Showing papers by "Johann W. Kolar published in 2018"

Accurate Transient Calorimetric Measurement of Soft-Switching Losses of 10-kV SiC mosfets and Diodes

Abstract: The characterization of soft-switching losses (SSL) of modern high-voltage SiC mosfet s is a difficult but necessary task in order to provide a sound basis for the accurate modeling of converter systems, such as medium-voltage-connected solid-state transformers, where soft-switching techniques are employed to achieve an improved converter efficiency. Switching losses (SL), in general, are typically measured with the well-known double pulse method. In the case of SSL measurements, however, this method is very sensitive to the limited accuracy of the measurement of the current and voltage transients, and thus is unsuitable for the characterization of fast-switching high-voltage mosfet s. This paper presents an accurate and reliable calorimetric method for the determination of SSL using the example of 10-kV SiC mosfet modules. Measured SSL curves are presented for different dc-link voltages and switched currents. Furthermore, a deeper analysis concerning the origin of SSL is performed. With the proposed measurement method, it can be experimentally proven that the largest share of the SSL arises from charging and discharging the output capacitance of the mosfet module and especially of the antiparallel junction barrier Schottky diode.

On-state voltage measurement of fast switching power semiconductors

Abstract: The on-state resistance R ds,on is a key characteristic of unipolar power semiconductors and its value depends on the operating conditions, e.g. junction temperature, conducted current and applied gate voltage. Hence, the exact determination of the R ds,on value cannot rely on datasheet information and requires the measurement of current and on-state voltage during operation. Besides the determination of the conduction losses, the on-state voltage measurement enables dynamic R ds,on analysis, device temperature estimation, condition monitoring and consequently time-to-failure prediction. However, in contrast to a switch current measurement, several challenges arise in the design of an on-state voltage measurement circuit (OVMC), i.e. high measurement accuracy (mV-range) during on-state, high blocking voltage capability (kV-range) during off-state and fast dynamic response (ns-range) during switching transitions are demanded. Different OVMC concepts are known from IGBT applications, however, the more severe requirements introduced from the high switching frequency and low OV characterizing the operation of fast switching power semiconductors, prevent their usage. Off-the-shelf products hardly satisfy the mentioned specifications, whereas the performance of state-of-the-art OVM research prototypes require further investigations and/or improvements. With this aim, an innovative OVMC concept is designed, analyzed, calibrated and tested in this paper. Furthermore, the conduction losses of different power semiconductors are measured as function of their operating conditions to validate the performance and highlight the potential of the proposed OVMC.

Generic Derivation of Dynamic Model for Half-Cycle DCM Series Resonant Converters

Abstract: The series resonant converter (SRC) operated in the half-cycle discontinuous-conduction-mode (HC-DCM) provides galvanic separation and a tight coupling of its input and output voltages in an open-loop operation, i. e., with minimum control complexity, which renders it suitable for a wide range of applications that require galvanic isolation. This letter first explains the “DC transformer” behavior of the converter and then provides a generic derivation of a passive equivalent circuit that accurately models the converter's terminal behavior. The proposed generic derivation yields the known analytic results for idealized cases, but additionally allows for a parametrization of the equivalent circuit based on simulated or measured waveforms, thereby facilitating high accuracy also in nonideal cases. Furthermore, it is shown that a noninfinite magnetizing inductance results in an additional load-independent difference between the input and the output voltage, and a corresponding extension of the equivalent circuit model is proposed. The considerations are verified by simulations and measurements of a 10 kW/350 V/350 V HC-DCM SRC system.

