High performance dc-dc converter for wide voltage range operation

This paper presents a galvanic isolated multi-level dc-dc converter using different modulation strategies for wide voltage range operation. The dc gain of the converter can be changed in three steps by the proposed modulation schemes and thus achieve the change in output voltage among V_out, 0.5×V_out and 0.25×V_out. The operating frequency of the resonant tank is the same in all three cases. Therefore, the converter can be designed to operate at the optimal operating point to maintain high efficiency for a wide voltage range operation. The output voltage can then be adjusted within the three output voltages depending on the application by either varying the switching frequency of the converter or cascading a second stage dc-dc converter. The operating principle and the dc characteristics of the proposed modulation scheme are discussed and demonstrated with a 17kW five-level half-bridge LLC resonant dc-dc converter prototype operating at 150 kHz. The input and output voltage of the converters are 750V voltage and, 650V, 325V and 163V output voltages, respectively. The experimental results are closely matched with the theoretical predictions.