Tricks of the trade for driving bright LEDs

Up to 24 of Cree’s XLamp MPL high-brightness LED chips can fit into a tiny package. Cree says its XLamp MPL-EZW eliminates traditional chromaticity binning and maximizes lumen density, so luminaire and bulb retrofit manufacturers can deliver consistent color and light output. XLamp MPL-EZW LEDs reduce LED-to-LED color variation to within a four-step MacAdams ellipse around the desired color temperature, which is 75% smaller than the total area of the corresponding ANSI C78.377 color region.

First the good news: Prices for high-brightness LEDs (HBLEDs) have dropped by a third over the last few years. As a result, they're increasingly being spec'ed into illumination for office buildings, parking lots, city streets, airports, stadiums, bridges and tunnels. The trend is opening up opportunities for engineers who understand the complex design, layout, thermal management, and driving issues of this new lighting technology.

Now the bad news: High-brightness white LEDs are still more than an order of magnitude more expensive to mass produce than their dimmer brethren. They're also less efficient. At low driving currents of a few milliamperes, white LEDs can produce outputs of about 250 lumens/W compared with about 16 lumens/W for incandescents and 100 lumens/W for fluorescents. But when an LED is driven by hundreds of milliamperes, the level needed to produce high lumen outputs, efficiencies decline to below 100 lumens/W.

This decline in LED efficiency is caused by a malady dubbed the “droop.” It may come from electron leakage within the LED structure. At least that's the theory of Rensselaer Polytechnic Institute researchers who have produced high-output LEDs converting 22% more electricity to light than those with a conventional make-up. But the topic is still under active investigation.

More bad news: It can be tricky to design a circuit able to drive HBLEDs properly. That's because the character of the light output is quite sensitive to operating parameters such as drive current, ambient temperature, and operating voltage. HBLED system design is a multidisciplined effort that must factor in colorimetry, optics, power electronics, thermal management, and control issues. These are complicated requirements. They essentially mean the design of an HBLED lamp involves more than just hooking up a simple power supply.

Robert Kollman, Texas Instruments senior applications manager and a member of the technical staff, additionally points out that isolation and power-factor correction (PFC) can be important. This applies particularly for European markets, where PFC is required for lighting over 25 W.

LEDs work best when driven with a constant current, so LED manufacturers rate LED qualities like output lumens and beam pattern at a specified forward current. This is the rationale for wiring HBLEDs in series so each is driven with the same current. This approach is particularly useful for down-lighting applications, as when illuminating a work surface.

Here’s one way to adjust the brightness of an HBLED, courtesy of Maxim Integrated Products. Phototransistor Q1 senses ambient light level, which is eventually compared with a reference and fed to the dimming input of the MAX16820 PWM LED driver.
Select figure to enlarge.

LEDs can also be driven in parallel. This approach is useful for cove lighting systems (indirect lighting of valances, ledges, and so forth) that employ strings of LEDs in different lengths. It provides a regulated voltage for each LED, regardless of the current each draws. There are also applications that may require a combination of both serial and parallel approaches.

One example of a recent high-efficiency PWM LED driver is the MIC3201 from Micrel Inc. This step-down driver for HBLEDs incorporates high-current-sense and can drive up to four 1-A LEDs in series at more than 90% efficiency.

Continue to next page.