Showing papers by "Johann W. Kolar published in 2021"

Three Levels Are Not Enough: Scaling Laws for Multilevel Converters in AC/DC Applications

Abstract: Single-phase inverters and rectifiers in 230 V $_{\text{rms}}$ applications, with a dc-side voltage of 400 V, achieve ultrahigh efficiency with a simple two-level topology. These single-phase designs typically utilize a line-frequency unfolder stage, which has very low losses and essentially doubles the peak-to-peak voltage that can be generated on the ac side for a given dc-link voltage. For certain applications, however, such as higher power grid-connected photovoltaic inverters, electric vehicle chargers, and machine drives, three-phase converters are needed. Because of the three-phase characteristic of the system, unfolders cannot be similarly used, leading to a higher minimum dc-link voltage of the three-phase line-to-line voltage amplitude, which is typically set to 800 V for 230 V $_{\text{rms}}$ phase voltage systems. Previous demonstrations indicate that significantly more levels—and the associated higher cost and complexity—are required for ultrahigh-efficiency three-phase converters relative to their single-phase counterparts. In this article, we seek to determine the fundamental reason for the performance difference between three-phase 800 V dc-link converters and single-phase 400 V converters. First, we build a 2.2 kW dc/ac hardware demonstrator to confirm the necessity of higher complexity converters, showing a simultaneous reduction in efficiency and power density between a two-level 400 V benchmark (99.2% peak efficiency at 18.0 kW/L) and a three-level 800 V inverter phase-leg (98.8%, 9.1 kW/L). With the motivation confirmed, we derive general scaling laws for bridge-leg losses across the number of levels and dc-link voltage, finding the efficiency-optimal chip area and the minimum semiconductor losses. With commercially available Si or GaN power semiconductors, the scaling laws indicate that six or more levels would be required for an 800 V three-phase ac/dc converter to meet or exceed the bridge-leg efficiency of a two-level 400 V GaN benchmark for a fixed output filter. With a complete Pareto optimization, we find that at least seven levels are necessary to recover the efficiency of the two-level 400 V benchmark, and we validate this theory with a seven-level 800 V 2.2 kW hardware prototype with a power density of 15.8 kW/L and a peak efficiency of 99.03%. Finally, two practical solutions that make use of the benefits of unfolder bridges familiar in single-phase systems are identified for three-phase systems.

Comparative Evaluation of a Full- and Partial-Power Processing Active Power Buffer for Ultracompact Single-Phase DC/AC Converter Systems

Abstract: One of the key technical challenges of the Google and IEEE Little Box competition, an international contest to build the world’s smallest 2-kW single-phase inverter in 2015, was to shrink the volume of the energy storage required to cope with the twice mains-frequency (120 Hz) pulsating power at the ac side and meet the stringent 2.5% input voltage ripple at the dc side. In this article, first, a full-power processing buck-type converter active buffer approach, selected by the first prize winner of the Little Box Challenge (LBC), is analyzed in detail. Being relieved from strict voltage ripple requirements, a larger voltage ripple is allowed across the buffer capacitor significantly reducing the capacitance requirement. Second, a partial-power active buffer approach, selected by the second prize winner of the LBC, where conventional passive capacitive buffering of the dc-link is combined with a series-connected auxiliary converter, used to compensate for the remaining 120-Hz voltage ripple across the dc-link capacitance, is studied in detail. In this article, both the selected concepts are comparatively evaluated in terms of achievable efficiency, power density, and ripple compensation performance under both stationary and transient conditions. Novel control schemes and optimally designed hardware prototypes for both considered buffer concepts are presented and accompanied by experimental measurements to support the claimed efficiency and power density and assess the performance of the implemented control systems. Finally, by means of comparison with conventional passive dc-link buffering using only electrolytic capacitors, it is determined at what voltage ripple requirement it actually becomes beneficial in terms of volume to employ the considered active buffer concepts.

EMI Filter Design for a Three-Phase Buck–Boost Y-Inverter VSD With Unshielded Motor Cables Considering IEC 61800-3 Conducted and Radiated Emission Limits

Abstract: The standard converter concept employed in variable speed motor drives is the two-level three-phase Si insulated-gate bipolar transistor voltage source inverter with its switch nodes connected to the motor terminals via shielded cables to avoid excessive high-frequency noise emissions. However, high $dv/dt$ pulses of the inverter pose substantial stresses on the motor, which are further intensified by the ever-faster switching speeds of wide band-gap semiconductors, hence promoting interest in inverters with full-sinewave output filters, which potentially enable the use of inexpensive unshielded motor cables. However, the IEC 61800-3 standard dictates stringent conducted and radiated emission limits on unscreened power interfaces. In this article, a dc input and ac output filter structure allowing operation with unshielded cables is derived for a phase-modular 11-kW buck–boost Y-inverter motor drive system employing 1.2-kV SiC MOSFETs with a switching frequency of 100 kHz. First, regulations and measurement techniques for conducted and radiated emissions of motor drives are analyzed. Next, the operating principle of the Y-inverter is described and an electromagnetic interference equivalent circuit is derived, followed by a systematic filter design. Finally, measurements are conducted on an ultracompact hardware prototype of the converter system with 12 kW/dm3 (197 W/in3) power density, where the results indicate full compliance with the IEC 61800-3 conducted and radiated emission limits for operation with unshielded dc supply and motor cables in a residential area.

