Off-line LED driver with bidirectional second stage for reducing energy storage

This paper presents a technique for reducing energy storage capacitance and achieving high efficiency in off-line LED drivers. A bidirectional second stage is utilized to reduce the required energy storage, allowing use of high quality, long lifetime capacitors without significantly increasing cost. The bidirectional structure reduces losses when compared to traditional cascaded stages since less than one third of the total input energy is processed by the bidirectional stage. A two-stage LED driver is designed and tested, with a boost converter operating at critical conduction mode (CRM) as power factor correction (PFC) first stage and a synchronous boost converter with soft switching operating at 400 kHz switching frequency as the bidirectional second stage. Experimental results are presented for a 100-W off-line LED system. The total losses from the bidirectional second stage are less than 3% of the input power, and the overall efficiency including the PFC stage is greater than 94%.