Georgios C. Christidis

Weighted Efficiency Optimization of Flyback Microinverter Under Improved Boundary Conduction Mode (i-BCM)

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.

Hybrid Discontinuous/Boundary Conduction Mode of Flyback Microinverter for AC–PV Modules

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 that 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.

Innovative waste heat recovery systems in rotorcrafts

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.

Modeling and analysis of an innovative waste heat recovery system for helicopters

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.

Comparative study of MPPT algorithms for thermoelectric generators

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.

Comparative study of the dc/dc boost converter with SiC and Si power devices

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.

Analysis of a Flyback Current Source inverter under hybrid DCM-BCM operation

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.

Optimum design of a flyback PV microinverter under hybrid DCM/BCM operation

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.

Reliability analysis for a waste heat recovery power electronic interface applied at all-electric aircrafts

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).