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An improved twin-bus converter using n + 1 active switches is proposed for the n-string LEDs to be dimmable from true zero to the rated current level. The n-switches that perform the current-loop regulation can operate at very high frequency because of significantly reduced voltage stresses, while achieving high efficiency over a wide load range. The dimming switch, however, can operate at a low frequency to ensure high-efficiency operation without light flicker. The detailed power stage operating principle and control scheme of the individual string current for the presented converter are described. Experimental results of a 3-string 50-W LED driver hardware prototype show that the independent string current can be regulated from zero to the desired 350 mA for dimming control. A peak efficiency of 98.3% was achieved with 1.08-MHz switching frequency.
Date of Publication: Aug. 2011