Dominik Neumayr

ηρ-Pareto optimization and comparative evaluation of inverter concepts considered for the GOOGLE Little Box Challenge

In recent years, driven by worldwide growing environmental awareness the research in power electronics was focusing on the development of highly efficient but mostly bulky converter systems e.g. for interfacing renewable energy to the grid. The GOOGLE Little Box Challenge was impulse to give the power density again more attention by motivating engineers worldwide to design a single-phase solar inverter system at the cutting edge of what is technically possible. In this paper a comparative evaluation of inverter concepts considered by a team of ETH Zurich, FH-IZM and Fraza company for the GOOGLE Little Box Challenge is given. Based on the lessons learned from the participation in the competition, for the considered inverter concepts the achievable efficiency, power density and the optimal modulation scheme are identified with a multi-objective ηρ-Pareto optimization. This provides a sound basis for the redesign of the existing system pushing the forefront of power density even further.

Ultra-compact Power Pulsation Buffer for single-phase DC/AC converter systems

In single-phase power conversion systems, typically bulky electrolytic capacitors are installed in order to cope with the intrinsic double-line frequency power pulsation. However, since the voltage ripple at the dc bus is typ. limited to just a few percent of the nominal voltage, only a small fraction of the actually stored energy in the capacitors is used for the power decoupling. In this paper an auxiliary buffer converter is employed, shifting the double-line frequency power pulsation away from dc bus to a buffer capacitor. Being relieved from strict voltage ripple requirements, a larger voltage ripple is allowed across the buffer capacitor, significantly reducing the capacitance requirement. In this paper an ultra compact Power Pulsation Buffer (PPB) is designed for a 2kW PV-inverter application by means of a comprehensive Pareto optimization. Besides compensating the power pulsation, the PPB must be able to quickly stabilize the dc bus in case of abrupt load variations and maintain an average buffer capacitor voltage. In this paper, a novel cascaded control structure is presented, meeting all aforementioned control objectives. A constructed prototype of the optimized PPB is presented in the paper and experimental measurements verify the outstanding performance of the proposed control system.

Comprehensive evaluation of GaN GIT in low- and high-frequency bridge leg applications

In power electronics applications with power ratings around several kilowatts, wide band gap semiconductors are more and more replacing state-of-the-art Si MOSFET. SiC MOSFETs with blocking voltage rating up to 1200V and low-voltage GaN devices are already commercially available on the market since a couple of years. Now also 600V GaN devices are entering the market, which are a cost-effective solution in many 400V key applications in order to increase the system performance in terms of achievable efficiencies or power density. Besides the employed semiconductor devices also the design of the appropriate gate drive circuit is important. In this paper a simple and reliable gate drive circuit for driving GaN switches is presented. In addition, the proposed gate drive is used to evaluate the switching performance of a GaN Gate Injection Transistor (GIT) under soft- and hard-switching condition, which provides a basis for further optimization of totem-pole converter systems.

Comprehensive large-signal performance analysis of ceramic capacitors for power pulsation buffers

In high power density single-phase PV inverter systems, active auxiliary circuits are installed, shifting the double line-frequency power pulsation away from the dc link to a dedicated buffer capacitor. Being relieved from strict voltage ripple requirements, a larger voltage ripple is allowed at the buffer capacitor, significantly reducing the capacitance requirements and consequently the overall volume of the converter system. Since the capacitance density of electrolytic capacitors must be derated because of imposed current limitations, ceramic capacitors become the preferred choice in power pulsation buffer applications. In particular, TDKs class II X6S MLCC with BaTiO3 ceramic and the recently launched CeraLink with PLZT ceramic are promising candidates due to their high energy density. The actual prevailing capacitance of these two types of ceramics, strongly depends on the operating point, that is, a dc bias voltage and a superimposed large-signal amplitude ac voltage. Unfortunately, the large-signal behavior and performance of these ceramic capacitors is not specified by the manufacturer. In this paper, a comprehensive characterization of the X6S MLCC and the CeraLink large-signal performance is carried out by means of experimental measurements. The acquired data enables an accurate dimensioning of the power pulsation buffer capacitor and an estimate of the capacitor losses during operation. Since the device temperature has a strong impact on the ceramic properties, results for 30 °C, 60 °C and 90 °C are presented.

