Valtchev, S., B. Borges, K. Brandisky, and B. J. Klaassens,
"Resonant Contactless Energy Transfer With Improved Efficiency",
IEEE Transactions on Power Electronics, vol. 24, no. 3: IEEE, pp. 685–699, 2009.
AbstractThis paper describes the theoretical and experimental results achieved in optimizing the application of the series loaded series resonant converter for contactless energy transfer. The main goal of this work is to define the power stage operation mode that guarantees the highest possible efficiency. The results suggest a method to select the physical parameters (operation frequency, characteristic impedance, transformer ratio, etc.) to achieve that efficiency improvement. The research clarifies also the effects of the physical separation between both halves of the ferromagnetic core on the characteristics of the transformer. It is shown that for practical values of the separation distance, the leakage inductance, being part of the resonant inductor, remains almost unchanged. Nevertheless, the current distribution between the primary and the secondary windings changes significantly due to the large variation of the magnetizing inductance. An approximation in the circuit analysis permits to obtain more rapidly the changing values of the converter parameters. The analysis results in a set of equations which solutions are presented graphically. The graphics show a shift of the best efficiency operation zone, compared to the converter with an ideally coupled transformer. Experimental results are presented confirming that expected tendency.
Valtchev, S.,
"Resonant power conversion in contactless battery charging for Electric Vehicle ? Hybrid Electric Vehicle",
2010 International Symposium on Electric Vehicle and 2-nd Annual Conference of Polish Society for Environment Friendly Vehicles, July, 2010.
AbstractThe electromagnetic resonance became irreplaceable tool for wirelessly transfer energy and information. Since long this effect is widely used in communications but recently it is deeply studied and applied at the contactless transfer of energy. The electric?hybrid car battery charging is an urgent need and the knowledge about the resonant contactless transfer became very important. The study of the Series Loaded Series Resonant converter shows it as well suitable for the contactless energy transfer. The idealized Series Resonant Power Converter is used as a base for defining the best (most efficient) modes of operation. Based on the magnetic parameters of the loosely coupled transformer (magnetic link), the characteristics of the contactless power converter are described in approximated form, thus permitting an easier and faster calculation of the converter variables. This is used for instantaneous control of the converter, free from previously known defects.
Valtchev, S.,
Power supply for 2 kW ? 25 kV CO2 gas laser,
: Ministry of Science and Technology Bulgaria ? USSR (limited access), September, 1980.
AbstractInternational project (1251?78 and 1252?78) in which the Technical University of Sofia (S.Valtchev, N.Stefanov) has sucessfully acomplished the task of constructing and implementing in production a high precision (0.01% regulation) power supply for a cirurgical laser tool (1977-1980). The construction involved a vacuum tube cascaded with MOS-FET to obtain very high quality current source necessary for the stable characteristics of the gas discharge in the laser tube.
Valtchev, S.,
"Contactless Power Transfer",
Seminar of S.Valtchev on Contactless Power Transfer, July, 2008.
AbstractThis seminar was a presentation of S.Valtchev on the methods of wireless energy transfer. The seminar was invited by the colleagues of the Power Processing Laboratory of TU Delft, the Netherlands.
Valtchev, S., and B. J. Klaassens,
"Efficient Resonant Power Conversion",
IEEE Transactions on Industrial Electronics, vol. 37, no. 6: IEEE, pp. 490–495, 1990.
AbstractThe DC analysis of a series-resonant converter operating above resonant frequency is presented. The results are used to analyze the current form factor and its effect on the efficiency. The selection of the switching frequency to maximize the efficiency is considered. The derived expressions are generalized and can be applied to calculations in any of the switching modes for a series-resonant circuit. For switching frequencies higher than the resonant frequency, an area of more efficient operation is indicated which will aid in the design of this class of converters and power supplies. It is pointed out that (especially for power MOSFETs where ohmic losses dominate) it is more attractive to select switching frequencies that are higher than the resonant frequency because of the possibility of nondissipative snubbers. Slowing down the rise of the gate voltage and, hence, the slow decrease of ON resistance during turn-on is also not a drawback to high-frequency switching. Because of this safer operation, the standard intrinsic diode of the power MOSFET could be used at high frequencies instead of the more expensive FREDFET
Valtchev, S., B. Borges, K. Brandisky, and J. B. Klaassens,
"Resonant contactless energy transfer with improved efficiency",
IEEE Transactions on Power Electronics, vol. 24, no. 3, pp. 685-699, 2009.
