High efficiency LLC converter design for universal battery chargers

In order to support different types of rechargeable batteries (e.g. Li-Ion, Lead-Acid, NiMh), the design of universal battery chargers must focus on wide conversion efficiency instead of traditional peak efficiency design. Wide efficiency is the ability to maintain high performance within the nominal output power while supporting the charging cycle voltage of different battery technologies. The objective of this paper is to tackle this new wide efficiency technical challenge and provide a design methodology that focuses on multiple operating points rather than obtaining peak efficiency at one operating point. The universal battery charger is expected to provide a demanding output voltage range between nominal and 1.5 times nominal and sustaining maximum power delivery with high efficiency. The proposed LLC converter design procedure successfully selects the resonant tank elements and operating frequencies to maximize efficiency for the maximum power region. The design procedure employs analytical equations and a Tabu Search algorithm (TS) for a 96V DC, 960W universal battery charger implementation. The experimental results exhibit the excellent performance of the designed converter, which has an average efficiency of 96.1% within the nominal output power delivery range (between 96V DC and 144V DC output voltage range) with extreme regulation capability.