Two resonant topologies for isolated bi-directional DC-DC conversion
Twiname, Ross Peter
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A bidirectional dual active bridge is a structure comprising two full bridges which enables power to be transferred between two dc levels via an intermediate ac link. This thesis introduces two types of bi-directional dc-dc resonant converters, referred to as the LCL and the CLC, whose names relate to the resonant network connected between their dual active bridges. These converters provide high efficiency conversion over a wide range of dc supply voltages and for a wide power range. By virtue of their network’s resonant operation the magnitudes of the reactive components of the bridge currents are reduced. As a result of the reduced currents lower rated switching devices may be used, and the conduction losses in the bridges are significantly smaller than those of conventional converters. Although the conceptual nature of the LCL and CLC converters is the same, they have slightly different operating characteristics as a result of the realisation of their tee networks. This thesis provides a comprehensive analysis and comparison of these two converters, each of which has different merits. In comparison with a conventional converter, which, aside from the isolation transformer, only has an inductor connected between the bridges, additional components are required in the resonant network of the LCL and CLC converters. It will be shown that the extra expenditure on the resonant network is repaid many times in the life of the converter. After each converter is modelled, its currents and its power throughput are calculated by the summation of harmonic components calculated in the frequency domain. Theoretical calculations are then used to derive the power throughput, the current harmonic content and the switching status as a function of modulation. Circuit waveforms are calculated and then verified through simulations. The theory is validated with experimental results. Comparisons of the key metrics are made between the proposed resonant converters and a conventional converter for standard designs operating under fixed operating conditions. Finally, comparisons are made, on a cost basis, between optimised converters operating under varying operating conditions. The results show the advantage of the proposed converters.