Modulation and Control of a Three-Phase Phase-Modular Isolated Matrix-Type PFC Rectifier

Abstract: Three-phase phase-modular isolated power factor correction rectifiers are an interesting alternative to phase-integrated three-phase rectifiers as matrix-type phase modules allow a single-stage isolated energy conversion between the three-phase mains and a dc bus. Therefore, this paper presents a phase-modular isolated matrix-type rectifier which can be connected to the mains either in star (Y) or delta ( $\Delta$ ) configuration, enabling a wide input voltage range. Additionally this allows to select the voltage and current stresses of the phase module switches according to the used semiconductor technology, for example ${\text{650 V}}$ Si or GaN devices could be used in rectifiers powered from the ${400}$ or ${\text{480 V}}_{\text{rms}}$ mains. A detailed analysis of the operating principles and switching behavior of the converter is presented, showing that zero voltage switching can be achieved in the phase modules. Additionally a third harmonic current injection concept is proposed which allows an up to ${15} \%$ higher output voltage in $\Delta$ -mode. The concepts are validated with measurements taken on a ${\text{7.5}}\;{\text{kW}}$ , ${\text{400}}\;{\text{V}}$ dc output voltage prototype converter achieving ${97.2} \%$ efficiency and a total harmonic distortion of $ at rated power.

Minimum loss operation of high-frequency inductors

Abstract: This paper investigates the losses of a power inductor employed in a 2kW, 400V input DC-DC converter, in dependency of key operating parameters, i.e. switching frequency and current ripple. Based on detailed high-frequency winding and core loss models, including the implications of DC-bias, temperature and frequency on the core losses, and an analytic thermal model, a minimum loss inductor is designed for each combination of switching frequency, f, and current ripple, r. In the course of the optimization, the core (E55, N87) and the winding (litz wire, 100 μm) are considered given. Surprisingly, the evaluation of the losses calculated in the f-r domain reveals that nearly minimum inductor losses are obtained for a current ripple that is inversely proportional to the frequency, i.e., for a constant inductance. Further detailed investigations of the calculated inductor losses reveal a decrease of the losses for increasing frequencies up to a very high frequency of 500 kHz. In this regard, at f = 100kHz, minimum total losses of 4.0W result for r = 45%, which can be reduced to 1.8W at f = 500 kHz for r = 10%. Finally, the sensitivities of the losses with regard to different litz wires (71 μm, 200 μm) and a different core (E42, N87) are examined and a design guidance is extracted that summarizes the main findings of the detailed investigation. A calorimetric measurement set-up is used to measure the losses of a realized inductor at different operating points in order to confirm the theoretical considerations.

Scaling laws for electrodynamic suspension in high-speed transportation

Abstract: Electrodynamic suspension (EDS) relies on the repulsive force created by eddy currents in a stationary conductive body (rail) and a magnetic field generated by an excitation system on a moving vehicle (pod). The excitation system in this paper consists of permanent magnets in a Hallbach array. EDS generates lift forces that levitate the pod reliably at high speeds of the vehicle since no mechanical suspension is required. Therefore, it gains interest for high-speed transportation applications such as the Hyperloop project, driven by the Space Exploration Technologies Corporation (SpaceX). Electrodynamic fields and forces have been analysed in detail in the literature; however, the sophistication and/or limited applicability of analytical approaches or the computational burden of FEM/numerical methods render those impractical for the initial design of EDS systems. Therefore, power and loss scaling laws for EDS systems are derived in this study. A 3D simulation for a design example shows that the scaling law is within 10% deviation. Finally, the drag coefficient of EDS systems is compared with other forms of commercial high-speed ground and air transportation systems. A pod with EDS running in vacuum has the potential of decreasing energy consumption significantly above the cruising speeds of modern subsonic airliners.

ZVS Modulation Scheme for Reduced Complexity Clamp-Switch TCM DC–DC Boost Converter

Abstract: DC–DC boost converter zero-voltage-switching (ZVS) modulation schemes such as triangular current mode (TCM) offer a highly efficient operation but suffer from large switching frequency variations, which are complicating the EMI filter design and the digital control. As a solution, a tri-state boost converter operated in ZVS mode, referred to as clamp-switch TCM (CL-TCM) operation can be introduced, which allows to limit the switching frequency variation significantly. This paper presents two variations of the CL-TCM boost converter with reduced number of active switches in the circuit, which are suitable for high input-to-output voltage conversion ratios. In addition, the ZVS modulation schemes, its limitations, the converter design and the controller implementation are presented and analyzed in detail for both converter topologies. The timing calculations for the switching signals are provided for two operating modes, either offering a minimized switching frequency variation and minimized RMS inductor current or a constant switching frequency operation, which in turn comes at the expense of an increased RMS inductor current. The ZVS operation and the operating modes are experimentally verified using a hardware prototype.

