Modern car industry is developing hybrid vehicles, which have two batteries with different voltages. Mild hybrid vehicles have typically a smaller 12 V battery and a larger 48 V battery. In such electric system there is a DC/DC converter, which connects both voltage levels and allows energy transfer from one to another battery. The purpose of this master thesis is to find the best topology for the DC/DC converter and to analyze its operation based on simulations using LT Spice program.
Typical hybrid system, its advantages and role of the DC/DC converter are described in this paper. Most frequently used topology of a cascaded buck-bust converter has its limits, when we want to increase switching frequency. Hard switching of transistors produces very big electromagnetic interference, because of fast and rapid changes of current and voltage in the circuit. Further on losses of switching MOSFETs and possibilities for improving the converter efficiency are presented. One of the possibilities is introduction of soft switching, which can be realized by a different topology or with a more complex MOSFET driving technique.
Solution based on a topology of a multilevel converter with separating diodes and a resonant circuit was selected for investigation. Its advantages and disadvantages are described in this work. The operation of multilevel converter and comparison to a cascaded converter are presented and explained based on the simulations. It turns out, that the multilevel converter for the selected voltages operates with lower efficiency. This is because the number of elements in the circuit, which produce conduction losses is bigger. It also has some advantages like lower switching losses, smaller voltage on transistors and consequently lower EMC.
Nevertheless, for a mild hybrid system with rather low voltages cascaded buck-boost converter is a better solution for achieving higher efficiency. Soft switching can also be achieved with a complex MOSFET driving technique. On top of that, it also has a smaller number of elements, which reduces the manufacturing cost.
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