Loss Modelling of Three-Level Inverters controlled with Space Vector Modulation Technique

The objective of this master’s thesis is to investigate the loss behavior of three-level ANPC inverter and compare it with conventional NPC inverter. The both inverters are controlled with mature space vector modulation strategy. In order to provide the comparison both accurate and detailed enough NPC and ANPC simulation models should be obtained. The similar control model of SVM is utilized for both NPC and ANPC inverter models. The principles of control algorithms, the structure and description of models are clarified. The power loss calculation model is based on practical calculation approaches with certain assumptions. The comparison between NPC and ANPC topologies is presented based on results obtained for each semiconductor device, their switching and conduction losses and efficiency of the inverters. Alternative switching states of ANPC topology allow distributing losses among the switches more evenly, than in NPC inverter. Obviously, the losses of a switching device depend on its position in the topology. Losses distribution among the components in ANPC topology allows reducing the stress on certain switches, thus losses are equally distributed among the semiconductors, however the efficiency of the inverters is the same. As a new contribution to earlier studies, the obtained models of SVM control, NPC and ANPC inverters have been built. Thus, this thesis can be used in further more complicated modelling of full-power converters for modern multi-megawatt wind energy conversion systems.

Positron measurements in Mg-doped GaN and development of digital Doppler broadening spectroscopy

In this work parameters of Mg-doped GaN samples were studied using positron annihilation spectroscopy and analyzed. It is shown that gallium vacancies exist in an unintentionally doped sample. Next, the sample with higher concentration of Mg and low growth temperature contains vacancy clusters. In case of low concentration of Mg the growth temperature does not affect the formation of defects. Analog electronics can be replaced by a modern digital device. While promising a high quantity of benefits, the performance of these digitizers requires thorough adjustment. A 14-bit two channel digitizer has been tested in order to achieve better performance than the one of a traditional analog setup, and the adjustment process is described. It has been shown that the digital device is unable to achieve better energy resolution, but it is quite close to the corresponding attribute of the available analog system, which had been used for measurements in Mg-doped GaN.

Power cycling lifetime estimation of IGBT power modules based on chip temperature modeling

In this doctoral thesis, methods to estimate the expected power cycling life of power semiconductor modules based on chip temperature modeling are developed. Frequency converters operate under dynamic loads in most electric drives. The varying loads cause thermal expansion and contraction, which stresses the internal boundaries between the material layers in the power module. Eventually, the stress wears out the semiconductor modules. The wear-out cannot be detected by traditional temperature or current measurements inside the frequency converter. Therefore, it is important to develop a method to predict the end of the converter lifetime. The thesis concentrates on power-cycling-related failures of insulated gate bipolar transistors. Two types of power modules are discussed: a direct bonded copper (DBC) sandwich structure with and without a baseplate. Most common failure mechanisms are reviewed, and methods to improve the power cycling lifetime of the power modules are presented. Power cycling curves are determined for a module with a lead-free solder by accelerated power cycling tests. A lifetime model is selected and the parameters are updated based on the power cycling test results. According to the measurements, the factor of improvement in the power cycling lifetime of modern IGBT power modules is greater than 10 during the last decade. Also, it is noticed that a 10 C increase in the chip temperature cycle amplitude decreases the lifetime by 40%. A thermal model for the chip temperature estimation is developed. The model is based on power loss estimation of the chip from the output current of the frequency converter. The model is verified with a purpose-built test equipment, which allows simultaneous measurement and simulation of the chip temperature with an arbitrary load waveform. The measurement system is shown to be convenient for studying the thermal behavior of the chip. It is found that the thermal model has a 5 C accuracy in the temperature estimation. The temperature cycles that the power semiconductor chip has experienced are counted by the rainflow algorithm. The counted cycles are compared with the experimentally verified power cycling curves to estimate the life consumption based on the mission profile of the drive. The methods are validated by the lifetime estimation of a power module in a direct-driven wind turbine. The estimated lifetime of the IGBT power module in a direct-driven wind turbine is 15 000 years, if the turbine is located in south-eastern Finland.

Galvanomagnetic effects in InGaAs nanostructure

Investigation of galvanomagnetic effects in nanostructure GaAs/Mn/GaAs/In0.15Ga0.85As/ GaAs is presented. This nanostructure is classified as diluted magnetic semiconductor (DMS). Temperature dependence of transverse magnetoresistivity of the sample was studied. The anomalous Hall effect was detected and subtracted from the total Hall component. Special attention was paid to the measurements of Shubnikov-de Haas oscillations, which exists only in the case of magnetic field aligned perpendicularly to the plane of the sample. This confirms two-dimensional character of the hole energy spectrum in the quantum well. Such important characteristics as cyclotron mass, the Fermi energy and the Dingle temperature were calculated, using experimental data of Shubnikov-de Haas oscillations. The hole concentration and hole mobility in the quantum well also were estimated for the sample. At 4.2 K spin splitting of the maxima of transverse resistivity was observed and g-factor was calculated for that case. The values of the Dingle temperatures were obtained by two different approaches. From the comparison of these values it was concluded that the broadening of Landau levels in the investigated structure is mainly defined by the scattering of charge carriers on the defects of the crystal lattice

