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This paper introduces techniques to reduce energy storage in off-line light-emitting diode (LED) drivers. Rather than targeting the ideals of unity input power factor (PF) and constant LED current, a topology is selected to minimize the required energy storage with the more practical targets of 0.9 PF and a selectable LED current ripple. The objectives are accomplished using a combination of the following: first, constant-input-current regulation, which results in 0.9 PF and 34% less required energy storage than when using unity power factor, and is achieved with a simple reference signal that does not require input voltage sensing; second, a two-stage approach that isolates the LED from the capacitor to allow full use of the stored energy; third, a bidirectional second stage (LED string is between the two stages) that processes only the necessary energy storage; and fourth, a selectable regulation band on the LED current ripple to pass through the maximum allowed double-line-frequency ripple. The topology is applied to the series-input modular architecture to utilize low-voltage high-frequency circuits and low-profile components in off-line applications. Experimental results are presented for a series-input system with three modules each driving eight LEDs with 30% double-line-frequency current ripple using small filter capacitors.
Date of Publication: Oct. 2011