Zero-voltage switching control of an bi-directional buck/boost converter with variable coupled inductor

Upravljanje mekim prekidanjem kod dvosmernog buck/boost pretvarača zasnovano na elementu sa strujno regulisanim koefcijentom magnetne sprege Glavni ciljevi istraživanja predstavljenog u okviru ove disertacije su razvoj nove magnetno simetrične strukture, sa strujno regulisanim koefcijentom sprege i simetrične raspodele magnetnog fluksa, pogodne za primenu u više-faznim pretvaračima i razvoj novog načina upravljanja mekim prekidanjem energetskih prekidača u slučaju dvosmernog buck/boost pretvarača, putem strujno regulisanog elementa upravljive magnetne sprege. Postojeće realizacije magnetnih regulatora sa strujno regulisanom magnetnom spregom mogu se podeliti u dve grupe. U prvu grupu spadaju magnetno nesimetrični elementi spregnutih induktivnosti sa strujno regulisanim koefcijentom sprege, kod kojih vrednost indukovanog napona, usled promene fluksa, nije jednaka u svim namotajima elementa i koji se kao takvi ne mogu smatrati pogodnim za primenu u višefaznim energetskim pretvaračima. Drugu grupu elemenata čine magnetno simetrične strukture, kod kojih magnetno polje usled struje predmagnećenja uvećava jednosmernu radnu tačku magnetnog materijala u celom jezgru i time značajno uvećava gubitke u magnetnom materijalu. Magnetno simetrična struktura razvijena u okviru ovog istraživanja sačinjena je od namenski izrađenih feritnih jezgra, gde su kontrolne magnetne grane smeštene simetrično, na jednakim udaljenostima od radnih grana. Takođe, vazdušni procepi u magnetnom materijalu su distribuirani i postavljeni u obe radne magnetne grane, kako bi na taj način magnetno polje koje potiče od struje predmagnećenja bilo ograničeno samo u kontrolnim magnetnim granama strukture. Realizacijom strukture na prethodno opisan način, omogućena je regulacija vrednosti koefcijenta sprege između radnih namotaja putem struje predmagnetizacije, dok je distribucija jednosmernog magnetnog polja ograničena samo u kontrolnim granama magnetne strukture. Distribucija jednosmernog magnetnog polja u predloženoj magnetnoj strukturi, verifkovana je primenom simulacije, metodom konačnih elemenata (Finite Element Method – FEM). Električne osobine predložene strukture predstavljene su pomoću analitičkog modela, gde je korelisanje parametra modela sa realizovanom strukturom izvršeno na osnovu eksperimentalno dobijenih rezultata. Maksimalan opseg promene kontrolisanevrednosti koefcijenta sprege predstavljen je u odnosu na različite vrednosti odnosa reluktance radne i kontrolne magnetne grane. Takođe, u okviru istraživanja, predstavljena je analiza uticaja promenljive vrednosti koefcijenta sprege elementa spregnutih induktivnosti na performanse dvo-faznog boost DC-DC pretvarača, koji radi u kontinualnom režimu vođenja energetskih prekidača. Pored predstavljene analize uticaja promenljive vrednosti koefcijenta sprege, naveden je i primer određivanja parametara predložene magnetne strukture u slučaju dvo-faznog boost DC-DC pretvarača.Main goals of the research presented in this dissertation are development of new symmetrical magnetic structure, with current regulated value of the coupling coefcient and symmetrical distribution of the magnetic flux, suitable for use in the multi-phase power converters, and development of new approach for soft-switching control in case of the bi-directional buck/boost power converter, by utilizing the coupled inductor with the variable coupling coefcient. Existing realizations of the magnetic regulators with the current controlled magnetic coupling can be divided into two groups. The frst group includes magnetically asymmetric coupled elements with the current controlled magnetic coupling, in which value of induced voltage, due to variable magnetic flux, isn’t equal in all windings of the element, and which as such cannot be considered suitable for use in multiphase power converters. The second group of elements consists of the magnetically symmetric structures, in which the magnetic feld, due to the DC bias current, increases the bias magnetic feld in the magnetic material, throughout the magnetic core, and thus signifcantly increases the losses. The magnetically symmetric structure developed in this research is implemented using custom made ferrite cores, where the control magnetic legs are arranged symmetrically, at equal distances from the main magnetic legs. Also, the air gaps in the magnetic material are distributed and positioned in both main magnetic legs, so that the magnetic feld originating from the DC bias current is limited only in the control magnetic legs of the structure. The realization of the structure in the manner described above enabled the regulation of the value of the coupling coefcient between the main windings by the mean of DC bias current, while the bias magnetic feld was limited only in the control legs of the magnetic structure. The distribution of the DC bias magnetic feld in the proposed magnetic structure has been verifed using a Finite Element Method (FEM) by simulation. The electrical properties of the proposed structure are presented using an analytical model, where correlation of the model parameters with the realized structure was performed based on the experimentally obtained results. The maximum range of variation of the control value of the coupling coefcient is presented in relation to different values of the ratio of the reactance of the main and the control magnetic legs. Also, within this research, an analysis of the influence of the variable value of the coupling coefcient on the performance of a two-phase boost DC-DC converter, operating in the continuous conduction mode is presented. In addition to the presented analysis of the influence of the variable value of the coupling coefcient, an example of determining the parameters of the proposed magnetic structure in the case of a two-phase boost DC-DC converter is given