Synthesis, characterization and chemical sensor application of conducting polymers

Polymeric materials that conduct electricity are highly interesting for fundamental studies and beneficial for modern applications in e.g. solar cells, organic field effect transistors (OFETs) as well as in chemical and bio‐sensing. Therefore, it is important to characterize this class of materials with a wide variety of methods. This work summarizes the use of electrochemistry also in combination with spectroscopic methods in synthesis and characterization of electrically conducting polymers and other π‐conjugated systems. The materials studied in this work are intended for organic electronic devices and chemical sensors. Additionally, an important part of the presented work, concerns rational approaches to the development of water‐based inks containing conducting particles. Electrochemical synthesis and electroactivity of conducting polymers can be greatly enhanced in room temperature ionic liquids (RTILs) in comparison to conventional electrolytes. Therefore, poly(para‐phyenylene) (PPP) was electrochemically synthesized in the two representative RTILs: bmimPF6 and bmiTf2N (imidazolium and pyrrolidinium‐based salts, respectively). It was found that the electrochemical synthesis of PPP was significantly enhanced in bmimPF6. Additionally, the results from doping studies of PPP films indicate improved electroactivity in bmimPF6 during oxidation (p‐doping) and in bmiTf2N in the case of reduction (n‐doping). These findings were supported by in situ infrared spectroscopy studies. Conducting poly(benzimidazobenzophenanthroline) (BBL) is a material which can provide relatively high field‐effect mobility of charge carriers in OFET devices. The main disadvantage of this n‐type semiconductor is its limited processability. Therefore in this work BBL was functionalized with poly(ethylene oxide) PEO, varying the length of side chains enabling water dispersions of the studied polymer. It was found that functionalization did not distract the electrochemical activity of the BBL backbone while the processability was improved significantly in comparison to conventional BBL. Another objective was to study highly processable poly(3,4‐ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) water‐based inks for controlled patterning scaled‐down to nearly a nanodomain with the intention to fabricate various chemical sensors. Developed PEDOT:PSS inks greatly improved printing of nanoarrays and with further modification with quaternary ammonium cations enabled fabrication of PEDOT:PSS‐based chemical sensors for lead (II) ions with enhanced adhesion and stability in aqueous environments. This opens new possibilities for development of PEDOT:PSS films that can be used in bio‐related applications. Polycyclic aromatic hydrocarbons (PAHs) are a broad group of π‐conjugated materials consisting of aromatic rings in the range from naphthalene to even hundred rings in one molecule. The research on this type of materials is intriguing, due to their interesting optical properties and resemblance of graphene. The objective was to use electrochemical synthesis to yield relatively large PAHs and fabricate electroactive films that could be used as template material in chemical sensors. Spectroscopic, electrochemical and electrical investigations evidence formation of highly stable films with fast redox response, consisting of molecules with 40 to 60 carbon atoms. Additionally, this approach in synthesis, starting from relatively small PAH molecules was successfully used in chemical sensor for lead (II).