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

Abstract: 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.

Three-Phase Two-Phase-Clamped Boost-Buck Unity Power Factor Rectifier Employing Novel Variable DC Link Voltage Input Current Control

Abstract: Battery chargers supplied from the three-phase mains are typically realized as two-stage systems consisting of a three-phase PFC boost-type rectifier with an output DC link capacitor followed by a DC/DC buck converter if boost and buck functionality is required. In this paper, a new modulation scheme for this topology is presented, where always only one out of three rectifier half-bridges is pulse width modulated, while the remaining two phases are clamped and therefore a higher efficiency is achieved. This modulation concept with a minimum number of active half-bridges, denoted as 1/3 rectifier, becomes possible if in contrast to other modulation schemes the intermediate DC link voltage is varied in a six-pulse voltage fashion, while still sinusoidal grid currents in phase with their corresponding phase voltages and a constant battery output voltage are obtained. In this paper, a detailed description of the novel 1/3 rectifier's operating principle and the corresponding control structure are presented and the proper closed loop operation is verified by means of a circuit simulation. Finally, the performance gain of the 1/3 rectifier control scheme compared to conventional modulation schemes is evaluated by means of a virtual prototype system.

Parasitic Extraction Procedures for SiC Power Modules

Abstract: This paper presents an overview of the procedures performed both in academia and industry for estimating the parasitic behavior of power semiconductor packages. The modeling features and limitations of the state-of-the-art software tool, ANSYS Q3D Extractor, and the measurement methods typically used for the parasitic inductance analysis of silicon carbide (SiC) power modules are comprehensively analyzed on the example of a TO-247-3 package with a single 80 mΩ, 1.2 kV SiC power MOSFET, and of a half-bridge wire-bondless module with two 25 mΩ, 1.2 kV SiC power MOSFETs.

Ultrafast rotation of magnetically levitated macroscopic steel spheres

Abstract: Our world is increasingly powered by electricity, which is largely converted to or from mechanical energy using electric motors. Several applications have driven the miniaturization of these machines, resulting in high rotational speeds. Although speeds of several hundred thousand revolutions per minute have been used industrially, we report the realization of an electrical motor reaching 40 million rpm to explore the underlying physical boundaries. Millimeter-scale steel spheres, which are levitated and accelerated by magnetic fields inside a vacuum, are used as a rotor. Circumferential speeds exceeding 1000 m/s and centrifugal accelerations of more than 4 × 108 times gravity were reached. The results open up new research possibilities, such as the testing of materials under extreme centrifugal load, and provide insights into the development of future electric drive systems.

High-Efficiency Weight-Optimized Fault-Tolerant Modular Multi-Cell Three-Phase GaN Inverter for Next Generation Aerospace Applications

Abstract: The aircraft industry demands a significant increase in terms of efficiency and gravimetric power density of power converters for next generation aerospace applications. Between the two minimum targets, i.e. an efficiency > 98% and a gravimetric power density > 10 kW/kg, the specification concerning the converter weight is the most challenging to fulfill. Since cooling systems and magnetic components dominate the weight breakdown of conventional converter concepts, multi-cell topologies, enabling improved semiconductors performance and reduced filtering requirements, are foreseen as promising solutions for the power electronics on board of More Electric Aircraft. On the other hand, the necessary simultaneous operation of a high number of cells inevitably limits the reliability of multi-cell converters if redundancy is not provided. In this paper, a favorable scaling trend of power density with respect to reliability, aiming to guarantee fault-tolerant operation without affecting the performance figures, is identified in modular multi-cell converters. Thus, a 45 kW weight-optimized modular multi-cell three-phase inverter featuring a redundant power stage is optimized, achieving an efficiency of 99% and a gravimetric power density of 22.8 kW/kg.

