A novel 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. With the proposed twin-bus structure, 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. With operating frequency exceeding 1 MHz in the test case, the proposed LED driver demonstrates ultra compact size inductor and fast slew rates of the LED current. The dimming controlled 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.Index Terms--High efficiency, DC-DC converter, light emitting diode, Dimmable LED drive, offline LED lighting 978-1-4244-5287-3/10/$26.00 ©2010 IEEE

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Abstract-Emerging

A lossless current sensing technique is proposed, which takes advantage of the on-resistance of the sync FET used in the buck converter to sense the current flowing through the device and to reconstruct an emulated version of the inductor current. The current sensing circuit is integrated into the MOSFET driver and co-packaged with a set of FETs in a stacked die arrangement with common lead-frame shared between driver and sync FET to enable accuracy equivalent to or better than DCR sensing. In addition to steady-state operation, modern multiphase controllers are required to drive the buck converter in other operating modes including: Diode Emulation Mode (DEM), body-braking, tri-state (phase off). These additional modes are also correctly emulated by the current sense logic.