With the efforts for a global transition to renewable energy sources, there has been an accelerated development in the field of photovoltaics. Progress can be seen, among other things, in increasing the power of photovoltaic (PV) modules, as currently the most powerful commercially available modules already reach over 800 W. The development must also be followed by the measurement systems used to monitor the operation of PV modules in real conditions and to determine their characteristics, energy yield and lifespan. Among other things, the measuring equipment ensures that the PV modules operate with the maximum available power. This is taken care of by the maximum power point tracker or MPPT, where switching converters in different topologies are used for good energy conversion efficiency, among which flyback converters are the most prevalent.
In this assignment, we asked ourselves a research question, if it would be possible to replace the existing flyback converters with resonant converters in LLC topology for use in the MPP trackers and thereby improve the efficiency and prepare a more modular base that allows for further upgrades. With the LLC topology, we also wanted to significantly reduce the switching losses compared to the rest of the switching converters and achieve a conversion efficiency of over 93%. At the same time, we were also interested in whether we can use a transformer for the LLC DC-DC boost converter, which is intended for the LLC buck-converter, which we connect in such a way that the primary winding of the transformer takes the role of the secondary and vice versa.
Classic designs of switch mode power supplies disconnect the entire load current with each switching cycle. This leads to switching losses that increase with increasing switching frequency. The reduction of switching losses is most easily achieved by switching the switches (transistors) when the voltage or current through the switch passes through zero. Topologies of switching converters where the voltage or current passes through zero (ZVS or ZCS) most often take advantage of the properties of the LC resonant circuit, therefore they are broadly classified as resonant converters.
Based on simulations and data sheets, we designed a resonant switching converter of the LLC topology. Which has a resonant circuit consisting of two inductances (L_r and L_m) and one capacitance (C_res). We made a prototype circuit, in which we managed to achieve the set goals through iterative improvements. The converter can operate between voltages of 5 V and up to 60 V, and at the same time provides a voltage of 250 V at the output. During testing, we achieved a power of 400 W, which is the maximum power of the transformer. The set circuit conversion efficiency of 93% was achieved, since at the resonant point of the circuit the efficiency varied between 88% and 93%, with a maximum of 93.2%.
During the measurements, we found that in order to replace the existing gate converters inside the MPPT devices, it would be necessary to optimize the operating points with the aim of better efficiency in the entire range of loads and voltages and not only in the area above 50% of the load. With the paper, we have shown that transformers, which are primarily intended for step-down conversion, can also be successfully used in boost converters.
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