Digital Current Mode Control for Buck-Converter Based on Average Inductor Current Measurement

This paper introduces a digitally performedcurrent mode and voltage control for the dc-dc step downconverter based on voltage to frequency converter where theaverage values of inductor current and output voltage areobtained. These current and voltage measurement arerealized by the voltage control oscillator (VCO) andcounters (digital integrator). Such an approach enables fulldigitalization of current and voltage control loops

SINGLE-STAGE DC-AC CONVERTER FOR PHOTOVOLTAIC SYSTEMS

V diplomskem delu so predstavljene posamezne komponente fotonapetostnih sistemov, podpore proizvodnji električne energije iz obnovljivih virov in osnove DC-DC ter DC-AC pretvornikov. Opisano je načrtovanje in preizkušanje enostopenjskega DC-AC pretvornika, ki se lahko uporablja kot omrežni razsmernik v sistemu sončne elektrarne. Za proženje stikal je uporabljena pulzno-širinska modulacija, ki je izvedena z uporabo digitalnega signalnega procesorja. Z namenom uvrstitve pretvornika v fotonapetostni sistem je določena prodajna cena izdelka, ki temelji na njegovi lastni ceni. Za potrebe določitve lastne cene je izvedena stroškovna analiza. V končni fazi je predstavljena še ekonomska upravičenost proizvodnje električne energije z omrežno priključenim fotonapetostnim sistemom, ki uporablja enostopenjski DC-AC pretvornik.This report presents design and experimental testing of single-stage DC-AC converter for photovoltaic systems. Its main attribute is the fact that it generates an AC output voltage larger than the DC input, depending on instantaneous duty cycle. Output voltage is controlled with pulse-width modulation, which is done by using of a digital signal processor. With intention of placing single-stage DC-AC converter into a residential photovoltaic system the cost analysis of the converter is made and its selling price has been evaluated. Economic justification of electricity production with a micro photovoltaic power plant is rated in the final chapter. This work also presents the main components used in photovoltaic systems, basics of DC-DC and DC-AC converters and a short summary of legislation and support schemes for electricity produced from renewable energy sources in Slovenia

UPORABA TEHNIKE MEHKEGA PREKLAPLJANJA ZA IZBOLJŠANJE UČINKOVTOSTI IN POVEČANJE GOSTOTE MOČI ENOFAZNEGA PRETVORNIKA S KOREKCIJO FAKTORJA MOČI

