Vasechko Y. Two-stage DC/AC converters of increased efficiency

Vasechko E. V. Two-stage DC/AC converters of the increased efficiency. – Qualifying scientific work on the rights of the manuscript. Dissertation for the degree of Doctor of Philosophy in specialty 171 –Electronics (17 – Electronics and telecommunications). – National Technical University «Kharkiv Polytechnic Institute», Kharkiv, 2024. The dissertation is devoted to the solution of an actual scientific and applied problem related to the development of new and improvement of existing circuit solutions and control algorithms in the DC-DC conversion stage and the use of special non-sinusoidal voltage forms in the DC-AC stage. The purpose of the work is to improve the energy and quality characteristics of two-stage voltage converters, namely: reducing the power of static losses in the active elements of the DC-DC converter; compensation of input current pulsations; reducing the mass and dimensions of the converter; reducing the power of dynamic losses in the active elements of the DC-AC converter due to the use of non-sinusoidal voltage forms; reduction of the coefficient of harmonics of the output quasi-sinusoidal voltage when using non-sinusoidal forms in single-phase and three-phase execution of the DC-AC conversion stage. The object of the study is the process of energy conversion in two-stage DC/AC voltage converters. The subject of the study is circuit engineering solutions, work algorithms, non-sinusoidal forms of voltages in two-stage DC/AC converters. Scientific novelty of the results: 1. For the first time, the method of an equivalent element is proposed for comparing the power of static losses in active elements of DC-DC converters (MOSFET keys, rectifier diodes) with fundamentally different numbers and operating voltages in topologies, which allows to ensure the equivalence of elements and bring the results of comparative evaluations closer to real ones. 2. For the first time, a comparative analysis of the power of static losses in the active elements of isolated DC-DC converters of different topologies, including with different number of switching phases, under the conditions of a range of input voltage changes and using the equivalent element method, which ensures the adequacy of the comparison of topologies with different numbers of active elements, was carried out. 3. For the first time, a compensatory method for reducing low-frequency pulsations of the input current in a single-circuit voltage stabilization system in a DC-DC converter in a two-stage topology was proposed and investigated. The method makes it possible to increase the output resistance of the alternating current in the converter in a simple algorithmic way and thereby reduce the low-frequency pulsation of the input current and power losses in the DC-DC conversion stage. 4. The obtained dependences of the level of higher harmonics in the output voltage spectrum of the bridge inverter on the duration of the trapezoidal voltage switching intervals in half-bridge converters with linear and sinusoidal fronts. These dependencies make it possible to choose the optimal duration of the switching intervals to obtain the minimum value of the output voltage harmonics coefficient while simultaneously reducing the power of dynamic losses in IGBT switches during the formation of single-phase and three-phase quasi-sinusoidal voltage. A new analytical function of a special form of trapezoidal voltage fronts is proposed, when using which complete suppression of higher harmonics is achieved and a sinusoidal voltage is formed while simultaneously reducing the power of dynamic losses. 5. For the first time, the use of complementary forms with non-sinusoidal voltages in half-bridge converters with combined modulation was proposed in a single-phase bridge DC-AC converter, which allows reducing the mass and dimensions of the inductor in the output filter. The studied conditions for achieving the minimum level of dynamic losses in switching elements at an acceptable level of common-phase interference. The results of the dissertation work were used at the scientific production enterprise «Impuls» (Zaporizhia) in the development and production of two-stage DC/AC voltage converters with a capacity of 1-10 kW (the act on the use of the materials of the dissertation work in the production of converters dated February 21, 2024, presented in the appendix).