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White LED Driver Circuits for Off-Line Applications using Standard PWM Controllers

Feb 12, 2009

Intersil


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Introduction
As high-efficiency ultra-bright white LEDs come down in
cost, they are approaching cost parity with conventional
mercury vapour, HID quartz metal halide and high/low
pressure sodium lighting on a cost per lumen basis. They
are becoming 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. Just as conventional lighting
sources require a ballast, LED lighting sources have an
analogous circuit. This Application Note discusses
techniques for powering LEDs directly from the AC mains,
not only to develop the requisite voltage and current, but
also to deliver power from the AC mains 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...



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