Analytical Calculation of the Residual ZVS Losses of TCM-Operated Single-Phase PFC Rectifiers

Abstract: Triangular-current-mode (TCM) modulation guarantees zero-voltage-switching across the mains cycle in AC-DC power converters, eliminating hard-switching with a minor ${\approx} {30}{\%}$ penalty in conduction losses over the conventional continuous current mode (CCM) modulation scheme TCM-operated converters, however, include a wide variation in both switching frequency and switched current across the mains cycle, complicating an analytical description of the key operating parameters to date In this work, we derive an analytical description for the semiconductor bridge-leg losses in a TCM AC-DC converter, including the rms current and/or conduction losses, switching frequency, and switching losses For SiC mosfet s, we introduce a new loss model for switching losses under zero-voltage-switching, which we call “residual ZVS losses” These losses include the constant $C_\text{oss}$ losses found in previous literature but must also add, we find, turn-off losses that occur at high switched currents The existence and modeling of these turn-off losses, which are due to currents flowing through the Miller capacitance and raise the inner gate source voltage to the threshold level and accordingly limit the voltage slew rate, are validated on the IMZA65R027M1H 650V SiC mosfet The complete loss model – and the promise of TCM for high power density and high efficiency – is validated on a 22 kW hardware bridge-leg demonstrator, which achieves a peak 996 $\%$ semiconductor efficiency at full load The proposed, fully-analytical model predicts bridge-leg losses with only 12 $\%$ deviation at the nominal load, accurately including residual ZVS losses across load, modulation index, and external gate resistance

Transient Calorimetric Measurement of Ferrite Core Losses up to 50 MHz

Abstract: An accurate and fast transient calorimetric ferrite core-loss measurement method is proposed in this article In contrast to electrical measurements, the accuracy of the calorimetric approach is largely independent of the magnetic excitation and operating frequency However, accurate values of the thermal capacitance and the temperature of the core under test (CUT) are required Accurate measurement of the specific heat capacity of the core material can be achieved with a differential scanning calorimeter (DSC) or by using the CUT as a dc electric conductor and measuring its thermal response for known Joule heating Accurate temperature measurements can be realized with NTC temperature sensors A thorough uncertainty analysis of the presented method is conducted by identifying the impact of each source of uncertainty in the course of a sensitivity analysis For the considered reference case (R 221/137/79 toroidal core with N49 ferrite material by EPCOS-TDK - 500 kHz/100 mT), the method achieves a total uncertainty with a worst-case value of less than 12% or, in case of a more realistic approach considering a Gaussian distribution of each source of uncertainty, a mean value of −43% with a 95% confidence interval of $\mathbf {\pm }$ 32% The results are verified by means of finite element method (FEM) simulations and experiments Furthermore, a step-by-step description of the workflow for preparing and conducting the experiments is provided The proposed method is tested experimentally and compared to a state-of-the-art electrical loss measurement method for MnZn N87 and N49 ferrite cores of EPCOS-TDK In addition, it is used to measure the loss-map of the NiZn ferrite material 67 from Fair-Rite for very high frequencies up to 50 MHz, which enables the computation of the material's Steinmetz parameters

High-Bandwidth High-CMRR Current Measurement for a 4.8 MHz Multi-Level GaN Inverter AC Power Source

Abstract: The control of very high switching frequency power electronic converter systems featuring latest generation wide bandgap (WBG) devices requires current measurements with a very high bandwidth (BW) to achieve high closed-loop control dynamics. One example is a ultra-high BW 4.8 MHz parallel-interleaved multi-level GaN inverter AC power source with a target output BW of 100 kHz. This work investigates the combination of state-of-the-art Hall-effect current sensors with a suitable high-frequency (HF) sensor to extend the BW of the commercially available current sensor by a factor of 20 – 50, i.e., up to 10 − 20 MHz. The main focus lies on a small form factor and a low realization effort. HF current sensors based on a Rogowski coil, an inductor integrated voltage sensing and a current transformer (CT) are analyzed and compared. Additionally, their respective performance limitations are highlighted. Furthermore, a precise combiner network to combine the low-frequency (LF) and HF signal is analyzed. The combiner circuit is designed in a way that component tolerances have no influence on the behavior in the transition frequency range from LF to HF. Thereby, also the immunity to Common-Mode (CM) disturbances, i.e., the high dv/dt occurring for the switching transitions of WBG semiconductors is considered. Finally, a hardware demonstrator featuring the two most promising current sensor approaches, i.e., the inductor voltage sensing and the CT, is presented and verified with comprehensive measurements in frequency and time domain. A BW from DC up to 35 MHz is measured. The realized sensors are further tested with a hardware prototype of the aforementioned AC power source switching 600 V at an effective switching frequency of 1.6 MHz. The measurements clearly reveal that both proposed sensor concepts are well suited for accurate measurements in fast switching converter systems with negligible additional volume.