This thesis investigates the possibilities for increasing the power conversion efficiency and power density of a single-phase single-stage AC-DC converter with power factor correction capability. Initially, the limitations are investigated for simultaneous increase of power density and efficiency in hard switched bidirectional converters. The switching frequency dependent turn-on losses of the transistors have been identified as the main limiting factor. In order to avoid the increase in total power losses with increasing the switching frequency, a control approach is proposed for achieving zero voltage switching transitions within the entire operating range of a bidirectional converter that utilizes power transistors in a bridge structure. This approach is based on operation in the discontinuous conduction mode with a variable switching frequency. Operation in the discontinuous conduction mode ensures the necessary reversed current that naturally discharges the parasitic output capacitance of the transistor and thus allows this transistor to be turned on at zero voltage. On the other hand, the varying switching frequency ensures that the converter operates close to the zero voltage switching boundary, which is defined as the minimum required current ripple at which zero voltage switching can be maintained. Operation with the minimum required current ripple is desirable as it generates the lowest magnetic core losses and conduction losses within the power circuit. The performance and effectiveness of the investigated approach were initially verified in a bidirectional DC-DC converter. A reliable zero voltage switching was confirmed over the entire operating range of a bidirectional DC-DC converter, as well as the absence of the reverse recovery effect and the unwanted turn-on of the synchronous transistor. In order to justify its usage and demonstrate its superior performance, the proposed zero voltage switching technique was compared with a conventional continuous conduction mode operation which is characterized by hard switching commutations. After successful verification and implementation in a bidirectional DC-DC converter, the investigated zero voltage switching approach was adapted for usage in an interleaved DC-AC converter with power factor correction capability. Comprehensive analysis of the converter\u27s operation in discontinuous conduction mode with a variable switching frequency was performed in order to derive its power loss model. The latter facilitated the design process of the converter\u27s power circuit. A systematic approach for selecting the converter\u27s power components has been used while targeting for an extremely high power conversion efficiency over a wide operating range and a low volume design of the converter. The final result of the investigations performed within the scope of this thesis is the interleaved AC-DC converter with power factor correction capability. Utilization of interleaving allows for increasing the converter\u27s power processing capability, reduces the conducted differential mode noise and shrinks the range within which the switching frequency has to vary. The proposed zero voltage switching control approach was entirely implemented within a digital signal controller and does not require any additional components within the converter\u27s circuit. The experimental results have confirmed highly efficient operation over a wide range of operating powers. A peak efficiency of 98.4 % has been achieved at the output power of 1100 W, while the efficiency is maintained above 97 % over the entire range of output powers between 200 W and 3050 W.Doktorska disertacija raziskuje možnosti za povečanje izkoristka močnostne pretvorbe in gostote moči enofaznega enostopenjskega mostičnega usmernika s korekcijo faktorja moči. Najprej smo preučili omejitve, ki preprečujejo hkratno povečanje gostote moči in izkoristka dvosmernih trdo-preklapljanih mostičnih pretvornikov. Izgube pri vklopu močnostih tranzistorjev, ki so odvisne tudi od stikalne frekvence, prispevajo največji delež k celotnim izgubam mostičnih pretvornikov in posledično predstavljajo največjo oviro za hkratno povečanje gostote moči in izkoristka. Z namenom preprečitve povečevanja izgub pri močnostni pretvorbi z višanjem stikalne frekvence smo predlagali napreden način vodenja pretvornika, s katerim se doseže mehko preklapljanje tranzistorjev pri ničelni napetosti v celotnem območju delovanja enofaznega mostičnega pretvornika. Predlagani način vodenja temelji na obratovanju v nezveznem režimu delovanja in sprotnem prilagajanju stikalne frekvence. Z ustreznim obratovanjem v nezveznem režimu tako zagotovimo povratni tok, ki je potreben za praznjenje izhodnega parazitnega kondenzatorja močnostnega tranzistorja pred njegovim vklopom. Po izpraznitvi parazitnega izhodnega kondenzatorja je možen vklop tranzistorja pri ničelni napetosti. S časovno in bremensko-odvisnim spreminjanjem stikalne frekvence zagotovimo obratovanje na meji preklapljanja pri ničelni napetosti. Slednja predstavlja stikalno frekvenco, pri kateri dosežemo minimalni potrebni povratni tok, pri katerem je mogoče vzdrževati preklapljanje pri ničelni napetosti. Delovanje z minimalnim povratnim tokom je zaželeno z vidika minimizacije izgub v magnetnih jedrih in prevodnih izgub v močnostnem tokokrogu. Podali smo tudi navodila za načrtovanje takšnega načina vodenja, ki bo zagotovil zanesljivo preklapljanje tranzistorjev v mostični strukturi pri ničelni napetosti znotraj celotnega območja delovanja pretvornika. Delovanje in učinkovitost predlaganega načina vodenja je bilo prvotno preverjeno na dvosmernem DC-DC pretvorniku. Potrjeni so bili tako zanesljivi preklopi pri ničelni napetosti kot tudi odsotnost okrevanja notranje diode tranzistorjev in neželenega vklopa. Izvedli smo tudi primerjavo predlaganega načina vodenja v nezveznem režimu z najpogosteje uporabljanim delovanjem v zveznem režimu. Pri slednjem se preklopi tranzistorjev izvedejo pri polni zaporni napetosti. Rezultati primerjave so pokazali, da z uporabo predlaganega načina vodenja s preklapljanjem pri ničelni napetosti dosežemo višji izkoristek močnostne pretvorbe in ugodnejšo ter bolj enakomerno porazdelitev izgub v pretvorniku. Izrazito manjši je predvsem delež stikalnih izgub, ki dovoljuje uporabo manjših komponent za hlajenje in posledično omogoča povečanje gostote moči pretvornika. Izpostaviti je treba tudi dejstvo, da je za delovanje v nezveznem režimu zahtevana manjša induktivnost močnostne dušilke. Takšno induktivnost lahko dosežemo z uporabo manjšega magnetnega jedra, kar nudi možnost za dodatno zmanjšanje dimenzij pretvornika. Po uspešnem testiranju na dvosmernem DC-DC pretvorniku smo predlagali še način vodenja v nezveznem režimu s konstantno amplitudo povratnega toka in spremenljivo stikalno frekvenco, prilagojen za uporabo v AC-DC pretvorniku s korekcijo faktorja moči. V doktorski disertaciji je podrobno predstavljeno načrtovanje obravnavanega močnostnega stikalnega pretvornika, ki je bil tudi izdelan in eksperimentalno preizkušen. Proces načrtovanja je bil usmerjen v doseganje čim višjega izkoristka močnostne pretvorbe s pretvornikom čim manjših dimenzij. Eksperimentalni rezultati potrjujejo izredno visoko učinkovitost delovanja, saj znaša maksimalni izkoristek močnostne pretvorbe 98.4 %. Slednji je dosežen pri izhodni moči 1100 W. Ob visokem maksimalnem izkoristku pa pretvornik odlikuje tudi visok izkoristek v širšem območju delovanja, ki presega 97 % v celotnem območju izhodnih moči med 200 W in 3050 W

