Control and design of a modular converter system for vehicle applications

This thesis presents a new family of modular high frequency converters. The initiative idea of the circuit topology of this family of modular high frequency converters (MHFC) has been introduced in recent literatures, whereas not much about control strategy of this family of MHFCs has been revealed. In the proposed MHFC system, each of the sub-modules is consist of an input capacitor and an H-bridge, which are interconnected in parallel. The load of each sub-module is fed through the H-bridge. The sub-modules are connected in series in respect to each other. The combination of the sub-modules is connected to the source (battery) side, which, in this case, is consisting of a constant voltage source together with an inductor. Consequently, the first, and the main, step towards controlling the proposed MHFC system could be developing of a control strategy for obtaining the desired voltage on the input capacitors of each sub-module. Towards this aim, in this thesis work, based on the concepts of DC/DC converters, equivalent models of the proposed MHFC system is developed. By utilizing the equivalent models, dependencies of the acquired voltages on the capacitors of the sub-modules could be found. Then, by devising proper switching patterns for input switches of the sub-modules, the desired voltages on the input capacitors of submodules are obtained. One ancillary advantage of the devised switching pattern is that by adopting this pattern, the inductor of the battery side senses a virtual switching frequency which is N (the number of sub-modules of the system) times higher than the real switching frequency; this may lead to reduction of the required inductance by the factor of N. In this way, based on the parameters of the system in relatively lower real switching frequencies, the need for utilizing an external inductor might be eliminated and the intrinsic inductor of the battery might be sufficient for proper performance of the system. After devising the mentioned control strategy, some feedback loops have been devised to increase the performance of the system. In addition, during the investigations of this thesis, some deficiencies of the initiative idea of the circuit topology of this family of MHFCs were exposed. These deficiencies are due to a low frequency ripple which appears on the voltage of the sub-modules' input capacitors. For suppression of the mentioned ripple, dependencies of that ripple are completely investigated; the mentioned ripple is presented by an equation in this thesis. Since the mentioned ripple is presented by an equation, the magnitude and the dependencies of it are known in every instant; a feed-forward control strategy could be devised to eliminate the mentioned ripple. It is shown that the successfulness of the feed-forward strategy depends on the load’s power factor. It is also shown that the problem of the low frequency ripple for loads with poor power factors would not be solved by control strategies. In these cases, the circuit of the proposed MHFC system needs to be enhanced for dealing with loads with poor power factors. Otherwise this family of MHFCs would not be completely suitable for vehicle applications and nor power grid applications in which the power factors could be highly low. Based on the theoretical investigations, the source of the ripple was found. Then, some new circuit topologies devised. It is shown that by taking advantage of the suggested circuit topologies for this family of MHFCs, the low frequency ripple could be completely suppressed.<br>As the low frequency ripple is almost completely suppressed, suppressing of the remaining high frequency ripple, which is in the frequency of switching, could lead to increasing the optimality of the system. In this thesis the dependencies of the high frequency voltage ripple on the input capacitors of sub-modules are investigated as well. Then, by adopting a proper switching strategy, this ripple is suppressed notably as well. Based on the obtained relationships which are represented clearly by general theoretical equations in this thesis, the values of the circuit components of the proposed MHFC system could be calculated for any specific range of applications.