Abstractn/a
Valtchev, S.,
"Super-resonant power converter operation in soft-switching mode",
Seminar of S.Valtchev on the new PhD thesis chapter, February, 1995.
AbstractThis new chapter of the PhD thesis was an addition to the presented already material at TU Sofia, Bulgaria. This was necessary to obtain the permission to prepare the thesis for its final version. Unfortunatelly this thesis was never presented but it served for the core to the presented in IST, Portugal, later on.
Valtchev, S., and S. Valtchev,
"Improved strategy for an instantaneous super-resonant converter regulation",
12th International Conference on Optimization of Electrical and Electronic Equipment (OPTIM), pp. 631–638, July, 2010.
AbstractThe proposed regulation strategy consists in obtaining at the right time the correct information about the energy contained in the resonant tank. This information allows a stable operation of the switches and a higher efficiency of any Series Loaded Series Resonant (SLSR) power converter, especially when contactless energy transfer is concerned. The strategy is based on guaranteeing the correct portion of energy transported by the resonant tank to the load. The portion has to vary corresponding to the error signal taken from the output voltage and remains unchanged if the error signal is at its minimum. In a certain way this method is similar to the Current Mode Control of the classical power converters. The viability of the idea is demonstrated by simulation of a realistic analogue circuit (on preparation for a digital implementation in the near future).
Valtchev, S., B. J. Klaassens, and M. van Wesenbeeck,
"Super?Resonant Converter with Switched Resonant Inductor with PFM?PWM Control,",
IEEE Transactions on Power Electronics, vol. 10, no. 6: IEEE, pp. 760–765, November, 1995.
Valtchev, S.,
Eng. Stanimir Valtchev, Top MSc Graduate of the year 1974, Award and Engraved Wrist-Watch,
: Rector of TU Sofia, Bulgaria, September, 1974.
AbstractEng. Stanimir Valtchev, Top MSc Graduate of the year 1974, Specialized in Technology of Electronic Equipment and Integrated Circuits
Valtchev, {S. S. }, and {J. J. } Pamies-Teixeira,
"Culture of the European Student with an Insight to the Future Needs of the Global (European) Labour Market",
Proceedings of the European Politechnical University, vol. 1, no. NA, pp. 78–82, 1, 2012.
AbstractRecent issues of global heating and energy shortages are imposing a need to change our paradigm around transportation. Somehow, electric vehicles are progressively standing as a strong and necessary alternative for the society. Technically and technologically the acceptance of the EV is easier now than ever but the psychology of the consumers and the running business of internal combustion vehicles, the whole existing infrastructure are too much conservative to be changed easily. The changes in technology require changes in the engineering society and its human resources. The objective of this paper is to give a contribution to the discussion and reflection of potential future scenarios where EV/HEV‘s are spread across the society. It gives an overview of the range of knowledge and competences necessary for a sustainable and streamlined development of those. In fact, it is expected that a new kind of professional profiles need to be created or developed to supply the work market with the right human resources. The paper provides some discussion on the creation of new profiles or adaption of existing ones. Among different possible scenarios the creation of post-graduation courses for students holding undergraduate profiles in the fields referred to earlier would be an interesting and viable solution for fast response. The post graduation would be focused in specific key areas of the EV/HEV. Several factors are pointed out to endorse this scenario
Valtchev, S.,
Series Resonant Power Converter for Contactless Energy Transfer with Improved Efficiency,
: UTL-IST (Portugal), TU Delft (the Netherlands), April, 2008.
AbstractThe development of more efficient power converters is the most important and challenging task for Power Electronics specialists. In the same time, many currently existing or yet to appear future applications require full mechanical independence between the transmitter and receiver of the electrical energy. This contactless form of energy transfer is the concern of the presented work. The work is based on the study of the Series Loaded Series Resonant converter which prove to be the best suitable for the contactless energy transfer. The work investigates the idealized Series Resonant Power Converter with the objective to find the best efficiency zones of operation. Generalized expressions obtained are original and useful. Based on the magnetic parameters of the loosely coupled transformer (magnetic link), the characteristics of the contactless power converter are described in approximated form. The approximation permits easier and faster calculation of the converter variables, thus predicting a shift of the maximum efficiency zone compared to the ideal converter case. The approximated form of the equations permitted to present a new instantaneous form of regulation which combines the frequency and pulse width modes which is free from the previously known defects. The method is based on calculating the energy portions supplied to the load during each half period. Measurements performed on industrial converters and on the laboratory experimental converter, confirm the predicted theoretically behaviour of the converter.
