Research On Two-frequency Controlled For Combined Three-phase Three-level DC-DC Converter

Three level (TL) converter, which can reduce the stress of switch to half the input voltage, iswidely used in high voltage input applications. With the widening of the input voltage range, the dutycycle or switching frequency of converter varies widely, which makes the converter performancedifficult to be optimized. This thesis focuses on the high power dc-dc converter with high and widerange input voltage.A new method to derive the TL converter topology was proposed in this thesis, which is made ofTL clamping unit and tradional converter unit. As the converters can work in the half voltage mode orfull voltage mode through controlling the switch timing of TL clamping unit, the performance will beoptimized over a wide input voltage range. In particular, for wide input voltage range, high-voltageand high-power applications, a combined three-phase TL dc-dc converter with its control strategy wasproposed. It consists of TL clamping unit and three-phase full bridge unit. The TL clamping unitensures the voltage stress on switches to be half the input voltage and the three-phase structurereduces the switch current rating as well as the output filter.The operating principle and characteristics of the combined three-phase TL dc-dc converter withsymmetrical and asymmetrical duty cycle control were analyzed in this thesis. The filter inductor, thevoltage stress on switches and rectifier diodes are compared over different switching points, also theimplemention circuit of different modes switching was presented in detail. In order to improve theefficiency of converter, the loss analysis with asymmetrical duty cycle control was carried out, and theinfluence of different switching points was discussed.Finally, a200V~600V input and48V/20A output prototypes adopting symmetrical duty cyclecontrol and asymmetrical duty cycle control in different switching points, respectively, werefabricated and tested in the lab, the experimental results verifies the theoretical analysis and theperformance of the proposed converter.