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Constant power LED driver architecture reduces cost for offline LED ballasts

May 14, 2008

In his fourth article on ballasts, Iain Mosely describes how the constant power operation inherent in a peak-current-controlled discontinuous mode flyback converter can deliver good drive characteristics for LED applications.

For high brightness LED technology to succeed in replacing standard incandescent bulbs for high volume markets, the cost of the drive electronics required to power the LEDs must be kept to a minimum. This article, the fourth in a series discussing LED ballast design, presents a solution that exploits a novel constant power flyback converter architecture. This approach provides an ultra low cost 7W offline LED ballast that will run from 85–265Vac at 50/60Hz.

Traditionally, a mains-derived, isolated, constant current ballast suitable for driving LEDs is achieved using a flyback converter operating under closed loop control (Fig. 1a). The actual LED current is measured using a sense resistor and is used to control the duty cycle of the primary side switching element. Although this gives excellent LED current regulation, the feedback circuitry requires an optocoupler, shunt reference, and sense resistor, all of which add cost and reduce overall efficiency.

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This article was published in the April 2008 issue of LEDs Magazine.

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