Design and Experimental Analysis of 166 kW Medium-Voltage Medium-Frequency Air-Core Transformer for 1:1-DCX Applications

Abstract: The galvanic isolation of solid-state transformers (SSTs) is typically realized with a medium-frequency (MF) magnetic-core transformer (MCT). Previous demonstrations indicate that achieving highly power dense and lightweight MCTs imposes several challenges on the design because of stringent requirements related to insulation and cooling. This work investigates the achievable efficiency of an optimized 166kW / 7 kV air-core transformer (ACT), which is a core part of a DC Transformer (DCX), i.e., an unregulated DC-DC SST with a voltage scaling defined by the transformer turns ratio. The ACT features relatively low complexity of the construction, comparably high coupling values, and high efficiency. Modeling, optimization, and construction of the realized ACT are explained and guidelines regarding insulation, cooling, and shielding of the magnetic stray flux are discussed in detail. Furthermore, the prototype is experimentally validated to demonstrate its full functionality. In the investigated DCX, which is based on a series resonant converter (SRC) topology, the realized ACT is found to achieve a full-load efficiency of 99.5% and an unprecedented gravimetric power density of 16.5 kW/kg. With the use of 10 kV SiC MOSFETs, the complete DCX is estimated to reach an efficiency of 99% at 166kW output power.

Contactless Picking of Objects Using an Acoustic Gripper

Abstract: Acoustic levitation forces can be used to manipulate small objects and liquids without mechanical contact or contamination This work presents analytical models based on which concepts for the controlled insertion of objects into the acoustic field are developed This is essential for the use of acoustic levitators as contactless robotic grippers Three prototypes of such grippers are implemented and used to experimentally verify the lifting of objects into an acoustic pressure field Lifting of high-density objects (ρ > 7 g/cm3) from acoustically transparent surfaces is demonstrated using a double-sided acoustic gripper that generates standing acoustic waves with dynamically adjustable acoustic power A combination of multiple acoustic traps is used to lift lower density objects (ρ≤025g/cm3) from acoustically reflective surfaces using a single-sided arrangement Furthermore, a method that uses standing acoustic waves and thin reflectors to lift medium-density objects (ρ≤1g/cm3) from acoustically reflective surfaces is presented The provided results open up new possibilities for using acoustic levitation in robotic grippers, which has the potential to be applied in a variety of industrial use cases

Input/output EMI filter design for three-phase ultra-high speed motor drive gan inverter stage

Abstract: Pairing wide-bandgap (WBG) inverters with highspeed motors results in compact and efficient motor drives, but requires special attention on electromagnetic interference (EMI) aspects. This paper focuses on electromagnetic compatibility (EMC) of high-speed motor drives, supplied by a DC source. In order to protect the nearby equipment from the EMI noise of the WBG inverter, a filter that complies with conducted EMI regulations is placed at the inverter DC input-side. However, there is no clear mandate requiring from inverters to comply with conducted EMI regulations at the AC output-side, where only the motor is placed. For this reason, there is no full consensus whether it is necessary to use an output filter, and if so, what type of output filter would be suitable, i.e., if differential-mode (DM), common-mode (CM) or both DM/CM output filter would fit best. A full sine-wave output filter (FSF) is proposed in this paper, that features both DM and CM attenuation, and capacitors connected to the DC link. Besides the several well established benefits of a FSF, such as purely sinusoidal motor currents and the protection of the motor against high dw/dt originating from the fast switching of the semiconductor devices, a FSF at the inverter output-side, also reduces the CM EMI emissions at the inverter input-side. Namely, since the inverter housing, the motor housing and the interconnecting shielded cable are all grounded, CM emissions generated at the inverter output-side are directly mapped to the inverter input-side, i.e., there is an input-to-output CM noise interrelation. A FSF reduces the output-side CM EMI emissions and thus mitigates the input-to-output CM noise mutual influence. Two types of FSF (c-FSF and d-FSF) are comparatively evaluated, in terms of volume, losses and EMI performance. The theoretical consideration are tested within the context of a high-speed 280 krpm, 1 kW motor drive, with 80 V DC supply. The experimental results validate the good performance of the proposed filter concept.

Performance Evaluation of Future T-Type PFC Rectifier and Inverter Systems with Monolithic Bidirectional 600 V GaN Switches

Abstract: Three-phase PFC rectifier and inverter systems can be realized using identical main converter stages (MCSs) that comprise three three-level T-Type (TT) bridge-legs and a DC-bus referenced LC-filter stage. TT bridge-legs advantageously realize three output voltage levels by extending a half-bridge with the possibility to connect the switch node to the capacitive DC-link midpoint. To do so, a switching device with bipolar voltage blocking and bidirectional current carrying capability is required. A novel monolithic bidirectional 600 V/140 mΩ GaN e-mode transistor is an ideal candidate to realize the required midpoint connection in a chip-area efficient way. We therefore comprehensively characterize this monolithic bidirectional switch (M-BDS) in an 800 V DC TT bridge-leg realized with two additional 1200 V/140 mΩ SiC MOSFETs. Continuous operation of the M-BDS in all four voltage/current quadrants is demonstrated at ±400 V, and results of calorimetric measurements of the hard- and soft switching losses are provided. This facilitates the evaluation of the TT MCS’ achievable performance in CCM and TCM rectifier and inverter designs regarding efficiency and power density. For MCSs with (three-phase) nominal power ratings of up to 2 kW, we find efficiencies of > 99 % for power densities of up to about 15kW/I.

