Optimal design of a non-isolated high power density buck-boost converter

Semiconductor lifetime and power density are considered to be two important development directions of power converter design in the future. Especially in some high dynamic application, the operation condition is changing frequently causing the temperature swing of semiconductor, and it leads to thermal expansion and contraction which affects the lifetime or reliability of semiconductor. A high maximum temperature or a large temperature swing leads to short lifetime. Meanwhile, high power density is a general trend. If the size of a whole system is limited, a high power density converter enables more functionality of the system. Also, high power density also leads to high portability. Works have been done to use interleaved and high switching frequency converter to get a high power density. However, those converters such as interleaved buck converter do not help to improve semiconductor reliability because it can only operate in synchronous conduction mode (SCM) and triangular current mode (TCM). In this work, a $20kW$ DC/DC converter for highly dynamic application is designed. The four-switch buck-boost converter is chosen because of its flexibility and its possibility to get longer semiconductor lifetime and higher power density. It is possible to offer a free-wheeling interval in which no voltage is applied on the inductor, the transferred power can be regulated by adjusting the duty cycle of this interval. In addition, an optimal modulation scheme is proposed for this topology which further helps improve the reliability of semiconductor. An optimal power stage design is given which is a two-stage interleaved structure. Plus, a close-loop control strategy is given according to the design. The proposed modulation scheme is tested with a low voltage level prototype, and the performance prediction is verified.