Resumen de Three phase buck type rectifier integrated with current fed full bridge
In the last years, the importance of the electrical energy in avionic applications has been increasing with numerous news and ambitious projects as “The More Electrical Aircraft” (MEA). Since in avionics, the weight is very importance, in order to reduce the consumption of fuel, one of the main goals for the designers of electronic systems has been increase the power density of the electronic systems. The introduction of commutated power supplies allows to increase the power density throughout using new type of semiconductors allowing increase the switching frequency. This fact helps to decrease the size of the magnetics, whose represents the most important part of the total volume of the system. In the three phase rectifiers, which transform the AC energy coming from the generators to DC to supply several loads, the magnetics can be found in the rectifiers but also in the EMI filters used to attenuate the noise induced by the switching frequency of the rectifiers. This EMI filters are needed to comply with the standards of avionic applications. Therefore, the increment of the switching frequency is a technique used to achieve higher power densities in the designs.
In the review of the state of the art has been study several solutions of commutated three phase rectifiers. Several topologies have been analyzed considering the standards MIL-STD-461E, MIL-STD-704F. The EMI noise requirement are very strict because are independent of the power of the converter. In this chapter, several topologies have been simulated both for buck type and boost type. For the same conditions, the size of the inductors of both kind of topologies have been calculated to comply with the standards. Some of these topologies include galvanic isolation, which is a very common requirement in avionic applications. Since galvanic isolation is a requirement of this applications, in this chapter some of the most used topologies with isolation for hundreds to thousands of watts are reviewed. These topologies are mainly based on Bridges (Full Bridge and Half Bridge) with classic or resonant configurations, current or voltage fed. Once these topologies have been reviewed, several power architectures based on combinations between buck type rectifiers, which it is the most suitable topology for the voltage and power range specified in this thesis, and the DC/DC with isolation commented previously. These solutions are compared with the solutions of only one stage with galvanic isolation.
In this thesis have been analyzed in detail the combination based on the buck type three phase rectifier and the Current Fed Full Bridge. This combination allows to reduce the total number of magnetics of the system because the output inductor of the rectifier is shared with the input inductor of the Current Fed Full Bridge. This combination gives the opportunity to analyze different control strategies applied to both stages in order to improve the global response. Therefore, the analysis of the design of integrating both stages, analyzing the design and the control possibilities is the main motivation for this thesis.
In the third chapter is presented the main contribution of this thesis, the synchronization between the buck type stage and the Current Fed Full Bridge.
The concept of synchronization was proposed by other authors, connecting the buck type rectifier and a boost converter as a second stage. The different approach to this solution is the inclusion of a power transformer. The transformer allows to optimize the volts per second applied to an intermediate inductor and consequently, reduce its size.
Since this circuit has two variables of control (M and d), in this chapter the regulation possibilities are evaluated. There are three possible strategies. In the first strategy, the modulation index of the rectifier, M, is fixed and the system is modulated with d, the duty cycle of the Full-Bridge. In the second strategy, the duty cycle of the Full Bridge is fixed and the system is regulated with M. And the last strategy consists on regulate modifying both variables, M and d. The impact on the efficiency is analyzed in this chapter.
In the fourth chapter are analyzed some possible implementations of the digital control proposed. First at all, low cost solutions based on FPGA and DSP are compared. Then, SoC devices are introduced in the comparison. SoC are deviced which includes microprocessor, FPGA and ADC, in the same chip. In the chapter is presented the block diagram of the control where can be appreciated the necessities of measurements of the control for the modulation and regulation of the circuit. Several possible implementations of the control in the SoC are compared in execution time and complexity.
In the fifth chapter, the theoretical analysis presented during the thesis under a prototype are validated. In this chapter is explained how the prototype have been designed and the considerations to comply with the standards MIL-STD-461E and MIL-STD-704F that has been applied. This standard affects to the selection of the components and the design of the magnetics, where a derating should be considered. Additionally, the standards specify the limits for the harmonics induced at the switching frequency by the rectifier, which affects to the design of the EMI filter. The experimental results obtained are analyzed.
In the sixth chapter, the results obtained, both theoretical and experimental are analyzed, highlighting the original contributions and presenting the conclusions of the work.