New EV Battery Charger PFC Rectifier Front-End Allowing Full Power Delivery in 3-Phase and 1-Phase Operation

Abstract: A new universal front-end PFC rectifier topology of a battery charger for Electric Vehicles (EVs) is proposed, which allows fast charging at rated and/or full power level in case of 3-phase (Europe) as well as 1-phase (USA) mains supply. In this regard, a conventional 3-phase PFC rectifier would facilitate only one-third of the rated power in case of 1-phase operation. The new topology is based on a two-level six-switch (2LB6) 3-phase boost-type PFC rectifier, which is extended with a diode bridge-leg and additional windings of the Common-Mode (CM) chokes of the EMI filter. Besides this extension of the power circuit, the general design of the new converter is explained, and the generated Differential Mode (DM) and Common Mode (CM) EMI disturbances are investigated for 3-phase and 1-phase operation, resulting in guidelines for the EMI filter design. The EMI performance (CISPR 11 class-B QP) is experimentally verified for 1-phase and 3-phase operation at an output power of 4.5 kW, using a full-scale hardware prototype that implements the proposed extensions for a 2LB6 3-phase boost-type PFC rectifier and that is designed for output power levels of 22 kW and 19 kW in case of 3-phase and 1-phase operation, respectively. Compared to a conventional 2LB6 PFC rectifier, the volume of the extended system increases from 2.7 dm3 to 3.4 dm3, of which 0.5 dm3 is due to the additional dc-link capacitance for buffering the power pulsation with twice the mains frequency occurring for 1-phase operation.

CFD Assisted Evaluation of In Vitro Experiments on Bearingless Blood Pumps

Abstract: Objective: This paper presents a comparative study of computational fluid dynamics (CFD) simulations and in vitro hemolysis examinations of a bearingless centrifugal blood pump. The outcomes of the in vitro study are analyzed with the help of CFD hemolysis models. Methods: Several pump prototypes were manufactured and tested. Each model was implemented in a CFD framework and simulated with different Eulerian hemolysis models. The outcomes are compared to experimental data. The model achieving the highest correlation is used to explain the in vitro outcomes in detail. Results: It is shown that a double-stage model achieves the best correlation. The sensitivity of the simulation is considerably lower than that of in vitro tests. The CFD model reveals that most of the cell destruction is caused in the radial gap of the pump. Further critical regions are the bottom volume and the shroud clearance gap. Only 0.5% of the priming volume is subject to overcritical shear stress. Conclusion: Cell compatibility can be improved by increasing the radial gap, lowering the shroud and hub clearance gaps, and increasing the fillet radius of the inlet nozzle. CFD models can be used to examine the cell damage effects and help to further improve the pump design. Significance: This paper compares different Eulerian CFD hemolysis models, parameter sets, and equivalent shear stresses to several in vitro hemolysis tests. The sensitivity of the models is compared to that of in vitro studies. It is shown that CFD simulations have their limitations but can help with interpreting the outcomes of in vitro studies.

Comparative Evaluation of Harmonic Injection Techniques for a Phase-Modular Three-Phase Six-Switch Buck-Boost Y-Inverter

Abstract: Phase-modular buck-boost dc/ac inverters extend the voltage conversion range of conventional buck-type inverter topologies, and accordingly offer significant advantages for variable-speed motor drives powered from DC sources with wide-voltage-ranges like fuel cells or batteries. In this Letter, two new modulation schemes are applied to the Six-Switch Buck-Boost Y-Inverter (6YI) topology, with an analysis of the efficiency improvement for the proposed Third Harmonic Injection PWM (TPWM) and Discontinuous PWM (DPWM) schemes relative to conventional Sinusoidal Pulse Width Modulation (SPWM). TPWM substantially reduces conduction losses and DPWM extends this gain by eliminating high-frequency switching for 1/3 of the fundamental period. These gains are validated on a 1kW hardware demonstrator across a DC input voltage from 80V to 240V, with DPWM reducing total converter losses by 31% over SPWM for a peak efficiency at nominal load of 95.6% with a power density of 62.3W/in^3.

