Constant power LED driver architecture reduces cost for offline LED ballasts
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.
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.
This article was published in the April 2008 issue of LEDs Magazine.
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