Hard and Soft Switching Losses in Power Converters: Role of Transistor Output Capacitance

To best utilize power converters, a sound understanding of the relationship between the circuit topology and the power-semiconductor-device characteristics is required. This is especially important in high-frequency switching, where device parasitics start to largely interfere with the circuit. In this regard, the parasitic-output-capacitance of power devices plays a vital role in the losses of the system, which affect both the efficiency of the circuit and the system volume. This thesis studies the characterization of output capacitance as well as its interaction with power electronic topologies at a fundamental level and presents important insights, underlining limitations in the conventional knowledge on the subject.The thesis shows that the analysis of output-capacitance-related losses should be treated based on the type of switching employed, as the circuit-level effect of the output capacitance is different in soft- and hard-switching topologies. For both cases, it is highlighted that the charge-versus-voltage curve of device output capacitance provides the most complete information of its behaviour. First, the correct measurement methods to identify output capacitance losses for soft-switching operations are analysed and validated. Then, it is shown how a fundamentally different charge-discharge mechanism in device output capacitance takes place in hard-switching operation (in contrast to soft-switching), and a new measurement approach is devised to quantify the related loss. These measurement methods are then used to evaluate output-capacitance losses of commercially available Si, SiC, and GaN power transistors. The developed concepts are taken a step further and utilized to separately evaluate gate- and driver-related losses in driving a transistor undergoing zero-voltage-switching. The technique is then used to identify the performance limitations of HF and VHF gate-driver ICs. Finally, consolidating the ideas developed throughout the thesis, an experimental case study is provided for a soft-switching converter.We conclude that output capacitance is a critical device feature that should be considered in high-frequency converter design and be given the same level of attention as device on-resistance in the design stage of a device. We believe this is instrumental to the progress of modern power converters—especially, ones based on emerging wide-bandgap devices—towards their fullest potential.