LEDs and lighting design: What you need to know about photons

EEs work with electrons. They’ve never seen them but they have a degree in manipulating them. Photons are another matter. With LEDs appearing everywhere from cell phone screens to backlights to signage and industrial lighting applications, knowing how they work—or at least how to drive them—is a critical part of an engineer’s tool kit. Here are a few paragraphs to bring you up to speed:

LEDs are semiconductors that convert electrons into photons; as such they’re generally classified by wavelength—ultraviolet, visible and infrared. Commercially available LEDs with single-pixel output of at least 5 mW fall into the 360- to 950-nm range, with the color determined by the material from which they’re made.

White LEDs are generally made from blue LEDs pumping a yellow phosphor, with the blue and yellow wavelengths mixed in the package. This works well enough for general indoor and outdoor lighting, though the color rendering leaves a lot to be desired, since the green and red components are weak. Where color rendering is important—for example, when backlighting an RGB TFT-LCD screen—RGB LEDs are generally used. These combine red, green and blue (and, optionally, amber) chips to create a light that has a much higher color rendering index (CRI). The only problem is that red and amber LEDs are more sensitive than green and blue LEDs to changes in temperature, so the resulting light tends to become warmer (more red) at low temperature and colder (more blue) at high temperature. LED controllers and drivers have evolved to help address this problem.

LEDs are basically diodes, so they need to be operated in current mode. The easiest way is to use a current limiting resistor or (better) a constant-current source; this is hardly an elegant approach, but it works where neither the operating temperature nor the output of the LED is critical. Most LED drivers today use pulse-width modulation (PWM) to control the brightness of LEDs. Current-controlled PWM LED arrays are common in series-parallel configurations up to about 20V/5A. Higher voltage/current versions for industrial lighting applications present their own set of design challenges, but that’s a discussion we’ll leave to the experts (see below).

Lighting: The desktop trade show
In case that mini-tutorial wasn’t enough, Digi-Key and EE Times are presenting a virtual conference titled “Lighting and System Design” on Thursday, August 26th to help engineers catch up with the latest developments in lighting technologies and applications. The event will include four panels of lighting experts speaking on:

•   Today's Lighting Technologies
•   Lighting the Future
•   Procurement Issues
 
The fourth panel titled Solve My Lighting Problems: Ask the Experts will spend an hour tackling questions from the audience. The panels will include lighting experts from Digi-Key, Cree, OSRAM, TI, STMicro, Linear Tech, Oree, Qualcomm and Strategies Unlimited who will address the full range of lighting issues. If you have a knotty design issue, just sign up, log in and enjoy a bit of free consulting time. Or just lurk and learn.

In addition to the panels there will be three scheduled chats, the topics to be chosen by people who sign up for the conference (you get to vote when you register). Sponsors will have virtual booths, where even if you’re a garage shop you can get through to a helpful FAE to answer your questions.

Overall these events are both highly informative a lot of fun. Jump in with both feet or lurk and learn. Either way you’ll learn a lot about lighting technologies without having to deal with the usual trade show traffic and sore feet.