Constant duty cycle sinusoidal output inverter with sine amplitude modulated high frequency link

Despite the increasing performance of power semi-conductors and passives components, limited timing resolution in off-the-shelf available digital control hardware often prevents the switching frequency in kW-scale dc/ac power conversion to be increased above several MHz for the sake of extreme power densities. In this paper an alternative approach to generate a sinusoidal output voltage, based on constant duty cycle frequency shift control of a high frequency resonant inverter stage and a subsequent synchronous cycloconverter, is analyzed. The design of the presented converter is facilitated by means of a derived mathematical model. A novel closed-loop control system is proposed which achieves tight regulation of the output voltage by means of controlling the switching frequencies of the involved bridge legs operated in resonant mode. Characteristic waveforms of the dc/ac converter during steady-state and load transients are presented. Two distinct implementations of the resonant inverter stage, constituting an intermediate voltage or intermediate current link, are analysed and compared.

Behavior of the flying capacitor converter under critical operating conditions

The Flying Capacitor Converter (FCC) offers an attractive alternative to conventional 2-IeveI converter topologies due to the easily acquired high number of voltage levels and the increased effective switching frequency. However, balancing of the flying capacitor (FC) voltages is crucial in practice since a deviation from the nominal voltage levels increases harmonics in the output voltage and, more importantly, jeopardizes the integrity of the converter due to overvoltages across the power transistors. Modulation inherent FC balancing techniques (termed natural/passive balancing) have been thoroughly analyzed in literature, however only for stationary operating conditions. In this paper, the behavior of the FCC and the effectiveness of passive balancing will be analyzed in detail regarding specific operating conditions present in typical industry applications such as converter start-up, shut-down, standby and operation under fault conditions. The basis for the analysis is a 5-level, 2 kW FCC embedded in two typical industry applications: single-phase PV inverter and single-phase PFC rectifier.

Novel efficiency-Optimal Frequency Modulation for high power density DC/AC converter systems

State-of-the-art high power density AC/DC and DC/AC converter systems typically employ Triangular Current Mode (TCM) modulation or conventional PWM. TCM is characterized by a wide variation of switching frequency over the mains period and ensures soft-switching in all operating points, but results in increased conduction and high-frequency losses due to the necessary large current ripple. In contrast, PWM with constant switching frequency features a lower RMS current and thus reduced conduction losses, but cannot achieve soft-switching over the entire mains period and suffers from turn-on losses. In this paper it is investigated if an optimal combination of these two operating modes, i. e. an optimal adjustment of switching frequency and/or current ripple amplitude throughout the mains period, can lead to an increase in conversion efficiency and how such an Optimal Frequency Modulation (OFM) control scheme can be implemented in practice. The presented analysis is based on an ultra compact 2kW, 400V DC/AC converter system designed to overcome the GOOGLE Little Box Challenge.

Analysis and design of a multi-tapped high-frequency auto-transformer based inverter system

The Google Little Box Challenge, an open competition to build the world smallest 2 kW Photo-Voltaic (PV) inverter, triggered a trend in the power electronic community to design PV inverter systems with a very high power density. In particular, in this paper, a novel single-phase non-isolated PV inverter topology is presented, achieving both high efficiency and high power density. The proposed solution takes advantage of a highly efficient resonant DC-DC stage to generate multiple intermediate DC voltage levels; these levels are then modulated by means of a high-frequency operating tap-selector and the resulting waveform is filtered and unfolded to obtain a low-frequency sinusoidal voltage at the output. Only a fraction of the full DC-link voltage is switched in every commutation of the tap-selector, consequently the occurring switching losses are reduced with respect to single-stage PWM inverters. At the core of the DC-DC stage, a high-frequency auto-transformer with multiple output taps is employed. Since it is realized only with a single winding and a fraction of the input current directly flows to the output, its utilization increases the efficiency and reduces the volume with respect to an equivalent transformer based solution. A detailed analysis of the working principle of the investigated inverter is presented in this paper together with a Pareto optimal design, focusing in particular on the resonant operation and on the modeling of the autotransformer. The performance in terms of losses and volume is finally provided for the optimal solution.