Novel High-Speed Turbo Compressor With Integrated Inverter for Fuel Cell Air Supply

Abstract: Fuel cell technology is continuously gaining ground in E-mobility applications. Fuel cells require a constant supply of pressurized air, for which high-speed turbo compressors with air bearings are an optimal choice to reduce size, guarantee oil-free operation required for the lifetime of the fuel cell and increase efficiency. However, the inverter driving the electric motor of the turbo compressor does not scale down with increasing speed, therefore other technology advances are required to achieve an overall compressor system with low weight. New power electronic topologies (double-bridge voltage sources inverter), cutting edge power semiconductor technology (gallium nitride) and multi objective optimization techniques allow to reduce the inverter size, increase inverter efficiency and improve the output current quality and in return lower the losses in the electric motor. This enables the electrical, mechanical and thermal integration of the inverter into the compressor housing of very high-speed and compact turbo compressors, thereby reducing size and weight of the overall compressor system by a factor of two. Furthermore, a turbo compressor with integrated inverter reduces complexity and cost for operators by savings in casing, cables, coolant piping and connectors, and reduces EMI noise by shielding the high frequency motor currents with one housing. Beside its main application for fuel cell air supply, the advantages gained by an integrated inverter can also be used in other boosting and air handling applications such as advanced air and exhaust handling in combustion engines. The proposed integration concept is verified with a 280,000 rpm, 1 kW turbo compressor, targeted for the balance of plant of a 5-15 kW fuel cell. The experimental results show that the temperature limits on the power electronics parts can be kept below the limit of 90°C up to a coolant temperature of 55°C, and besides the advantage of lower cabling effort the efficiency of the compressor system (turbo compressor and integrated inverter) is increased by 5.5% compared to a turbo compressor without integrated inverter.

Three-Port Series-Resonant DC/DC Converter for Automotive Charging Applications

Abstract: In the energy distribution grid of electric vehicles (EVs), multiple different voltage potentials need to be interconnected, to allow arbitrary power flow between the various energy sources and the different electrical loads. However, between the different potentials, galvanic isolation is absolutely necessary, either due to safety reasons and/or due to different grounding schemes. This paper presents an isolated three-port DC/DC converter topology, which, in combination with an upstream PFC rectifier, can be used as combined EV charger for interconnecting the single-phase AC mains, the high-voltage (HV) battery and the low-voltage (LV) bus in EVs. The proposed topology comprises two synergetically controlled and magnetically coupled converter parts, namely, a series-resonant converter between the PFC-sided DC-link capacitor and the HV battery, as well as a phase-shifted full-bridge circuit equivalent in the LV port, and is mainly characterized by simplicity in terms of control and circuit complexity. For this converter, a simple soft switching modulation scheme is proposed and comprehensively analyzed, in consideration of all parasitic components of a real converter implementation. Based on this analysis, the design of a 3.6 kW, 500 V/500 V/15 V prototype is discussed, striving for the highest possible power density and as low as possible manufacturing costs, by using PCB-integrated windings for all magnetic components. The hardware demonstrator achieves a measured full-load efficiency in charge mode of 96.5% for nominal operating conditions and a power density of 16.4 kWL−1.

Analysis of the Influence of Measurement Circuit Asymmetries on Three-Phase CM/DM Conducted EMI Separation

Abstract: Electromagnetic interference (EMI) conducted emissions (CE) are of increasing concern in power electronics due to the high switching frequency and fast switching speeds of the latest generation of wide-bandgap semiconductors. The decomposition of the total conducted EMI noise into its common-mode (CM) and differential-mode (DM) part by means of a CM/DM noise separator is a useful tool that allows for a systematic EMI filter design. Carefully designed realizations achieve a CM rejection ratio and DM rejection ratio of ${50}\,$ dB at ${30}\,$ MHz. However, a very high-performance CM/DM noise separator is not sufficient. It is theoretically analyzed and experimentally proven that asymmetries in the EMI test setup result in an unwanted conversion between CM and DM EMI noise, and therefore significantly influence the CM/DM EMI separation. In particular, three main influences are identified: the line impedance stabilization network (LISN), the connection cables between LISN and the equipment under test (EUT), and the converter EMI filter. The unwanted noise conversion is pronounced for frequencies in the MHz range, where parasitic resonances occur. Experimental results show a CM-to-DM conversion of up to ${-30}\,$ dB at ${30}\,$ MHz (a degradation by ${20}\,$ dB or a factor of 10 compared to a high-performance separator alone) considering a connection cable length mismatch of roughly ${5}\,$ cm. Values as high as ${-21\,}$ dB result when standard commercial LISNs are used for the measurement. The impact of asymmetries in the EMI filter is most severe, and clearly limits the EMI noise splitting at high frequencies. A high-performance noise separator can, however, be used to investigate such filter asymmetries (component tolerances and/or layout), and therefore helps to improve the filter design process and facilitates the modeling of EMI noise sources. [COMP: Change bullet list to numbered]

Analysis and performance evaluation of a three-phase sparse neutral point clamped converter for industrial variable speed drives

Abstract: This paper analyzes the operation and characterizes the performance of a three-phase three-level (3-L) Sparse Neutral Point Clamped converter (SNPCC) for industrial variable speed drives (VSDs). The operating principle of the SNPCC, which advantageously employs a lower number of power transistors than a conventional 3-L inverter, is described in detail, focusing on the AC-side differential-mode and common-mode voltage formation and on the DC-side mid-point current generation processes. The degrees of freedom in the SNPCC modulation scheme are defined and several switching sequences are investigated. Afterwards, the stresses on the active and passive components (e.g. semiconductor losses, machine phase current ripple, DC-link capacitor RMS current, etc.) are calculated by analytical and/or numerical means, enabling a straightforward performance comparison among the identified switching sequences. The most suited modulation strategy for VSD applications is then selected and a chip area sizing procedure, aimed at minimizing the total semiconductor chip size, is applied to a 800V 7.5kW three-phase system. The performance limits of the designed SNPCC are evaluated and finally compared to the ones of conventional 2-L and 3-L solutions, highlighting the promising cost/performance trade-off of the analyzed topology.