Valtchev, S., R. Miletiev, R. Arnaoudov, and S. Valtchev,
"Control strategy for efficient operation of super-resonant SLSR (contactless) converters",
XLIV INTERNATIONAL SCIENTIFIC CONFERENCE ON INFORMATION, COMMUNICATION AND ENERGY SYSTEMS AND TECHNOLOGIES (before 1990 was: "Day of the Radio Conference"), pp. 527–530, June, 2009.
Valtchev, S.,
"High Efficiency Resonant Power Conversion",
High Efficiency Resonant Power Conversion, April, 1992.
AbstractMode of operation and area of rnhanced efficiency for series-resonant converters is presented. Frequency control on voltage source DC-DC converters is discussed and other principles of control are introduced. Optimum modes of operation for power transistor resonant converters are also considered. For even better efficiency a resonant current shaping is discussed.
Valtchev, S., and J. P. Teixeira,
"The Charging of Hybrid and Electrical Vehicles? Batteries: Contactless Energy Transfer as the key to the future",
2010 International Symposium on Electric Vehicle and 2-nd Annual Conference of Polish Society for Environment Friendly Vehicles, July, 2010.
AbstractWould it be necessary to charge rapidly the electric vehicle (EV) and hybrid electric vehicle (HEV)? It is not easy to answer without a thorough look at the capability of the modern propulsion battery and the power necessities of the EV and HEV. The different possible solutions are compared and maximum speed of charging is analysed. The wired connection for charging is compared to the wireless energy transfer and because of the limitations imposed by the modern and future batteries, the contactless energy transfer is chosen as the future charging method.
Valtchev, S., and S. Valtchev,
"Control for the Contactless Series Resonant Energy Converter",
Emerging Capabilities and Applications of Wireless Power Transfer: IGI Global, pp. 102–140, 2019.
AbstractThis chapter presents control methods applied in the operation of the series loaded series resonant (SLSR) power converters in a most efficient operation zone. The choice of the control method is affected by the objective to guarantee suitably the efficiency, being this method in the same time, relatively easy to apply. The first part of the chapter compares three basic principles of regulation: frequency mode (FM), pulse width mode (PWM), and their combination (PWM/FM). Finally, a new method for instantaneous regulation is developed. The proposed technique consists of a simplified observation of a state variable value to limit each portion of supplied energy, depending on the requirement for power in each half period. The result of this regulation is comparable to the current mode (CM) control applied to the hard-switching power converters. The viability of this new regulation method is demonstrated by simulations of its analogue circuit implementations and experimentally proved. The circuit is also prepared for the changes in the magnetic coupling (contactless energy transfer).
Valtchev, S., K. Brandisky, B. Borges, and B. J. Klaassens,
"Efficient Resonant Inductive Coupling Energy Transfer Using New Magnetic and Design Criteria",
IEEE 36th Power Electronics Specialists Conference, PESC '05, pp. 1293–1298, June, 2005.
AbstractThis paper describes some theoretical and experimental results obtained in an effort to optimize the Series Resonant Converter (SRC) when used with a loosely coupled transformer for Inductive Coupling Power Transfer (ICPT). The main goal of this work is to define precisely which mode of operation of the power stage is the most efficient. The results also suggest a way to choose the design criteria for the physical parameters (operation frequency, characteristic impedance, transformer ratio, etc.) to achieve that mode of operation. The analysis involves also the investigation of the separated in two halves pot core ferrite transformer, especially the way it changes its magnetizing and leakage fluxes and hence, inductances. It is shown that for the practical values of the separation distance, the leakage inductance remains almost unchanged. Nevertheless the current distribution between the primary and the secondary windings changes drastically due to the large variation of the magnetizing inductance. The analysis has lead to a set of equations with solutions that show graphically the way to an optimized operation of the converter, i.e. higher primary currents and higher transformer ratios to fit in the desired mode.
vanWesenbeeck, M. P. N., J. B. Klaassens, U. vonStockhausen, A. M. D. Ancíola, and S. S. Valtchev,
"A multiple-switch high-voltage DC-DC converter",
17th Telecommunications Energy Conference, INTELEC '95, pp. 322–329, October, 1995.
vanWesenbeeck, M. P. N., J. B. Klaassens, U. vonStockhausen, A. Muñoz De Morales Anciola, and S. S. Valtchev,
"A multiple-switch high-voltage dc-dc converter",
IEEE Transactions on Industrial Electronics, vol. 44, no. 6, pp. 780-787, 1997.
