Light sources such as light-emitting diodes (LEDs) are an attractive alternative to incandescent and fluorescent light bulbs in illumination devices due to their higher efficiency, smaller form factor, longer lifetime, and enhanced mechanical robustness. Broad-area lighting systems such as those designed to replace fluorescent tubes must uniformly distribute the light over the emitting aperture of the lighting system. In some cases, relatively high-power LEDs are utilized in such lighting systems, but use of such LEDs typically requires additional optics or mixing chambers to spread out and/or diffuse the light, which add cost and bulk and reduce efficiency.
LED-based lighting systems based on relatively large arrays of relatively low-power LEDs may be used as an alternative to the use of smaller numbers of high-power LEDs. Such systems may use packaged or unpackaged LEDs. Some systems may be formed using a low cost plastic substrate, while others may utilize more conventional printed circuit or wiring boards, such as FR4 or metal core printed circuit board (MCPCB). These systems may feature conductive traces formed over a low-cost plastic substrate (e.g., a plastic wiring board) interconnecting a large array of packaged or unpackaged LEDs. Such systems have been described in U.S. patent application Ser. No. 13/171,973, the entire disclosure of which is incorporated herein by reference.
One potential limitation of such systems is that once the pattern is formed on the circuit board or substrate, the size of the printed area and the pitch and layout of LEDs generally cannot be changed. This is a particular limitation when utilizing high-volume manufacturing, such as roll-to-roll processing, where very large amounts of a single design or layout of a printed sheet must be processed to achieve sufficiently low cost. Thus, supporting a large number of different products having different printed sheet configurations typically requires a large number of printed sheets in inventory and a correspondingly higher manufacturing cost.
A second limitation arises from the electrical topography used in such systems, which typically features large numbers of strings of series-connected LEDs. Because of inherent variations in the forward voltage of the LEDs, as well as potential variations in the resistance of the conductive traces, one generally cannot simply connect all of the strings in parallel and expect that the current will divide equally or substantially equally among the strings. In such a system, one string may have a relatively lower string voltage, and thus a higher current will flow through the string. As more current flows through the string, the LEDs in that string will heat up, causing the LED forward voltage to decrease, resulting in a further increase in current. This results in “current-hogging” in the relatively lower-voltage strings and may result in failure of one or more LEDs in a string, which may cascade into failure of the lighting system.
Conventional LED systems utilize a constant-current driver that provides a constant current to a series-connected string of LEDs, independent of the string voltage. While such an approach works for conventional LED systems, array-based lighting systems may have tens or hundreds of strings of LEDs. Using a separate constant current driver for each string in this situation may be prohibitively expensive. Furthermore, the number of connection points to each sheet in such a scenario is generally roughly equal to the number of strings. Again, this is very costly and potentially is a source of failures that may decrease reliability. Furthermore, providing for such a large number of connections requires a relatively large amount of space or volume, making such systems difficult to install and manage.
A third limitation is related to the fabrication of larger lighting systems by tiling multiple discrete lighting units together. In addition to the cost of assembling such a system, there is often an undesirable lower light intensity, a dark space, or a different color light in the region surrounding the joints between different lighting units. Such an undesirable characteristic at the joint may, for example, be a result of the need to provide additional space between different lighting units to accommodate means for mechanically supporting the lighting units, physically connecting the lighting units, electrically connecting the lighting units, and/or connecting the lighting units to the power source.
In view of the foregoing, a need exists for systems and techniques enabling the low-cost design and manufacture of reliable array-based lighting systems capable of supporting a large number of different products and having a cost-effective drive and interconnect system, as well as the ability to make uniform and reliable large-area lighting systems at low cost.