3-Φ Bidirectional Buck-Boost Sinusoidal Input Current Three-Level SiC Y-Rectifier

Abstract: Aiming for ever more compact and efficient Electric Vehicle (EV) battery chargers, in this paper a three-phase phase-modular buck-boost Three-Level Flying Capacitor Y-Rectifier (3L-FC-YR) is introduced. The unique circuit structure of the 3L-FC-YR requires time-varying flying capacitor voltages, making the safe and performant operation of such a converter system challenging. Accordingly, a special clamping modulation strategy is proposed which assures equal blocking voltage distribution among the power semiconductors. Further, control strategies and protection circuitry for system startup or failure-mode handling are discussed, such that all practical aspects of the 3L-FC-YR are covered. The concept is verified by means of closed-loop circuit simulations and the resulting waveforms are presented. Finally, a Pareto comparison considering the efficiency and power density limits of the 3L-FC-YR and the standard 2L-YR is conducted considering an extremely wide DC output voltage range of 200 V to 920 V according to the DC CCS HPC350 EV charging standard. The results indicate a significantly higher performance of the 3L approach and/or the feasibility of a CISPR 11 Class A compliant 3L-FC-YR prototype with a peak efficiency of 98.5 % and power density of 19 kW/dm3 (311 W/in3).

Dynamic Electromechanical Model and Position Controller Design of a New High-Precision Self-Bearing Linear Actuator

Abstract: Direct drive tubular linear actuators (TLAs) may be used in various applications that require fast movements and high-precision positioning, e.g., pick-and-place robots. This article presents the analysis, dynamic electromechanical model derivation, controller design, and system operation of a TLA with integrated magnetic bearings (MBs). Furthermore, the actuator operation is verified with extensive measurements on the prototype, which include axial position step response, standard deviation of the steady-state positions and mover tilting control. Compared with any conventional TLA, which may perform only linear motion, the mover tilting control is possible due to integrated MBs. This gives the new actuator a great advantage in high-precision positioning systems, since any thermal expansions of the parallel kinematics may be compensated.

Zero-Voltage-Switching Auxiliary Circuit for Minimized Inductance Requirement in Series-Resonant DC/DC Converter Systems

Abstract: For power electronic converter systems, the cost and the size of the magnetic components are typically limiting factors when it comes to overall production costs and power density. This especially holds for soft-switching high-frequency applications, where a resonant discharge of the output capacitances of the semiconductors relies on sufficient amount of energy stored in the main magnetic components, i.e., inductors or transformers, yielding a substantial increase in their volumes. In this article, an alternative solution is introduced, where a small auxiliary circuit is used to ensure soft-switching of the power transistors, whereby the aforementioned volume of the main magnetic components can be significantly reduced. The proposed auxiliary circuit provides zero-voltage-switching conditions independent of the applied voltages, the switching frequency, and the output power level, without significantly increasing the circuit complexity or the control effort. The new concept is first analyzed based on simple analytical calculations and circuit simulations, and is then experimentally verified by means of hardware demonstrators related to a 3.6-kW 250–500V/250–500V series-resonant dc/dc converter application. Finally, it is shown that the auxiliary circuit can also be used to precharge the output capacitor of a converter system, which allows to further reduce the size of the main magnetic components.

iGSE-C $_{\mathrm{x}}$ – a New Normalized Steinmetz Model for Class II Multilayer Ceramic Capacitors

Abstract: Ferroelectric Class II ceramic capacitors allow for highly compact converter realizations, but are showing relatively high losses for large-signal excitations which must be taken into account in the system dimensioning. Recent literature introduced the iGSE-C $_{\mathrm{Q}}$ , a Steinmetz model based on the macroscopic capacitor Q-U hysteresis, allowing to accurately predict the losses of X7R capacitors. However, the model is specific for each single device, i.e., is insufficient to characterize losses in devices of the same series and manufacturer, which are employing the same dielectric material but with different voltage rating or nominal capacitance value. In this publication, based on basic physical properties we propose a new Steinmetz model, the iGSE-C $_{\mathrm{X}}$ based on the relative dielectric material D-E hysteresis, which is applicable to all devices of a capacitor series. The iGSE-C $_{\mathrm{X}}$ loss modeling technique is demonstrated for the TDK X7R, the TDK X7T, as well as the Knowles Syfer X7R series. Finally, the iGSE-C $_{\mathrm{X}}$ is employed to estimate the large-signal losses of the capacitors of a three-phase inverter and shown to offer sufficient accuracy for a first power circuit design.

