re new to power supply design, this may be the best way to go, balancing the level of expertise required with the flexibility of a non-proprietary design. Some semiconductor giants, 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.

Check out these reference designs from Texas Instruments, STMicroelectronics, and Linear Technology for examples of tried-and-true starting points. But remember, if you’re modifying a supplier’s reference design it’s only fair to keep that supplier on your bill of materials throughout design, prototype, and production.

If you’re determined to design your own bespoke supply from scratch, the only real obstacle to overcome is your own level of skill and possible electrocution.

Nowadays, going it alone doesn’t even mean designing in a solitary vacuum, rather you can turn to online communities of peers and experts 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, as mentioned above, 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.

When you come to the design phase of your project, there are also good options available when it comes to economical schematic capture and layout software – and once you’re ready to start building, affordable PCB services are available at your fingertips, with instant quoting and extremely fast turnaround.

All things considered, there has never been a better time to be your own ODM, with access to a global knowledge base, affordable circuit design software, and world-class prototyping facilities just a few clicks away. Which is great given, for professional engineers and makers alike, going all-in is by far the most fun option.

Paul Ward is European opto product manager at Farnell element14

 

 

 

 

 

Tags: automotive, medical

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