Driver selection for high-power LED arrays

This page addresses the basics of driver selection for high-power LED arrays, specifically chip-on-board (COB) types such as those manufactured by Bridgelux, Citizen, Cree, and others, pictured above. Two assumptions are made. First, that the reader has a working knowledge of some basic electronic concepts; voltage, current, the relationship between the two, what a diode is, and what’s meant by a diode’s “forward voltage” or Vf. Second, that the reader is interested in a commercially-produced driver unit, rather than building their own from parts.

The first step in the driver selection process is to determine the expected Vf range for the chosen array, at the chosen drive current, in the chosen environmental setting. The easiest way to do this is to choose an array and drive current for which the manufacturer conveniently provides this information: look for a section in the datasheet called “driver selection voltages” or something to this effect. If this info is not provided by the manufacturer, another simple if inexact method is to simply apply a tolerance to the manufacturer’s stated “typical” Vf value; +/- 15% is a reasonable to conservative figure to use. (A more analytical approach is to use the expected device temperature limits and the thermal Vf coefficient to expand on the Min/Max Vf range given for the device.) And for the nonconformists who want to operate their LED at a drive current level other than that suggested by the test current value? Use the forward voltage vs. forward current plot (any decent datasheet will have one) to find a new “center” for the Vf range, and adjust the limits of the Vf range proportionally.

With voltage range information in hand, the rest of the process is pretty simple:

  1. Find a place to buy a constant-current LED driver.

  2. Select the desired drive current. Consider those with output current ratings a percent or two above/below the intended value for more options.

  3. Select devices whose output range includes the range of possible Vf values for the chosen LED array. Best results are usually obtained when the LED Vf range overlaps with the upper portion of the driver’s output range. Note that many dimming-capable drivers require that the Vf of the connected load remain within the stated output voltage limits, regardless of dimming level.

  4. Select devices with an input voltage range suitable for the application. it’s also a good time to make selections based on product lifecycle and other characteristics, such as physical size or IP rating.

  1. Make final selection(s) based on other considerations, such as dimming capabilities, price, availability, etc.

For sake of example, imagine that Scott wants more light in his cubicle, and therefore decides to build a desk lamp based on the BXRC-30E10K0-D-73, operating at the recommended test current. Standard 120V AC power is available, and the device temperature is expected to range from -40°C (It’s Minnesota, eh…) to 105°C. Consulting the datasheet, we find the recommended drive current to be 2.1A, and joy of joys, the manufacturer lists a Vf range of 34.0V to 43.0V for purposes of driver selection.

But what if that info wasn’t there? If we use the typical Vf value of 38.7V and simply give/take 15%, that yields a range of 32.9V to 44.5V—a bit wider actually than the manufacturer’s suggestion, but it’s usually a good thing for rules of thumb to err on the side of caution. Going the analytical route for comparison, the min/max Vf @25°C case temperature are given as 35.8 and 41.6V respectively, and the coefficient of forward voltage is given as -22.1mV/°C. The minimum expected Vf then, would be: 35.8V+(-0.0221V/°C)(105°C-25°C)=34.0V. The maximum similarly would be: 41.6V+(-0.0221V/°C)(-40°C-25°C)=43.0V. Not too surprisingly, these are the same numbers as those suggested by the manufacturer.