Identification of a Proton-Exchange Membrane Fuel Cell's Model Parameters by Means of an Evolution Strategy

http://dx.doi.org/10.1109/TII.2014.2317982This paper presents the parameter identification of an equivalent circuit-based proton-exchange membrane fuel cell model. This model is represented by two electrical circuits, of which one reproduces the fuel cell's output voltage characteristic and the other its thermal characteristic. The output voltage model includes activation, concentration, and ohmic losses, which describe the static properties, while the double-layer charging effect, which delays in fuel and oxygen supplies, and other effects provide the model's dynamic properties. In addition, a novel thermal model of the studied Ballard's 1.2-kW Nexa fuel cell is proposed. The latter includes the thermal effects of the stack's fan, which significantly improve the model's accuracy. The parameters of both, the electrical and the thermal, equivalent circuits were estimated on the basis of experimental data using an evolution strategy. The resulting parameters were validated by the measurement data obtained from the Nexa module. The comparison indicates a good agreement between the simulation and the experiment. In addition to simulations, the identified model is also suitable for usage in real-time fuel cell emulators. The emulator presented in this paper additionally proves the accuracy of the obtained model and the effectiveness of using an evolution strategy for identification of the fuel cell's parameters

Simplified Mathematical Model for Calculating the Oxygen Excess Ratio of a PEM Fuel Cell System in Real-Time Applications

10.1109/TIE.2013.2276331The oxygen starvation phenomenon is a dangerous operating condition that reduces the lifetime of PEM fuel cells. The detection and prevention of this undesired phenomenon require estimation of the oxygen excess ratio $lambda_{{rm O}_{2}}$. The mathematical complexities of the reported methods for obtaining $lambda_{{rm O}_{2}}$ complicate its real-time calculation and require high-performance computational devices, which significantly increase the costs of the system. In this paper, a mutual information approach is used in obtaining a simplified mathematical model for the calculation of $lambda_{{rm O}_{2}}$. The usage of such a simplified model requires much less computational power for real-time monitoring of the variable $lambda_{{rm O}_{2}}$, while it provides comparable results to those obtained by using the complex model. Therefore, it represents a cost-effective solution, suitable for usage within applications that require high sampling frequencies, like emulators, converter and air compressor control loops, simulations, etc. In order to validate the accuracy of this simplified $lambda_{{rm O}_{2}}$ calculation model, a real-time monitoring system was built and experimentally tested using both the simplified and complex models. The matching experimental results validate the proposed simplification and justify the use of this simplified model within real-time monitoring applications