Modulation techniques for AC/DC converters in ultrafast battery charger applications

Concerning today’s exigencies of electrical mains, the possibility to build a reliable technology of electric energy distributed storage in conjunction with the growing market of electric and hybrid vehicles (EV and HEV) address the actual producers’ interest mostly on batteries and charger station. Indeed, the UFC (ultrafast charger) technology has been becoming dominant in this ambit for last years, because these devices guarantee high density of power and a variety of combinations in terms of layout (bidirectional or unidirectional applications).
According to the previous motivations, this thesis regards the report of a project of an ultrafast charger conceived by PEIC (Power Electronics Innovation Center) of the Turin Polytechnic, patented in symbiosis with “VISHAY Semiconductor Italia”, a leader company in the electronic and electric components market.
The charger consists of two different converters: the first one is an AC/DC (active front end) converter while the second one a DC/DC converter (LLC resonant). This paper focuses on the AC/DC stage and highlights how to set up the most efficient control strategy for the structure. In particular, in this AC/DC application an AFE (active front end) converter has been realized. This topology represents an example of multi-level converter, with 3 output connections (p, m, n) instead of 2 (1, 0). It leverages bidirectional T-type switches for mid-point connection.
The principal points of the thesis deal with the advantages guaranteed by the converter (unitary cosφ, low impact on absorbed current by the grid so low THD and TDD, galvanic isolation of the battery, possibility to charge more vehicles contemporaneously, power sharing strategy, and so on) and also with the major issues affecting it (mid-point current control and DC-link balance).
The main problem concerning the control of the structure regards the mid-point balancing. A suitable modulation technique and a proper voltage control are presented as methods to overcome this problematic. This modulation technique is known in technical literature as ZMPC (Zero-Mid-Point-Current) PWM and it is feasible for this application since it acts by nulling the periodic value of the mid-point current on the central connection of the DC-link.
In the end, several considerations, supported by proper figures and comments, are presented in this paper and a final section reports further conclusions about the treated topics.