Magnetic equivalent circuit of MF transformers: modeling and parameter uncertainties

Abstract: Medium-frequency (MF) transformers are extensively used in power electronic converters. Accordingly, accurate models of such devices are required, especially for the magnetic equivalent circuit. Literature documents many different methods to calculate the magnetizing and leakage inductances of transformers, where, however, few comparisons exist between the methods. Furthermore, the impact of underlying hypotheses and parameter uncertainties is usually neglected. This paper analyzes nine different models, ranging from simple analytical expressions to 3D detailed numerical simulations. The accuracy of the different methods is assessed by means of Monte Carlo simulations and linearized statistical models. The experimental results, conducted with a $$100\,{\hbox {kHz}}$$ / $$20\,{\hbox {kW}}$$ MF transformer employed in a $$400\,{\hbox {V}}$$ DC distribution system isolation, are in agreement with the simulations (below 14% inaccuracy for all the considered methods). It is concluded that, considering typical tolerances, analytical models are accurate enough for most applications and that the tolerance analysis can be conducted with linearized models.

Exploration of the Design and Performance Space of a High Frequency 166 kW/10 kV SiC Solid-State Air-Core Transformer

Abstract: With the availability of 10 kV SiC MOSFETs with low Zero Voltage Switching (ZVS) losses, Medium-Voltage (MV) converters, e.g., Solid-State Transformers (SST), capable of operation at very high switching frequencies become feasible. However, the optimization of MV and Medium-Frequency (MF) transformer of the dc–dc converter stage of a high power SST reveals that only limited improvements in efficiency and weight result for switching frequencies exceeding 50 kHz. Therefore, air-core transformers are expected to enable a realization with lower weight and, at the same time, simplified insulation coordination. This paper presents a comprehensive exploration of the design and performance spaces (efficiency, mass, volume) of a conventional, i.e., a magnetic-core based, and an air-core transformer employed in a resonant dc–dc converter with input and output voltages of 7 kV and a rated power of 166 kW. As a result, comparable efficiencies are achievable for both transformers (99.3% and 99.0%), but the SST with air-core transformer at a switching frequency of 103.6 kHz features 41% of the mass (10.3 kg) of a conventional transformer (24.9 kg at a switching frequency of 48.5 kHz). Accordingly, air-core transformers are of special interest for future weight-critical SST applications, e.g., in More Electric Aircraft and More Electric Ships.

Comparative Evaluation of Y-Inverter against Three-Phase Two-Stage Buck-Boost DC-AC Converter Systems

Abstract: Modern motor drives feature output filtering capability in order to protect the motor from high converter output voltage du/dt rates and provide a sinusoidal current to the machine in order to minimize the rotor losses. The incorporation of such motor drive into a fuel-cell (FC) application is challenging since the power electronics converter has to cope with the power dependent variation of the FC voltage. In this paper three candidate converter concepts are comparatively evaluated i.e. a voltage source inverter with front-end DC-DC boost converter (boost VSI), a current source inverter with front-end DC-DC buck converter (buck CSI), and the recently proposed buck-boost Y-inverter topology. In a first step, the three implementations are assessed based on fundamental scaling laws, i.e. the semiconductor losses and/or the required chip area and the inductive components volume, which constitute a major part of the total system losses and volume, are analytically derived. In a second step, the exact performance of the different motor drive solutions in terms of efficiency η and power density ρ is quantified by means of a comprehensive multi-objective optimization. The optimization results reasonably mach the scaling-laws approach and hence further verify the practical value of the analytic calculations. Both the scaling-laws based analysis and the accurate optimization indicate a clear power density advantage in favor of the Y-inverter.

Novel Isolated Bidirectional Integrated Dual Three-Phase Active Bridge (D3AB) PFC Rectifier

Abstract: This Paper proposes a novel Dual Three-Phase Active Bridge (D3AB) PFC rectifier topology for a 400 V dc distribution system, which features galvanic isolation, bidirectional power conversion capability, a high level of component integration, and can be dimensioned with respect to high efficiency. In the course of a comprehensive and in-depth analytical investigation, the working principle of the D3AB PFC rectifier is described in order to enable converter modelling and the derivation of mathematical expressions and limitations needed for converter design and optimization. The developed converter models are verified by means of circuit simulations. An overall optimization of a system with 400 V line-to-line input voltage, 400 V dc output, and P out = 8 kW rated power with respect to efficiency and power density reveals the feasibility of a full-load efficiency of 98.1% and a power density of 4 kW/dm3 if SiC MOSFETs are used. The finally presented design is found to achieve efficiencies greater than 98 % for P out > 1.7 kW.

