Lukas Schrittwieser

99% efficient three-phase buck-type SiC MOSFET PFC rectifier minimizing life cycle cost in DC data centers

Due to the increasing power consumption of data centers, efficient dc power distribution systems have become an important topic in research and industry over the last years. Furthermore the power consumed by data centers is an economic factor, which implies that all parts of the distribution system should be designed to minimize the life cycle cost, i.e. the sum of first cost and the cost of dissipation. This paper analyzes a three-phase buck-type PFC rectifier with integrated active 3rd harmonic current injection for dc distribution systems. Switching frequency, chip area and magnetic components are selected based on a life cycle cost optimization, showing that a peak efficiency of 99% is technically and economically feasible with state-of-the-art SiC MOSFETs and magnetic components. Measurements taken on an 8 kW, 4 kW dm--3 hardware prototype demonstrate the validity and feasibility of the design.

Novel Principle for Flux Sensing in the Application of a DC + AC Current Sensor

This paper presents a new concept for measuring DC and AC magnetic flux densities within a ferromagnetic material. It is based on a measurement of the materials magnetostriction, which is its relative change in length due to the magnetic flux density inside the material. This dimensional change is converted to an electrical signal using a piezoelectric strain sensor. An additional sinusoidal AC excitation of the core material provides higher sensitivity of the length measurement and overcomes the inherent high-pass characteristic of the piezoelectric sensor. Therefore, flux density signals from DC to the kilohertz range can be measured. The concepts feasibility is demonstrated with the design and implementation of an isolated DC + AC current sensor with a measurement range of ±20 A and a bandwidth from DC to 20 MHz.

Novel SWISS Rectifier Modulation Scheme Preventing Input Current Distortions at Sector Boundaries

This paper describes a new modulation concept for the uni- and bidirectional SWISS rectifier, which mitigates ac input current distortions at the mains voltage sector boundaries. An analytical model is derived and compared to simulations, which allows an estimation of the distortions magnitude from design parameters, showing that these distortions increase the input current total harmonic distortion (THD) significantly. A modification of the original circuit topology is proposed, which decouples the operation of SWISS Rectifiers active third-harmonic current injection network and its dc–dc converter switches. An algorithm is presented which allows the calculation of a temporary pulse width modulation for the SWISS Rectifiers current injection network to mitigating the distortions. The concept is verified for both power flow directions and for operation with unsymmetrical and distorted mains voltages by measurement results taken on a bidirectional 7.5-kW SWISS Rectifier prototype. An ac input current THD of 1.3% results for symmetric sinusoidal mains voltages and 1.4% and 1.6% for operation with distorted and unsymmetrical mains voltages.

Control of the input characteristic and the displacement factor of uni- and bidirectional SWISS rectifier for symmetrical and unsymmetrical three-phase mains

This paper introduces a phase-oriented control strategy for the uni- and bidirectional three-phase, bucktype SWISS Rectifier. It allows phase shifted sinusoidal input currents which enable the generation of capacitive or inductive reactive power at the converters AC grid interface. Furthermore, the operation of the SWISS Rectifier with unsymmetrical AC mains voltages is analyzed. Modifications of the control structure, allowing constant AC input power or ohmic mains behavior even with unsymmetrical AC voltages are presented. Simulations and measurements taken on a 7.5 kW bidirectional SWISS Rectifier hardware prototype demonstrate the validity of the theoretical considerations.

Three-phase buck-boost Y-inverter with wide DC input voltage range

Driven by the needs of the continuously growing fuel-cell industry, a promising three-phase inverter topology, the Y-inverter, is proposed, which comprises three identical buck-boost DC/DC converter modules connected to a common star point. Each module constitutes a phase-leg and can be operated in similar fashion to conventional DC/DC converters, independent of the remaining two phases. Therefore, a straightforward and simple operation is possible. In addition, the Y-inverter allows for continuous output AC voltage waveforms, eliminating the need of additional AC-side filtering. Due to the buck-boost nature of each phase leg, the AC voltages can be higher or lower than the DC input voltage. This is an essential feature for fuel-cell applications, which suffer from a wide DC input voltage range. This paper details the operating principle of the Y-inverter, outlines the control system design and verifies its functionality by means of simulation results. The Y-inverter performance in terms of efficiency η and power density ρ is briefly analyzed by means of a multi-objective optimization and a converter design is selected which is compared to a benchmark system realized with a conventional inverter solution.

