Combined Three-Level DC/DC Converter And Its Soft Switching Techniques

A combined three-level (TL) converter is proposed in this thesis, which reduces the voltage stress on the switches and the size of the output filter inductance, the proposed converter is very suitable for high input voltage with wide range applications. The control strategies, soft-switching techniques and magnetics integration schemes for the combined TL converters are also investigated.Similar to the combined full-bridge converter, two half-bridge TL converters are combined into one configuration. Through a sequence of deduction and simplification, the combined TL converters are obtained, which is composed of a half-bridge TL cell and a full-bridge cell, and the transformer voltages in the two cells are added at the secondary side to reduce the high-frequency content across the output filter inductance. The advantages of the proposed converter are described as follows: all the switches only sustain one half of the input voltage, the output filter inductance and the voltage stress on rectifier diodes can be reduced significantly by properly designing the transformers turns-ratios. So the combined TL converters will achieve a higher power density and rapid dynamic response, which is very suitable for high input voltage with wide range applications. Meanwhile, the corresponding control strategies and realization methods are proposed, and the combined TL converter should employ double phase-shift control. According to the phase-shift sequence of each cell, the converters have two control strategies, and 1# control strategy is superior to 2# control strategy.In order to increase the power density of the converters, a general approach is to push the switching frequency as high as possible, which needs to adopt soft-switching techniques to reduce the switching losses. In this thesis, two kinds of soft-switching techniques, i.e., zero-voltage-switching (ZVS) and zero-voltage and zero-current- switching (ZVZCS) are introduced into the combined TL converters, and the operation principles and parameters design of two soft-switching PWM combined TL converters are described in detail.ZVS PWM combined TL converter can realize ZVS for switches with the use of the leakage inductance of the transformer and the intrinsic capacitors of the switches as the manner of phase-shift full-bridge converter. The demerit is that in two-level mode, one pair of leading switches lose ZVS and their body diodes have reverse recovery, degrading the conversion efficiency. ZVZCS PWM combined TL converter can realizes ZVS for the leading switches and ZCS for the lagging switches in a wide load range, and the body diode reverse recovery of one pair of leading switches is avoided due to ZCS. The experimental results from two prototypes are presented to validate the theoretical analysis and demonstrate the performance of the proposed converters.In ZVS PWM combined TL converter, the rectifier diodes still suffer the voltage oscillation resulted by the reverse recovery and excessive voltage overshoot occurs when the rectifier diode snap off. In order to eliminate the voltage oscillation, two basic clamping tanks are introduced into the ZVS PWM combined TL converter and a family of voltage clamping schemes are proposed. Two feasible schemes among them are effective which can not only eliminate the voltage oscillation and spike both in three-level mode and two-level mode, but keep all the advantages of the original converter, thus additional lossy RC snubber can be removed.In order to further reduce the volume of the magnetic components, integrated magnetic (IM) schemes for the combined TL converters are proposed. Based on the comparison of different schemes, the appropriate one is adopted, which combines the two decoupled reversed transformers into a single magnetic structure. The experimental results from the prototype indicate that the IM can reduce the core size and the core losses; and as a result, the power density and efficiency of the combined TL converters are improved.