Analysis of Voltage Auto-Balance Characteristic For Three-Level LLC Converter with Clamped Diodes and Flying Capacitor

Three-level LLC converter with only clamped diodes or flying-capacitor is difficult to realize voltage balance on two split input capacitors and the main switches. In this paper, the auxiliary circuits combining clamped diodes and flying capacitor is analyzed addressing the voltage balance issue. The operation principle of the converter under unbalanced voltages on input capacitors is analyzed in detail, based on which the mechanism for voltage auto-balance is elaborated. Moreover, the effects of the auxiliary circuits and their interactions are derived accordingly. Based on that, the design of the flying capacitor is also involved. Finally, a three-level LLC prototype is built and the experimental results demonstrates the validity of theoretical analysis.