Abstractn/a
vanWesenbeeck, M. P. N., J. B. Klaassens, U. vonStockhausen, A. M. D. Anciola, and S. S. Valtchev,
"A multiple-switch high-voltage DC-DC converter",
IEEE Transactions on Industrial Electronics, vol. IE-44, no. 6: IEEE, pp. 780–787, December, 1997.
AbstractSeries connection of power devices has evolved into a mature technique and is widely applied in HV dc systems. Static and dynamic voltage balance is ensured by shunting individual devices with dissipative snubbers. The snubber losses become pronounced for increased operating frequencies and adversely affect power density. Capacitive snubbers do not exhibit these disadvantages, but they require a zero-voltage switching mode. Super-resonant power converters facilitate the principle of zero-voltage switching. A high-voltage dc-dc power converter with multiple series-connected devices is proposed. It allows the application of nondissipating snubbers to assist the voltage sharing between the multiple series-connected devices and lowers turn-off losses. Simulation results obtained with a circuit simulator are validated in an experimental converter operating with two series-connected devices. The behavior of the series connection is examined for MOSFET's and insulated gate bipolar transistors (IGBT's) by both experimental work with a 2-kW prototype and computer simulation. Applications can be found in traction and heavy industry, where the soft-switching converter is directly powered from a high-voltage source.
Vasilev, V., V. Batchev, M. Milev, S. Valtchev, and A. Tatzov,
"An Electronic System for Rowers' Propulsion Motion Activities Studies",
Problems of the Physical Culture and Sport (now: "Sport and Science Magazine"), no. 2, pp. 13–17, February, 1986.
Vassilenko, V., S. Valtchev, {J. J. } Pamies-Teixeira, and S. Pavlov,
"Energy harvesting: an interesting topic for education programs in engineering specialities",
Proceedings of the tenth international scientific-practical conference «Internet-Education-Science» (IES-2016): ВНТУ, pp. 149–156, 2016.
AbstractNowadays some possible alternative methods to replace the batteries as power source, or to achieve better maintenance of existing (or smaller) batteries, are the so called Energy Harvesting (EH) methods, i.e. to obtain energy from the environment or recover and store energy generated by the human body in its usual activities. However, this requires specific technology and materials and usually a multidisciplinary team. For this reason, inclusion a topic on energy harvesting in educational programs of several engineering specialties at technical universities is of great importance.In this paper we present some results from interdisciplinary collaboration at Faculty of Science and Technology at Nova University of Lisbon on the energy harvesting. Experiments were performed in order to calculate the power that could be generated from the chest movements during breathing and from the feet during walking, etc. For the experiments, mostly piezoelectric effect was explored.
Vassilenko, V., {S. S. } Valtchev, and {J. J. } Pamies-Teixeira,
"Energy Harvesting and the Human Health",
Education Science Inovations, vol. 1, pp. 451–460, 1, 2012.
AbstractCurrently, the energy supply for the portable and autonomous equipment comes almost exclusively from the battery. Unfortunately the maintenance of those sources of energy brings disadvantages due to the need for frequent recharging or replacement. In many cases the battery brings extra weight and volume to the electronic equipment, limiting its autonomy. Some possible alternative methods to replace the batteries as power source, or to achieve better maintenance of existing (or smaller) batteries, are the so called Energy Harvesting (EH) methods, i.e. to obtain energy from the environment. For the medical equipment, there is also a possibility to recover and store energy generated by the human body in its usual activities. To harvest energy from the human body or from the environment requires specific technology and materials. The electronic circuits must have extremely high efficiency both in energy conversion and energy consumption. Experiments were performed in order to calculate the power that could be generated from the chest during breathing, from the feet, during walking, etc. For the experiments, mostly piezoelectric effect was explored.