Distortion Minimization for Ultra-Low THD Class-D Power Amplifiers

Abstract: The effects of key sources of total harmonic distortion (THD) in power output signals of digitally controlled switchmode (Class-D) converters/amplifiers are analyzed. Extensive measurements with a 400 V amplifier prototype, based on gallium nitride (GaN) power transistors, support the investiga-tions. First, the semiconductor loss model and a comprehensive circuit simulation of the converter with its closed-loop feedback system is presented to provide insights on distortion caused by junction temperature variation of the power transistors. Half-bridge interlock time is identified as a significant source of nonlinearity and hence, three simple and effective methods to reduce its deteriorating effect on THD are presented. Another important contribution to linearity arises from the closed-loop feedback controllers, which benefit from small delays and/or, high converter switching frequencies. It is also shown how a Kalman filter, which can be used to significantly reduce converter output noise, deteriorates the THD due to its linear system model. Finally, a method to reduce harmonic distortion and other disturbances caused by a non-ideal DC supply is also demonstrated. By rigorously eliminating distortion sources and applying the presented compensation methods, amplifier output current THD values below -100 dB (0.001%) are achieved and experimentally verified in wide load current ranges.

A 150 000-r/min Bearingless Slice Motor

Abstract: In this paper, the design and operating principle of a slotless bearingless slice motor for high rotational speeds are presented. The performance of the proposed concept is evaluated using an experimental prototype. Measurement results demonstrate a stable dynamic behavior during acceleration and an achievable rotational speed of 150 000 r/min, which is, to the knowledge of the authors, the highest rotational speed attained by a bearingless slice motor to date. The system performance is outlined at its maximum speed, and power loss measurements are carried out over the entire speed range.

Inductive Power Transfer Efficiency Limit of a Flat Half-Filled Disc Coil Pair

Abstract: The efficiency limit for an inductive power transfer between two flat half-filled disc coils is derived based on a model for the eddy current losses in the coils and the losses due to electromagnetic radiation. Analytic approximations for the coupling factor of the coils and eddy current losses are proposed and experimentally verified. It is shown that the approximative terms allow us to express the maximum efficiency of the coil pair analytically. If the strand diameter of the coil is sufficiently small, the efficiency depends only on the strand diameter, diameter of the coils, and the gap between the coils—but not on the operating frequency. Therefore, increasing the frequency does not result in higher efficiency but allows to reduce the coil thickness.

A New Bidirectional Three-Phase Phase-Modular Boost-Buck AC/DC Converter

Abstract: In this paper, a new phase-modular bidirectional three-phase boost-buck AC/DC converter topology is introduced. Each of its three phase modules is operated independently and consists of a boost-buck converter, allowing to directly convert an AC voltage into an arbitrary DC voltage by only modulating one of the two converter stages at a time, where the AC voltages are applied against a reference point common to all phases. Hence, single-stage high-frequency energy conversion is enabled, resulting in a highly compact and highly efficient converter system realization. In a first step, the basic structure of the phase-modular converter (PMC) is derived from the well known cascaded arrangement of a three-phase boost-type rectifier and a subsequent DC/DC buck converter, followed by a discussion of its operating principle and characteristic waveforms. Furthermore, the corresponding DC output voltage control scheme is presented which allows a seamless transition between buck and boost operation in each phase module. Finally, the phase-modular converter and the conventional two-stage system are compared by means of simple indices as well as a two-dimensional Pareto optimization concerning efficiency $\eta$and power density $ \rho $.

