At the recentAPEC (Applied Power Electronics Conference) event, two papers addressed DC power distribution to LED lighting. The technology offers potential power savings and perhaps a more flexible wiring scheme. Those papers seem especially relevant given the launch of Redwood Systems and their DC approach to commercial office lighting that also includes Power Line Control (PLC) of fixtures.
At APEC, a researcher from Carnegie Mellon University presented a paper that modeled the potential of DC power distribution for both fluorescent and LED lighting and found that DC offered efficiency advantages for LEDs. Meanwhile, a team from Zhejiang University in China authored both a paper and a poster session describing a low-voltage DC bus for powering and controlling LED lights via a PLC scheme.
Brinda Thomas of Carnegie Mellon evaluated fluorescent and LED fixtures powered by a 277-VAC line and then similar fixtures powered from 250 VDC. DC power distribution has been widely considered in the past few years as an energy-saving measure. For instance, the IT industry is evaluating DC distribution in computer data centers. DC distribution eliminates the AC/DC stage of a power supply and the associated efficiency hit in the power supply.
Note that the DC scheme studied by Carnegie Mellon is still quite different from the Redwood Systems technology. The Carnegie scheme would still rely on a DC/DC converter in the light fixture, whereas the Redwood technology does all power conversion in a central system and provides the precise DC power needed to drive LEDs in the remote fixture.
The choice of a 250-VDC distribution scheme matches the typical DC/DC converter topologies on the market in IC form. Those converters are typically designed to operate globally where AC line voltage can vary from the 110V line in the US to 240V lines in other regions. So most power supplies include an AC/DC stage that outputs DC voltage in the range of 250V.
Thomas concluded that fluorescent offers the lowest annualized costs with AC distribution. LEDs, however, could offer an advantage in annualized cost by 2012 in a DC distribution system. But the paper notes that such a system is mostly realistic in an installation where incentives have resulted in an enterprise installing a distributed DC system based on solar energy,
The paper and poster sessions from Zhejiang University described technology that more closely resembles the Redwood Systems scheme. A master-control system would both power and control remote or slave LED modules over a pair of wires.
The architecture essentially splits a boost power converter architecture between the master and slave modules. The master module includes the inductor that typically stores energy from the output of the rectifier in a boost converter. The slave includes the switch, diode, and capacitor that comprise a boost converter.
The master also includes a switch that's used to modulate data using a variable pulse width. Power to the salve is temporarily unavailable during communications, but the capacitor in the slave can store the power required for continuous operation. The paper also described a way for the slave to signal data back to the master.
Because the papers were presented at an IEEE sponsored conference, you won't likely find links to either freely available. But the APEC 2010 proceedings will soon be available for purchase.