Publications

Journal Articles

  1. Investigation of Parallel Active Filters' Limitations for Power Decoupling on Single Stage/Single Phase Micro-Inverters (paper)
    G. C. Christidis, A. Ch. Kyritsis, N. P. Papanikolaou, E. C. Tatakis
    IEEE Journal of Emerging and Selected Topics in Power Electronics, vol. 4, no. 3, September 2016
    DOI 10.1109/JESTPE.2016.2552980
    Abstract During the last decade, micro-inverters have gained a considerable share in residential PV systems' market. However, in order to be accepted by the marketplace, the reliability of micro-inverters must be comparable with that of traditional PV string and multi string inverters. A typical reliability issue that these systems experience, particularly in single stage/single phase topologies, is the presence of a large electrolytic capacitor for decoupling the power pulsation caused by the single-phase power generation. For this reason, several decoupling topologies have been introduced, that manage to decouple the power pulsation by using high reliability components, i.e. small capacitors (usually film ones) and power MOSFET transistors. Among these topologies, the family of bidirectional buck-boost based Parallel Active Filter (PAF) converters has been highlighted due to their minimum counterparts and their robust design. The main contribution of the present work is the unified theoretical analysis and the investigation of the operational limitations of this family of PAF converters, as well as the introduction of a control technique which overcomes those restrictions. Simulation and experimental results validate the derived PAF Family analysis and also prove the effectiveness of the introduced control technique.

  2. Hybrid discontinuous/boundary conduction mode of flyback micro-inverter for ac-PV modules (paper)
    G. C. Christidis, A. Ch. Nanakos, E. C. Tatakis
    IEEE Transactions on Power Electronics, vol. 31, no. 6, June 2016
    DOI 10.1109/TPEL.2015.2470094
    Abstract The Current-Source Flyback Inverter has been proven an excellent solution for ac-PV modules. The two principal modes of operation of this converter are the discontinuous conduction mode (DCM) and the boundary conduction mode (BCM). Although BCM offers higher power density, it suffers at lower power levels, because of the significantly increased switching frequency that consequently leads to higher switching losses and lower overall converter efficiency. In this paper, a hybrid operating mode is presented, that combines both DCM and BCM during a utility grid period, so as to improve the converter performance as well as the efficiency and the power density on a wider power range. The mathematical analysis which describes the control specifics especially for a smooth transition between the two modes of operation is presented and demonstrated through computer simulations and experimental results on a laboratory prototype.

  3. Weighted efficiency optimization of flyback micro-inverter under improved boundary conduction mode (i-BCM) (paper)
    A. Ch. Nanakos, G. C. Christidis, E. C. Tatakis
    IEEE Transactions on Power Electronics, vol. 30, no. 10, Oct. 2015
    DOI 10.1109/TPEL.2014.2372005
    Abstract The flyback topology is proven to be a very strong candidate solution for use in ac-PV module applications. Operation in the Boundary Condition Mode (BCM) provides high power density while maintaining the characteristics of a current source inverter. In this paper, a design methodology is presented, that maximizes the weighted efficiency of the converter through an optimization algorithm. The inverter operation is investigated and the behavior under the improved BCM is documented by analytical equations followed by the power loss calculations for each component. This enables to accurately define the relation between the design parameters and the efficiency of the implemented converter and so, an optimization algorithm is established, that takes into consideration the design specifications and constraints. The proposed methodology is also verified with an experimental prototype.

Conference Proceedings

  1. Optimum design of a flyback PV microinverter under hybrid DCM/BCM operation (paper)
    G. C. Christidis, A. Ch. Nanakos, E. C. Tatakis
    18th European Conference on Power Electronics and Applications (EPE 2016), 5-9 Sept. 2016
    DOI 10.1109/EPE.2016.7695626
    Abstract The Current-Source Flyback microinverter is widely used in ac-PV module applications, either in Discontinuous Conduction Mode (DCM), Continuous Conduction Mode (CCM) or Boundary Conduction Mode (BCM). The recently proposed hybrid DCM/BCM operation inherits the merits of both discrete conduction modes and improves the converter performance. In this work a power loss analysis of the hybrid operation is conducted for each converter component and a design optimization algorithm is applied, focusing on the appropriate selection of each converter component and parameter, in order to achieve maximum weighted efficiency. The mathematical analysis is validated using a laboratory experimental prototype.

  2. Analysis of a flyback inverter under hybrid DCM-BCM operation (paper)
    G. C. Christidis, A. Ch. Nanakos, E. C. Tatakis
    17th European Conference on Power Electronics and Applications (EPE 2015), 8-10 Sept. 2015
    DOI 10.1109/EPE.2015.7309414
    Abstract The Current-Source Flyback micro-inverter is widely used in ac-PV module applications, operated either on Discontinuous Conduction Mode (DCM) or Boundary Conduction Mode (BCM). However, each mode has its performance disadvantages (either low power density for DCM or low efficiency due to increased switching losses for BCM). To overcome the above, a hybrid DBCM operation is proposed hereafter, taking into consideration that a smooth transition between the two operating modes needs to be achieved in order to have good output power quality. The mathematical analysis for the hybrid operating mode is presented and validated through simulation and experimental results.