Veden desinfiointi UV-LED-valolla

Veden riittämätön puhdistus aiheuttaa riskin veden käyttäjille. Miljoonia kuolemia vuosittain aiheuttavien vesiteitse leviävien sairauksien ehkäisemiseksi vaaditaan tehokkaita juomaveden desinfiointimenetelmiä. Kuivuuden ja väestönkasvun myötä veden tarve on lisääntynyt ja vedenkulutus tulee yhä kasvamaan. Tästä syystä mahdollisuus kierrättää vettä hyödyntäen sitä esimerkiksi kasteluun on saanut yhä enemmän huomiota. Kierrätettävä vesi on kuitenkin käsiteltävä huolellisesti sen sisältämän mikrobiologisen kontaminaatioriskin vuoksi. Ultraviolettisäteily luokitellaan fysikaaliseksi desinfiointimenetelmäksi. Sen tehokkuus perustuu mikro-organismien absorboimaan UV-säteilyyn, jonka aiheuttamien DNA:ssa tai RNA:ssa tapahtuvien muutoksien seurauksena mikro-organismi inaktivoituu ja estyy lisääntymästä. UV-desinfioinnissa on tyypillisesti käytetty elohopeahöyrylamppuja. Vaihtoehtoinen UV-säteilyn lähde ovat LEDit eli valoa emittoivat diodit. Matalapaine-elohopeahöyrylamppujen emittoima säteily on aallonpituudella 254 nm ja keskipaine-elohopeahöyrylamppujen emittoima säteily on laajakaistaista säteilyä. Energiatehokkuuden lisäksi LEDien etuna on, että niillä voidaan tuottaa kapeakaista säteilyä aallonpituudella, joka parhaiten absorboituu DNA:han. Tämän diplomityön tarkoituksena oli tutkia, onko UVC-alueen aallonpituuksien yhdistelmillä synergistisiä etuja LEDien desinfiointitehokkuuteen, kun desinfioidaan virtaavaa vettä useilla säteilyannoksilla ja indikaattorimikrobina käytetään kolibakteeria. Tavoitteena oli myös tutkia tällä hetkellä saatavissa olevien LEDien desinfiointitehokkuutta energiatehokkuuden näkökulmasta. Yksittäisistä aallonpituuksista desinfiointitehokkuudeltaan parhaimmaksi osoittautui 260 nm, aallonpituuksien yhdistelmistä tehokkain oli 265 nm:n ja 260 nm:n yhdistelmä. Muilla aallonpituuksien yhdistelmillä ei saavutettu odotettua parempaa desinfiointitehokkuutta. Optiselta teholtaan parhaimmat LEDit, 265 nm, 270 nm ja 275 nm olivat kokeiden perusteella myös energiatehokkuuden kannalta tarkasteltuina parhaimmat sekä yksittäin että yhdistelminä. UVC-aallonpituuksia emittoivien LEDien optisen tehokkuuden paraneminen on edellytys LEDien hyödyntämiselle desinfioinnissa.

Strong magnetic field pulse generator

High magnetic fields and extremely low temperatures are essential in the study of new semiconductor materials for example in the field of spintronics. Typical phenomenons that arise in such conditions are: Hall Effect, Anomalous Hall effect and Shubnikov de-Haas effect. In this thesis a device capable for such conditions was described. A strong magnetic field pulse generator situated in the laboratory of physics and the Lappeenranta University of Technology was studied. The device is introduced in three parts. First one is the pulsed field magnetic generator, which is responsible for generating the high magnetic field. Next one is the measurement systems, which are responsible for monitoring the sample and the system itself. The last part describes the cryostat system, which allows the extremely cold temperatures in the system.

Minimizing Conducted RF-Emissions in Switch Mode Power Supplies Using Spread-Spectrum Techniques

Switching power supplies are usually implemented with a control circuitry that uses constant clock frequency turning the power semiconductor switches on and off. A drawback of this customary operating principle is that the switching frequency and harmonic frequencies are present in both the conducted and radiated EMI spectrum of the power converter. Various variable-frequency techniques have been introduced during the last decade to overcome the EMC problem. The main objective of this study was to compare the EMI and steady-state performance of a switch mode power supply with different spread-spectrum/variable-frequency methods. Another goal was to find out suitable tools for the variable-frequency EMI analysis. This thesis can be divided into three main parts: Firstly, some aspects of spectral estimation and measurement are presented. Secondly, selected spread spectrum generation techniques are presented with simulations and background information. Finally, simulations and prototype measurements from the EMC and the steady-state performance are carried out in the last part of this work. Combination of the autocorrelation function, the Welch spectrum estimate and the spectrogram were used as a substitute for ordinary Fourier methods in the EMC analysis. It was also shown that the switching function can be used in preliminary EMC analysis of a SMPS and the spectrum and autocorrelation sequence of a switching function correlates with the final EMI spectrum. This work is based on numerous simulations and measurements made with the prototype. All these simulations and measurements are made with the boost DC/DC converter. Four different variable-frequency modulation techniques in six different configurations were analyzed and the EMI performance was compared to the constant frequency operation. Output voltage and input current waveforms were also analyzed in time domain to see the effect of the spread spectrum operation on these quantities. According to the results presented in this work, spread spectrum modulation can be utilized in power converter for EMI mitigation. The results from steady-state voltage measurements show, that the variable-frequency operation of the SMPS has effect on the voltage ripple, but the ripple measured from the prototype is still acceptable in some applications. Both current and voltage ripple can be controlled with proper main circuit and controller design.

