Presently, hard-switching buck/boost converters are dominantly used for automotive applications. Automotive applications have stringent system requirements for dc-dc converters, such as wide input voltage range and limited EMI noise emission. High switching frequency of the dc-dc converters is much desired in automotive applications for avoiding AM band interference and for compact size. However, hard switching buck converter is not suitable at high frequency operation because of its low efficiency. In addition, buck converter has high EMI noise due to its hard-switching. Therefore, soft-switching topologies are considered in this thesis work to improve the performance of the dc-dc converters.

Many soft-switching topologies are reviewed but none of them is well suited for the given automotive applications. Two soft-switching PWM converters are proposed in this work. For low power automotive POL applications, a new active-clamp buck converter is proposed. Comprehensive analysis of this converter is presented. A 2.2 MHz, 25 W active-clamp buck converter prototype with Si MOSFETs was designed and built. The experimental results verify the operation of the converter. For 12 V to 5 V conversion, the Si based prototype achieves a peak efficiency of 89.7%. To further improve the efficiency, GaN FETs are used and an optimized SR turn-off delay is employed. Then, a peak efficiency of 93.22% is achieved. The EMI test result shows significantly improved EMI performance of the proposed active-clamp buck converter. Last, large- and small-signal models of the proposed converter are derived and verified by simulation.

For automotive dual voltage system, a new bidirectional zero-voltage-transition (ZVT) converter with coupled-inductor is proposed in this work. With the coupled-inductor, the current to realize zero-voltage-switching (ZVS) of main switches is much reduced and the core loss is minimized. Detailed analysis and design considerations for the proposed converter are presented. A 1 MHz, 250 W prototype is designed and constructed. The experimental results verify the operation. Peak efficiencies of 93.98% and 92.99% are achieved in buck mode and boost mode, respectively. Significant efficiency improvement is achieved from the efficiency comparison between the hard-switching buck converter and the proposed ZVT converter with coupled-inductor.

For the solid-state transformer (SST) application, a three-stage configuration consisting of a PWM rectifier based AC/DC stage, a dual active bridge (DAB) converter based DC/DC stage and a PWM inverter based DC/AC stage offers several advantages. For single-phase SST, the instantaneous input and load power seen by the DC/DC stage varies from zero to twice the load average power at double the line frequency. Traditionally, with phase-shift control, large DAB DC link capacitors are used to handle the instantaneous power variation of the load, with the DAB converter processing only the load average power resulting in better soft-switching range and consequently high efficiency. However, the large electrolytic capacitors required adversely affect the power density and the reliability of SST. In this thesis, a PWM control is used for the DAB converter in SST, which extends the ZVS range of DAB and allows the DAB converter to handle the pulsating power while maintaining/improving efficiency. The impact of the output capacitance of switches with PWM control is discussed for practical implementation. A 40kHz, 500W DAB converter is designed and built, and the experimental results proves that the DAB converter with PWM control in SST can achieve comparable efficiency while the DC link capacitors of SST can be reduced to a value that electrolytic capacitors are not required.