Analysis and control of two-level and multilevel semi-bridge converters
Creators: name: Teixeira, Carlos
Publisher: name: RMIT University
Year: 2024

description: Semi bridge converters are formed by the removal of upper (or alternatively, lower) phase leg semiconductor switching devices of their full bridge counterparts. They offer substantial advantages over a complete full bridge arrangement, because of their reduced controlled switch count and shoot through free phase leg structure. However, in contrast to the bidirectional capability of a full bridge converter, a semi bridge structure can only transfer energy in one specific direction, which limits its use to unity power factor rectification only, and even then with an identifiable level of degradation of its input current harmonic performance.<br><br>The first objective of this thesis is to quantify the impact that the intrinsic topological constraint of a semi bridge rectifier has on its input current quality. A detailed analytical expression is developed for the input current under this restriction, incorporating second order effects such as supply voltage harmonics and semiconductor voltage drops. The result is used to identify the theoretical input current total harmonic distortion limits for the semi bridge rectifier as converter parameters and working conditions are varied.<br><br>The second objective is to mitigate the intrinsic limitation of semi bridge rectifiers using multilevel arrangements, because of their ability to reduce the filter inductance for a given operating switching frequency. For a semi bridge rectifier, the input current harmonic performance is directly proportional to the size of filter inductance, so that the smaller filter size allowed by a multilevel converter reduces the inherent low frequency input current distortion. Two novel multilevel semi bridge rectifier structures, and the specific pulse width modulation technique for the correct carrier harmonics cancelling in these topologies, are devised and extensively validated.<br><br>Finally, the third objective is to broaden the application of semi bridge converters to operate at any power factor. Recent work presents an alternative multilevel approach via the inclusion of a coupled inductor into the converter phase leg, between the upper and lower switching devices. This concept, initially conceived for the H bridge topology, was later extended to the diode clamped converter architecture. In the first instance, both these coupled inductor arrangements appear to be new inverter structures. However, more detailed analysis identifies that they are in fact the parallel connection of two complementary semi bridge converter phase legs. Although the two individual semi bridge phase legs offer unidirectional power flow only, when operating within this topological arrangement, this intrinsic limitation is overcome and the converter output offers bidirectional regenerative capability.<br><br>This recognition led to a number of benefits. Firstly, a third family member is readily derived, based on a flying capacitor multilevel structure. Secondly, it becomes straightforward to identify how to best modulate and control this family of topologically reduced converters, since there is a wealth of established knowledge about how to best control their full bridge counterparts. Finally, at least for the IGBT based variants, the identification of their common semi bridge origin allows additional unnecessary diodes to be removed from the reduced topologies, minimizing their silicon device count to exactly match their full bridge counterparts.<br><br>All theoretical analyses presented in this thesis were broadly confirmed by precisely matched switched simulations and experiments performed on a set of laboratory prototypes.<br>