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The Boost converter switching topology can be used to drive a series string of LEDs with constant current when the available system input voltage is lower than the required combined forward voltage drop of the LEDs.
For an introduction to switched-mode power converters, see LEDs Magazine July/August 2007 p9 or www.ledsmagazine.com/features/4/8/4.
Many security or emergency lighting applications use a lead-acid battery to provide power during a mains power failure. In these applications, the battery terminal voltage could be 12Vdc or 24Vdc and series string of LEDs may require a combined forward voltage in excess of this value.
The design presented in this article runs from 12Vdc to 24Vdc input, driving ten series-connected Luxeon K2 LEDs at 1A with an electrical conversion efficiency of 95% and 35W total output power capability.
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This article was published in the November/December 2007 issue of LEDs Magazine.
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