New PCB Winding ”Snake-Core” Matrix Transformer for Ultra-Compact Wide DC Input Voltage Range Hybrid B+DCM Resonant Server Power Supply

Abstract: Given the demand for higher power density, higher efficiency and lower realization cost solutions in data centers, dc-dc converters using PCB winding magnetics and resonant operation have been intensively studied in literature. In this paper, a new approach for 300V-430V dc input and 12V dc output server power supplies is proposed, which is based on a series resonant converter topology using a synergy between boundary and discontinuous conduction mode operation, whereby low peak-to-average ratio transformer current waveforms, soft-switching throughout the full power range and tight regulation of the output voltage for a wide input voltage range are achieved. Furthermore, a PCB winding ”snake-core” matrix transformer is introduced, which provides only a single path for the magnetic flux and therefore ensures an equal flux linkage and/or induced voltage of the parallel-connected secondary windings despite possible geometric PCB layout asymmetries. This approach avoids the emergence of circulating currents between parallel-connected secondary windings and guarantees at the same time equal secondary-side voltages in power supplies with multiple isolated outputs. Thus, a higher degree of freedom in the design of the primary windings of PCB winding-integrated transformers is achieved, whereby the design of these transformers is considerably simplified.

Towards a 99.5% Efficient All-Silicon Three-Phase Seven-Level Hybrid Active Neutral Point Clamped Inverter

Abstract: With the deployment of PV energy generation increasing, the need to optimize efficiency of the power conversion from the PV panel to the AC grid while minimizing the material effort and the realization cost has gained significant importance. Accordingly, this paper presents a 12.5kW 99.35% efficient three-phase inverter using commercially available low-voltage silicon power semiconductors. A hybrid sevenlevel topology is employed, with each phase comprising an Active Neutral Point Clamped (ANPC) front-end connected to a Flying Capacitor Converter (FCC) stage, which leads to a low overall volume and weight, i.e. a volumetric power density of 3.4kW/dm3(55.9W/in3) and a gravimetric power density of 3.2kW/kg. Additionally, this topology shows a smaller capacitance requirement and consists of less switches than the conventional FCC topology. Finally, a comprehensive optimization shows that with the next generation of 200V silicon devices it is feasible to achieve a peak efficiency of 99.5%.

Optimized Modulation of a Four-Port Isolated DC–DC Converter Formed by Integration of Three Dual Active Bridge Converter Stages

Abstract: Multi-port converters have gained more and more interest in research during recent years, due to the increasing field of possible applications, e.g., DC micro grids, energy distribution in electric vehicles and more electric aircraft, and power supplies for cascaded multi-cell converters. This paper presents an optimized modulation strategy for a bidirectional multi-port DC–DC converter, which consists of the Integration of Three (3) conventional Dual-Active Bridge (I3DAB) converters into a structure that combines the primary-side full bridges into a common three-phase two-level inverter. The resulting structure features one input port and three isolated output ports. By utilizing so far unused degrees of freedom for the control of the power converter, it is shown that a reduction of the power dissipation can be achieved by adapting the primary-side duty cycles to the output power levels. According to the outcome of a comparative evaluation of conventional and optimized modulation strategies for an example system with input and output port voltages of 700 V and 3 × 100 V, respectively, and with a total nominal power of 3 × 4 kW, reductions of the conduction losses of up to 23% and reduced additional hardware efforts to achieve ZVS operation (with regard to reduced transformers’ magnetizing inductances) within wide power ranges are achievable and are also expected for deviating port voltages. Thus, in combination with the presented optimized modulation strategy, the I3DAB converter is found to be well suited to multi-port applications that require bidirectional conversion capabilities, galvanic isolation, and are subject to unequal load conditions with substantially different power levels provided by the output ports.

Optimal Design of Highly Efficient and Highly Compact PCB Winding Inductors

Abstract: In industrial power electronic systems, power density and costs are key design criteria. Therefore, very high switching frequencies are commonly used in order to minimize the volume of the inductive components. However, especially in high-current applications, the well-known skin and proximity effects as well as the fringing field around the air gaps of inductors, impede the design of these components, as they are usually all reducing the effective cross-sectional area of solid inductor windings. Consequently, litz wire windings would have to be used, which is often impossible, mainly due to cost reasons. This paper introduces a new, simple and efficient approach on how to integrate inductor windings directly into the PCB, by mitigating the high-frequency losses due to a relocation of the air gap. In this way, its fringing field can be used to partly compensate the parasitic magnetic fields causing the skin and proximity effects. Hence, very low production costs and high power densities can be achieved. In a first step, simple design guidelines are derived and verified by means of simulations. Finally, a possible hardware implementation of the proposed concept is presented.