  3. Reliability Analysis for a Waste Heat Recovery Power Electronic Interface Applied at All-Electric Aircrafts (paper)
    S. Saridakis, N. Papanikolaou, D. Voglitsis, E. Koutroulis, E. Tatakis, G. C. Christidis, I. Karatzaferis
    3rd International Conference on Electrical Systems for Aircraft, Railway and Ship Propulsion (ESARS’15), 3-5 Mar. 2015
    DOI 10.1109/ESARS.2015.7101530
    Abstract Reliability is one of the most important parameters in aircrafts. In this paper, a method for the calculation of reliability (i.e. number of failures / 106 hours) of a three-phase full-bridge inverter, which is employed in the dynamic waste heat recovery system of an aircraft, is presented. The main factors for the reliability analysis that have to be considered is the topology of the inverter, the ambient temperature conditions, the power switches (type and modulation technique), and the harmonic filter. The power inverter reliability has been calculated using a software program developed under the Matlab platform, which was used to calculate the failure rate of each device of the inverter, such as the power switches, the DC-bus capacitor and the filter inductor, given the operating switching frequency value. The results indicate that at higher switching frequency levels the inverter of the DWHR system exhibits a high failure rate, thus resulting in a lower Mean Time Between Failures (MTBF).

  4. Modeling and Analysis of an Innovative Waste Heat Recovery System for Helicopters (paper)
    G. C. Christidis, I. Ch. Karatzaferis, M. Sautreuil, E. C. Tatakis , N. P. Papanikolaou
    15th European Conference on Power Electronics and Applications (EPE 2013), 3-5 Sept. 2013
    DOI 10.1109/EPE.2013.6634603
    Abstract The efficiency of helicopters is limited due to the high power losses of combustion engines, a fraction of which can be recovered and provided to the electrical bus by exploiting the heat found in the exhaust gases and through power electronics converters. The first system recovers energy through a thermoelectric generator, whereas the second through a permanent magnet generator rotated by a steam turbine. A third power converter, connected to a supercapacitor bank, compensates the peak currents demanded by the loads. Since extensive simulations have to be run in order to validate the feasibility of those systems, average models of the power converters are derived, and their accuracy is examined. Finally, based on those average models, the developed control strategy is examined.

  5. Comparative Study of MPPT Algorithms for Thermoelectric Generators (paper)
    P. Chatzidakis, G. C. Christidis, E. C. Tatakis
    15th European Conference on Power Electronics and Applications (EPE 2013), 3-5 Sept. 2013
    DOI 10.1109/EPE.2013.6634607
    Abstract In this paper, the modeling of the thermocouple as a power source is examined and a comparative study on suitable Maximum Power Point Tracking (MPPT) methods is conducted. Algorithms already applied in Photovoltaic (PV) modules are compared, as well as some novel improvements on them. This comparison is based on computer simulations as well as an experimental implementation of those methods applied on a Boost converter.
  6. Innovative Waste Heat Recovery Systems in Rotorcrafts (paper)
    G. C. Christidis, I. Ch. Karatzaferis, I. Perpinias, M. Sautreuil, N. P. Papanikolaou, M. Loupis, I. Spanoudakis, E. C. Tatakis
    International Conference on Electrical Systems for Aircraft, Railway and Ship Propulsion (ESARS’12), 16-18 Oct. 2012
    DOI 10.1109/ESARS.2012.6387452
    Abstract Research in modern helicopters is targeted into the increase of their efficiency due to economical and ecological pressures. This paper introduces two innovative methods of absorbing a ratio of the energy remains of the main engine exhaust gases and converting it to electrical energy. The recovered power is then injected to the electrical bus of the helicopter through power electronics converters. The first one uses thermoelectric generators whereas the second one an electromechanical generator. Both of these systems are analyzed, candidate power converter configurations and topologies are depicted and the results of simulations using SABER are evaluated.

  7. Comparative Study of the dc/dc Boost Converter with SiC and Si Power Devices (paper)
    Ap. Charalambous, G. C. Christidis, E. C. Tatakis
    International Conference on Electrical Systems for Aircraft, Railway and Ship Propulsion (ESARS’12), 16-18 Oct. 2012
    DOI 10.1109/ESARS.2012.6387422
    Abstract In this paper, the design and implementation of a 500W dc/dc boost converter using a SiC VJFET and a SiC Schottky Barrier Diode (SBD) is investigated. Firstly, the converter is designed for stepping up a voltage of 48 V to 100 V. It is compared with an identical, more conventional boost converter that employs a Si MOSFET and a Si pn diode. An efficiency of 95.12% has been achieved by the SiC boost converter under nominal conditions. Secondly, the duty cycle is cranked up, as to investigate the maximum voltage step-up that can be attained by the implemented converter. A record-high voltage conversion ratio of 7.65 has been demonstrated, bringing the input of 48 V to 367.4 V, under a duty cycle of 0.88 and an efficiency of 90.91%. The great efficiency and voltage step-up allow for this simple boost converter, to be an affordable and appealing solution for every application where a voltage step-up is required, since it can save up energy, space and weight.