Modification of Photocatalyst with Enhanced Photocatalytic Activity for Water

Increasing demand and shortage of energy resources and clean water due to the rapid development of industry, population growth and long term droughts have become an issue worldwide. As a result, global warming, long term droughts and pollution-related diseases are becoming more and more serious. The traditional technologies, such as precipitation, neutralization, sedimentation, filtration and waste immobilization, cannot prevent the pollution but restrict the waste chemicals only after the pollution emission. Meanwhile, most of these treatments cannot thoroughly degrade the contaminants and may generate toxic secondary pollutants into ecosystem. Heterogeneous photocatalysis as the innovative wastewater technology attracts many attention, because it is able to generate highly reactive transitory species for total degradation of organic compounds, water pathogens and disinfection by-products. Semiconductor as photocatalysts have demonstrated their efficiency in degrading a wide range of organics into readily biodegradable compounds, and eventually mineralized them to innocuous carbon dioxide and water. But, the efficiency of photocatalysis is limited, and hence, it is crucial issue to modify photocatalyst to enhance photocatalytic activity. In this thesis, first of all, two literature views are conducted. A survey of materials for photocatalysis has been carried out in order to summarize the properties and the applications of photocatalysts that have been developed in this field. Meanwhile, the strategy for the improvement of photocatalytic activity have been explicit discussed. Furthermore, all the raw material and chemicals used in this work have been listed as well as a specific experimental process and characterization method has been described. The synthesize methods of different photocatalysts have been depicted step by step. Among these cases, different modification strategies have been used to enhance the efficiency of photocatalyst on degradation of organic compounds (Methylene Blue or Phenol). For each case, photocatalytic experiments have been done to exhibit their photocatalytic activity.The photocatalytic experiments have been designed and its process have been explained and illustrated in detailed. Moreover, the experimental results have been shown and discussion. All the findings have been demonstrated in detail and discussed case by case. Eventually, the mechanisms on the improvement of photocatalytic activities have been clarified by characterization of samples and analysis of results. As a conclusion, the photocatalytic activities of selected semiconductors have been successfully enhanced via choosing appropriate strategy for the modification of photocatalysts.

Observations of Acoustic Emission in Power Semiconductors

Industrial, electrical power generation, and transportation systems, to name but a few, rely heavily on power electronics to control and convert electrical power. Each of these systems, when encountering an unexpected failure, can cause significant financial losses, or even an emergency. A condition monitoring system would help to alleviate these concerns, but for the time being, there is no generally accepted and widely adopted method for power electronics. Acoustic emission is used as a failure precursor in many applications, but it has not been studied in power electronics so far. In this doctoral dissertation, observations of acoustic emission in power semiconductor components are presented. The acoustic emissions are caused by the switching operation and failure of power transistors. Three types of acoustic emission are observed. Furthermore, aspects related to the measurement and detection of acoustic phenomena are discussed. These include sensor performance and mechanical construction of experimental setups. The results presented in this dissertation are the outset of a research program where it will be determined whether an acoustic-emission-based condition monitoring method can be developed.

Pulssitettu laserablaatio mikrofluidisten kanavistojen sarjatuotantomenetelmänä

Puolijohteiden yleistyttyä vuodesta 1948 alkaen, ovat elektroniset laitteet pienentyneet jatkuvasti tehojen kuitenkin kasvaessa. Kasvaneet tehotiheydet kuitenkin vaikeuttavat laitesuunnittelua, sillä puoljohdekomponenttien suorituskyvylle ja eliniälle on oleellista lämpötilojen ja lämpötilavaihteluiden minimointi. Perinteisen ilmajäähdytyksen lähestyessä rajojaan niin kokonaistehon kuin järkevän energiatehokkuudenkin suhteen, on parhaaksi seuraavaksi teknologiaksi ennustettu kaksifaasijäähdytystä, jonka suorituskyky ja energiatehokkuus ovat vaaditulla tasolla. Kaksifaasijäähdytyksen optimaaliselle toiminnalle tärkeää on hyvin suunniteltu ja tarkasti valmistettu lämmönsiirtopinta, jota kutsutaan mikrokanavistoksi. Pulssitettu laserkaiverrus on edistynyt valmistustekniikka, jonka tarkkuus ja luotettavuus sopisivat mikrokanavistojen valmistamiseen. Laserkaiverruksella saavutettavat lopputulokset vaihtelevat kuitenkin materiaalista riippuen ja kupari – jota käytetään yleisesti lämmönjohteena – on eräs huonoimmin lasertyöstöön reagoivista materiaaleista ja siksi on oleellista selvittää laser-kaiverruksen toimivuutta kuparisten mikrokanavistojen valmistuksessa. Pulssitetun laser-kaiverruksen eri variaatioista nanosekunti-luokan pulssinpituuksilla toimivat laitteet ovat jatkuvan tuotannon kannalta paras vaihtoehto niiden hyvän tuottavuuden, saatavuuden sekä kohtuullisen alkuinvestoinnin vuoksi. Käytännön kaiverruskokeiden perusteella selvisi, että menetelmä on laatunsa ja tarkkuutensa puolesta sopiva varsinaiseen tuotantoon. Kaiverruksen tehokkuus kuparia työstettäessä on kuitenkin ennakoituakin heikompi ja niin valmistus- kuin suunnitelu-prosessikin vaativat vielä jatkotutkimusta ja -kehitystä.