Design and Experimental Analysis of a Selfbearing Double-Stator Linear-Rotary Actuator

Abstract: Linear-rotary actuators (LiRAs) are today used in industry applications where a controlled linear and rotary motion is necessary such as pick-and-place robots, servo actuation of gearboxes or tooling machines. However, in special industry applications that require high purity and/or high precision positioning, the usage of conventional LiRAs with mechanical bearings is limited. Therefore, in this paper a LiRA with integrated magnetic bearings, i.e. a selfbearing/bearingless LiRA, is analyzed. The actuator employs concentrically arranged linear and rotary stators placed inside and outside a cylindrically shaped mover, which results in a so-called selfbearing double-stator (SBDS) LiRA. A FEM geometry optimization of the SBDS LiRA is performed and Pareto performance plots concerning linear force and torque generation are obtained. A SBDS LiRA hardware demonstrator and an 18-phase inverter power supply hardware prototype are built and their operation is experimentally verified by rotary and linear position step response measurements.

Quad-Port AC–DC–DC–AC Operation of Isolated Dual Three-Phase Active Bridge Converter

Abstract: This paper investigates the operation of the Dual Three-Phase Active Bridge (D3AB) converter topology as a multi-port converter, by taking the three-phase ac port and dc port on the primary side (ac 1 and dc 1 ) and the galvanically isolated three-phase ac port and dc port on the secondary side (ac 2 and dc 2 ) simultaneously into account. The basic working principle of the D3AB topology under multi-port operation and a strategy to achieve independent power transfer between the four ports are described. It is found that the realization of a power transfer between ac 1 and ac 2 , which can be operated with different line voltages and line frequencies, is particularly challenging, as compared to dc 1 –dc 2 operation. Therefore, this mode of operation is further analyzed and suitable modulation schemes are developed to achieve sinusoidal ac currents, constant dc-link voltages, and reduced low-frequency power pulsations between the ac ports. The derived analytical results are verified by means of circuit simulations and experiments, for a primary-side ac rms phase voltage of 115 V and line frequency of 50 Hz, a secondary-side ac rms phase voltage of 57.5 V and line frequency of 77 Hz, and an output power of 675 W.

Single-Phase Full-Power Operable Three-Phase Buck-Boost Y-Rectifier Concepts

Abstract: Future electric vehicle chargers should feature full nominal output power in single-phase (1-Φ) and three-phase (3-Φ) operation, such that they can be employed in the USA as well as in Europe. Further, the converter system should have buck-boost capability to cover a wide DC output voltage range in order to provide compatibility with various nominal battery voltage levels. The phase-modular Y-Rectifier consists of three buck-boost converter modules, features unity power factor operation and allows for an ultra-compact and highly efficient converter realization, where so far only 3-Φ operation was investigated in literature. In this paper, circuit extensions adapting the Y-Rectifier for both 1-Φ and 3-Φ operation are analyzed and comparatively evaluated, where also a new six-module Y-Rectifier (6M-YR) topology is proposed. The required modulation and control techniques of the 6M-YR are presented for 1-Φ and 3-Φ operation. The discussed topologies are then compared by means of a component current conduction stress and a power stage performance analysis for a 6.6 kW, 3-Φ 400 V (Europe) 1-Φ 240 V (USA) AC, 200 V to 750 V DC application. Further, EMI filter design guidelines for the 6M-YR are presented. The results indicate the feasibility of a highly compact CISPR 11 Class B compliant 6M-YR with a power density of 10 kW/dm3 (164 W/in3) and a nominal efficiency of 98.1 % in 3-Φ and 97.6 % in 1-Φ operation.

Exploring the Physical Limits of Axial Magnetic Bearings Featuring Extremely Large Vertical Levitation Distances

Abstract: This work provides an analytical method, based on the Amperian model of permanent magnets (PM), for a fast calculation of the magnetic flux density in three-dimensional space applying Biot–Savart law, and the calculation of the forces using Lorentz's law. The applied approach enables the characterization regarding forces, torques, and stiffnesses of the levitating PM for any arbitrary position in space. Furthermore, it permits the extension of the investigation to any shape and configuration of ironless magnetic bearings (MB). In order to demonstrate the simple use of the analytical model, in this article, the dimensions of an ironless axial MB employing PMs are optimized with a multiobjective Pareto analysis, which reveals the physical limits concerning maximum achievable levitation height with respect to given constraints on, e.g., the required force and tilting stiffnesses, and the MB robustness defined by the maximum allowable payload on the levitating magnet. Moreover, the optimized axial MB and a corresponding test bench are realized to validate the proposed model with experimental results. For the sake of completeness, it should be mentioned that in a later stage, the optimized MB can also be scaled with simple scaling laws if the demanded specifications, e.g., concerning desired maximum levitation height or payload capability, would have changed.