  8. High DC Voltage Step-up Conversion for Marine Applications
    G. C. Christidis, A. Charalambous, E.C. Tatakis
    1st International Marinelive Conference on “All Electric Ship, 3-5 June 2012
    Abstract Within the concept of the “all electric ship”, more and more ship systems are converted in order to be powered by the ship electric system. What is more, new power generation sources are starting to be used in order to reduce the dependence on fossil fuels. As a result, more efficient and reliable power electronics converters are needed in order to fulfill these new demands, especially in the area of dc/dc step-up conversion. In this paper a typical boost converter using Silicon Carbide (SiC) power electronics devices will be presented. It will be shown, both theoretically and experimentally, that it can reach a very high voltage step-up ratio while achieving high efficiency, making it suitable for marine applications.

  9. Behavioral analysis of a single-switch step up converter (paper)
    E. C. Tatakis, M. Kalogeropoulou, G. C. Christidis
    14th European Conference on Power Electronics and Applications (EPE 2011), 30 Aug.-1 Sept. 2011
    Abstract This paper presents a detailed analysis of the behavior of a DC-DC boost-flyback converter which is a single-switch step up converter with a high voltage gain. The behavioral investigation of this converter is carried out supposing it consists of two converters, a boost and a flyback connected in series. The interesting characteristic of this boost-flyback converter is its modes of operation (continuous, discontinuous and mixed), which are depicted analytically. There are two different ways of operation: one in which the boost converter goes over first from the continuous conduction mode (CCM) to the discontinuous conduction mode (DCM) and another one in which the flyback goes over first from CCM to DCM. Each way of operation depends on the duty cycle δ and the ratio kL of both boost’s and flyback’s magnetic components’ inductances. In this paper, all four different modes of operation are investigated and the output voltage characteristic curves are described as well. Finally, simulation and experimental results are compared to those from theory for different values of the duty cycle δ and the load to validate the theoretical analysis.

  10. Μελέτη, σχεδίαση και υλοποίηση επεξεργαστή IDCT μεταβλητού μήκους λέξης για μείωση της κατανάλωσης ενέργειας
    Γ. Χρηστίδης, Θ. Στουραΐτης
    4o Συνέδριο Φοιτητών ΗΜΜΥ, 19-20 Νοεμβρίου 2010
    Abstract Στην παρούσα εργασία περιγράφεται μία τεχνική για τη μείωση της κατανάλωσης ενέργειας ενός επεξεργαστή με τη χρήση μεταβλητού μήκους λέξης για τις αριθμητικές πράξεις. Παρουσιάζεται μία μέθοδος για τον υπολογισμό της ακρίβειας ενός αλγόριθμου ως συνάρτηση του μήκους λέξης. Στην συνέχεια εφαρμόζεται στον αντίστροφο διακριτό μετασχηματισμό συνημιτόνου, ο οποίος συναντάται συχνά σε φορητές συσκευές εικόνας και βίντεο, όπου η κατανάλωση ενέργειας παίζει μεγάλο ρόλο. Έπειτα αναλύεται η σχεδίαση των βασικών αριθμητικών πράξεων με διαφορετικό μήκος λέξης. Ακολουθεί η σύνθεση του επεξεργαστή σε FPGA. Τέλος, συγκρίνεται η κατανάλωση του σε σχέση με την κατανάλωση ενέργειας ενός επεξεργαστή σταθερού μήκους λέξης και ίδιας αρχιτεκτονικής.

  11. Πειραματικές διατάξεις για τη μετάδοση αναλογικού και ψηφιακού σήματος μέσω οπτικού καναλιού
    Σ. Αλειφέρης, Α. Εμερετλής, Γ. Χρηστίδης
    4o Συνέδριο Φοιτητών ΗΜΜΥ, 19-20 Νοεμβρίου 2010
    Abstract Η πρόοδος στον τομέα της οπτοηλεκτρονικής έχει συμβάλει αρκετά στην ανάπτυξη των οπτικών τηλεπικοινωνιών. Εντούτοις η χρήση των οπτικών τεχνολογιών σε προπτυχιακό επίπεδο δεν είναι διαδεδομένη. Γι’ αυτό το λόγο, στην παρούσα εργασία, κατασκευάστηκαν δύο διατάξεις για τη μετάδοση αναλογικού και ψηφιακού σήματος μέσω οπτικής ζεύξης. Η πρώτη διάταξη υποστηρίζει τη μετάδοση σήματος στο ακουστικό φάσμα συχνοτήτων (20Hz - 20kHz) για χρήση σε ασύρματη μετάδοση ήχου χωρίς παραμορφώσεις. Η δεύτερη διάταξη επιτρέπει τη μετάδοση ψηφιακού σήματος μεταξύ δύο υπολογιστών με την οπτικοποίηση των σημάτων που μεταφέρονται μέσω της θύρας RS-232.