A two-stage LED driver for high-performance high-voltage LED fixtures

This paper presents a high-efficiency power-factor-corrected (PFC) dimmable two-stage driver for a high-efficacy light emitting diode (LED) fixture using three strings of high-voltage LEDs with precise color control and high color rendering index (CRI). Majority of off-line LED drivers employ flyback or buck converter topology with a low-voltage single-string LED configuration for low cost. However, that approach suffers from performance penalties that include low CRI and efficiencies in the 75-88% range. It is shown here that a driver efficiency of greater than 93% can be achieved at a power level of 12 W with a PFC boost - floating buck topology. One of the reasons for the high efficiency is the use of high-voltage (~180 V) LED strings at low currents. In addition it is shown that a high CRI and high efficacy are achieved at warm white color temperature at the fixture level. The paper describes the driver design and efficiency improvement techniques but also addresses the high-voltage LED technology, color mixing for high CRI fixtures and system design issues to give a comprehensive understanding of the requirements of a high-performance LED fixture.