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Address issues in driving parallel LED strings

Posted: 03 Dec 2013     Print Version  Bookmark and Share

Keywords:LED  parallel  heatsink  FET  controllers 

LEDs are being integrated in more products than ever before. Automotive lighting, TV backlights and tablets are just a few applications that require multiple LEDs. Driving a large number of LEDs with a constant current can be done with either a lengthy series connection or by driving multiple strings in parallel. But connecting many LEDs in a long series string poses high-voltage and single-point failure issues. Similarly, powering multiple strings in parallel requires multiple current regulators, one for each string. Ultimately, this leads to higher complexity and cost. Today's trend is to operate strings in parallel, and this article explores options and the rational for implementing circuitry to achieve this goal.

An LED is similar to a standard diode by virtue of being a current-driven device. It has an I-V curve in which the current and voltage are non-linear and a small change in its forward voltage can translate into a large current change. Since the LED current in nearly proportional to the LED's luminous flux, it is important in applications such as TVs to control the current accurately. But not all applications necessarily require high accuracy for brightness matching of the LEDs. If the LEDs are driven in a single string, there is inherent matching because each LED has the same current level. As the number of LEDs in use increases, paralleling strings becomes necessary, and a choice must be made as to how to control the current in each string.

Figure 1: The current mirror (B) offers advantages over simple resistor (A) current regulation.

A typical white LED can have a forward voltage of 3.3V with as much as a 20% variation at its rated current. If 10 LEDs are used in series, it's possible that one string may require 33V to adequately drive it, while a second string requires 39.6V at the same current. If these two strings are wired in parallel, the lower voltage string pulls significantly more current than intended and the second significantly less. The probability that all the LEDs in one string would fall at the high end of its forward voltage specification is rather small and this probability decreases as more LEDs are used.

Figure 2: Independent current string regulators provide redundancy.

In reality, balancing between these two strings is much better, but there could still be a difference of several volts. To help this situation, LED manufacturers use binning to sort parts into groups that accurately match the LEDs forward voltage (Vf) drops (as well as flux and wavelength) to allow better performance. Figure 1A shows a simple, low-cost implementation for paralleling two strings. A fixed-voltage source and a simple resistor to set the current level is all that is necessary.

The voltage across one sense resistor can be regulated by an external control circuit to adjust the output voltage higher or lower to accurately control the LED current. While this regulates the LED current in one string, it does not necessarily do a good job for the second. It can actually make the current in the second worse, as in the case where the control loop increases the output voltage for the regulated string, but the second string has the lower voltage drop of the two.

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