Multi-level topology evaluation for ultra-efficient three-phase inverters

Multi-level topologies reduce the requirements on inductors and filters, however, given the high number of series connected semiconductors, it is still unclear if they are a suitable option to achieve ultra-high efficiency while maintaining a reasonable power density. For this purpose, an extensive quantitative evaluation of different topologies is carried out, to determine the required volume for a targeted 99.5% efficiency of a 10kW three-phase inverter. This includes the EMI noise filtering, where the Common Mode filter is placed on the DC-side to save losses and the impact of the upcoming EMI regulations covering the range from 2 kHz to 150 kHz is discussed. With an evaluation of multilevel topologies, it is shown that even if a high number of levels can reduce the size of the magnetic components by an order of magnitude, the volume and losses of the capacitive components required to create the multi-level voltage output have to be considered. An evaluation is done to quantify the performance of topologies ranging from two-level to seven-level topologies, and detailed designs of the three-level T-type and seven-level Hybrid Active Neutral Point Clamped converters are presented, achieving a relatively high power density of 2.2 kW/dm3 and 2.7 kW/dm3 respectively.

99.3% Efficient three-phase buck-type all-SiC SWISS Rectifier for DC distribution systems

DC power distribution systems for data centers, industrial applications and residential areas are expected to provide higher efficiency, reliability and lower cost compared to ac systems. Accordingly they have been an important research topic in recent years. In these applications an efficient power factor correction rectifier, supplying a dc distribution bus from the conventional three-phase ac mains is typically required. This paper analyzes the three-phase buck-type unity power factor SWISS Rectifier showing that its input current THD can be improved significantly by interleaving. The dc output filter is implemented using a current compensated Integrated Common Mode Coupled Inductor which ensures equal current sharing between interleaved half bridges and provides common mode inductance. Based on the analysis an high efficient 8 kW, 4 kW dm−3 (64 Win−3) lab-scale prototype converter is designed using SiC MOSFETS. Measurements taken on a hardware prototype confirm a full power efficiency of 99.16 % and a peak efficiency of 99.26 %.

99% Efficient Three-Phase Buck-Type SiC MOSFET PFC Rectifier Minimizing Life Cycle Cost in DC Data Centers

Due to the increasing power consumption of data centers, efficient dc power distribution systems have become an important topic in research and industry over the last years and according standards have been adopted. Furthermore the power consumed by telecommunication equipment and data centers is an economic factor for the equipment operator, which implies that all parts of the distribution system should be designed to minimize the life cycle cost, i.e. the sum of first cost and the cost of the power conversion losses. This paper demonstrates how semiconductor technology, chip area, magnetic component volumes and switching frequency can be selected based on life cycle cost, using analytical and numerical optimizations. A three-phase buck-type PFC rectifier with integrated active filter for 380V dc distribution systems is used as an example system, which shows that a peak efficiency of 99% is technically and economically feasible with state-of-the-art SiC MOSFETs and nanocrystalline or ferrite cores. Measurements taken on an 8 kW, 4 kWdm-3 hardware prototype demonstrate the validity and feasibility of the design.

Novel principle for flux sensing in the application of a DC + AC current sensor

Magnetostriction describes the geometrical change in length of a ferromagnetic material in dependence of its internal magnetic flux density value. By detecting the vibrations caused by these dimensional changes with a piezo-electric transducer, the instantaneous value of the magnetic flux inside a magnetic core can be sensed from DC up to a few kilohertz. This principle, together with a high bandwidth current transformer, was utilized in order to construct a current sensor capable of measuring currents ranging from DC to several MHz. As will be shown in this paper, an additional sinusoidal AC-excitation of the core material provides higher sensitivity of the length measurement and overcomes the high-pass characteristic of the piezo sensor. In order to prove the principle and to demonstrate the capabilities of this new sensor, a series of experimental measurements and implementation results are presented.