Load-Independent Voltage Balancing of Multi-Level Flying Capacitor Converters in Quasi-2-Level Operation

Abstract: Quasi-2-level (Q2L) operation of multi-level bridge-legs, especially of flying-capacitor converters (FCC), is an interesting option for realizing single-cell power conversion in applications whose system voltages exceed the ratings of available power semiconductors. To ensure equal voltage sharing among a Q2L-FCC’s switches, the voltages of a Q2L-FCC’s minimized flying capacitors (FCs) must always be balanced. Thus, we propose a concept for load-independent FC voltage balancing: For non-zero load current, we use a model predictive control (MPC) approach to identify the commutation sequence of the individual switches within a Q2L transition that minimizes the FC or cell voltage errors. In case of zero load current, we employ a novel MPC-based approach using cell multiple switching (CMS), i.e., the insertion of additional zero-current commutations within a Q2L transition, to exchange charge between the FCs via the charging currents of the switches’ parasitic capacitances. Experiments with a 5-level FCC half-bridge demonstrator confirm the validity of the derived models and verify the performance of the proposed load-independent balancing concept.

Conceptualization and Analysis of a Next-Generation Ultra-Compact 1.5-kW PCB-Integrated Wide-Input-Voltage-Range 12V-Output Industrial DC/DC Converter Module

Abstract: The next-generation industrial environment requires power supplies that are compact, efficient, low-cost, and ultra-reliable, even across mains failures, to power mission-critical electrified processes. Hold-up time requirements and the demand for ultra-high power density and minimum production costs, in particular, drive the need for power converters with (i) a wide input voltage range, to reduce the size of the hold-up capacitor, (ii) soft-switching over the full input voltage and load ranges, to achieve low losses that facilitate a compact realization, and (iii) complete PCB-integration for low-cost manufacturing. In this work, we conceptualize, design, model, fabricate, and characterize a 1.5 kW, 12 V-output DC/DC converter for industrial power supplies that is required to operate across a wide 300 V–430 V input voltage range. This module utilizes an LLC-based control scheme for complete soft-switching and a snake-core transformer to divide the output current with a balanced flux among multiple secondary windings. Detailed loss models are derived for every component in the converter. The converter achieves close to 96% peak efficiency with a power density of 337 W in−3 (20.6 kW/dm−3), excellent matching to the derived loss models, and zero-voltage switching even down to zero load. The loss models are used to identify improvements to further boost efficiency, the most important of which is the minimization of delay times in synchronous rectification, and a subsequent improved 1.5 kW hardware module eliminates nearly 25% of converter losses for a peak efficiency of nearly 97% with a power density of 308 W in−3 (18.8 kW dm−3). Two 1.5 kW modules are then paralleled to achieve 3 kW output power at 12 V and 345 W in−3 (21.1 kW dm−3) with ideal current sharing between the secondary outputs and no drop in efficiency from a single module, an important characteristic enabled by the novel snake-core transformer.

Modular Multilevel Converter Circulating Current Control with Single Active Filter Module per Phase

Abstract: Phase-legs of DC/AC modular multilevel converters (MMC, M2LC) transfer power from the DC input to the corresponding phase output terminal by means of a current that circulates through the DC input and the phase-leg. In addition to the load-dependent DC component required for the power transfer, this circulating current contains significant harmonics if no countermeasures are implemented. Prominently, a large second harmonic appears, essentially because each converter arm performs a single-phase power conversion. This results in higher RMS values of the arm currents and ultimately in higher-than-necessary losses. One option to mitigate these undesired harmonics and the associated losses extends each arm of the MMC by an active filter module that controls the circulating current by injecting a common-mode component into the arm voltage. In this paper, we propose a new variant of an MMC topology with such active filter modules. In contrast to the state of the art, the proposed realization shows lower realization effort: it uses only a single active filter module per MMC phase-leg instead of two, which corresponds to a reduced effort in terms of both, power hardware components and also control and communication electronics. Furthermore, a single active filter module can operate fully self-contained if desired, i.e., without an external communication interface, thus simplifying system integration.

Comparative Evaluation of Overload Capability and Rated Power Efficiency of 200V Si/GaN 7-Level FC 3-Φ Variable Speed Drive Inverter Systems

Abstract: Variable speed drive systems in e.g. robotics applications are challenged with discontinuous operation cycles and short-time overload current requirements of 2-3 times nominal load. As the motor itself constitutes a large thermal time constant compared to the semiconductor devices in the inverter, latter create a bottleneck for the increased losses during overload operation. Hence, special focus has to be laid on the inverter overload capability, preferably without overdimensioning the system. In this paper a transient thermal model of optimized cooling approaches for 200V GaN and Si packages is empirically deduced. The model is then used to design a 7-Level Flying Capacitor inverter (7L FCi) aiming for 99% efficiency at nominal load for facilitated motor integration and 3 times overload capability. Consequently, the number of parallel switches, switching frequencies and the volume of passive components such as the flying capacitors and the output filter inductor is considered. In order to omit oversizing the inverter and the output inductor for overload operation, an unorthodox way of increasing the switching frequency during overload is proposed. It is concluded, that the small chip size of the 200V GaN devices compared the 200V Si devices poses additional challenges when dealing with overload operation.