Nikkei Electronics Asia -- June 2009
Solutions
White LED Driver Circuits for Off-Line Applications Using PWM Controllers

Jun 9, 2009 16:01 Nikkei Electronics Asia

As the cost of high-efficiency ultra-bright white LEDs comes down, they are approaching the cost parity with conventional mercury vapor, HID quartz metal halide and high/low pressure sodium lighting on a cost per lumen basis, which makes them viable replacements in industrial and commercial lighting applications. However, there are significant differences between conventional lighting sources and LEDs in terms of voltage and current operating requirements. In particular, LEDs require a constant current source from a low DC voltage source, but they must also operate from the AC mains. Conventional lighting sources require a ballast, whereas LED lighting sources have an analog circuit.

This article discusses the techniques necessary to power LEDs directly from the AC mains, not only to develop the requisite voltage and current, but also to deliver power with near unity power factor while using off-the-shelf constant frequency PWM controllers. The major difference in the control circuitry between a conventional DC-DC converter and the LED ballast is that the output current, rather than the output voltage, is the controlled parameter.

Three types of converter topologies suitable for LED driver applications are discussed. These converters, including boost, SEPIC, and flyback converters, are based on Intersil's ISL6745 doubled-ended PWM controller. The same loop configuration can be used in virtually all off-line AC LED applications. For situation when a single string of LEDs in series is not acceptable, while other applications may require more light intensity than it can be practically delivered by a single string of LEDs, additional LEDs strings can be added in parallel.

by Intersil Corp

Full article:
http://techon.nikkeibp.co.jp/NEA/solutions/0906002.pdf

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