Unified power flow analysis of string current diverters

Abstract: In this paper, a unified power flow analysis is proposed for current diverters which are used for balancing series-stacked voltage domains, e.g. employed in photovoltaic (PV) energy systems or auxiliary power supplies with very high DC input voltage. This analysis allows to easily derive the power levels processed by the current diverters for any given operating point of the attached sources and/or loads representing the voltage domains. The proposed analysis is applied to two examples; on the one hand, PV systems are investigated where it is revealed that power-limited current diverters can only offer a benefit for light shading scenarios; on the other hand, auxiliary power supplies with extremely high-voltage conversion ratios are investigated.

Analysis and Modeling of Eddy-Current Couplings for Auxiliary Power Generation on a Freight Train Wagon

Abstract: The subject of this paper is a non-coaxial eddy-current coupling, which can be utilized on a freight train wagon for generating auxiliary power in the range of several Watts. The coupling comprises a wheel with radially magnetized permanent magnets, which is positioned in the vicinity of the wagon’s wheel, and extracts kinetic energy when the train is in motion. A computational method for solving the 3-D problem of the eddy-current coupling is presented. Maxwell’s equations for calculating the excited eddy currents are solved in the Fourier domain with a semi-analytical method (SAM), resulting in computationally efficient simulations. In a case study, the SAM shows 500 times faster simulation times, compared to a 3-D transient eddy-current finite-element method simulation, carried out with a commercially available software. The SAM is verified with measurements taken on two hardware prototypes. Furthermore, in order to generalize the study, a $\rho \eta $ -Pareto optimization of the system is conducted for relaxed design space boundaries, an output power of $P= {\mathrm {10\,\,W}}$ , a C45E steel wheel with $v= {\mathrm {80\,\,km/h}}$ surface velocity, and $g= {\mathrm {3\,\,mm}}$ air gap. It is shown that a power density up to 0.8 kW/dm3 (13 W/in3) and a transfer efficiency up to 60% can be achieved using the proposed system.

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

Abstract: 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.

Next Generation Measurement Systems with High Common-Mode Rejection

Abstract: There is an increasing importance for floating measurements with high bandwidth in the field of power electronics, driven by the need to characterize power converters utilizing latest wide bandgap devices which offer very fast voltage transitions. In this paper, the required Common-Mode Rejection Ratio (CMRR) as the most important performance metric of a floating or isolated measurement system is analyzed and quantified for different converter realizations with the aim to limit the resulting time-domain error voltage. Afterwards, state of the art isolated single-ended probes are investigated and analyzed in terms of practical applicability and sensitivity regarding variations of the measurement setup, specifically looking at non-ideal connections between probe and converter, often inevitable in power electronic applications. It is found that the performance of isolated single-ended probes quickly degrades when additional connecting wires are used between the circuit under test and probe input. To overcome this inherent limitation of any single-ended approach, an isolated differential structure is presented that combines the advantages of conventional high-voltage differential probes and isolated single-ended probes. Finally, challenges involved when designing a matched differential system are discussed, in particular the influence of amplitude and phase imbalance on the maximum achievable CMRR. Especially the required phase accuracy which needs to be around six times higher than the amplitude accuracy is very hard to realize over the whole frequency range of interest. It turns out, however, that thanks to the passive common-mode filtering already present in state of the art single-ended approaches, CMRR values of around 30–40 dB in the difference-stage enable significant performance improvement for the differential structure.

Three-Phase Rectifiers with Current Compensation Schemes - Part I: Passive Circuits

Abstract: Three-phase rectifiers are widely used in several applications. The main feature desired in these power converters is a satisfying regulation of dc output voltage/current, but this ought to be achieved with high quality ac currents and unitary power factor. The most conventional schemes have unidirectional power flow and a compensation of the ac-side current should be implemented in high-performance systems. A small part of the power converted can be employed to enhance the characteristics in the mains side by using additional passive, active or hybrid circuits. This paper is the first of two parts, where the technology of ac-dc three-phase power converters with compensation techniques to attain ac currents with ohmic behaviour is reviewed. This part reports the passive networks used for this aim, describing the most popular topologies and features.