Markus Neubert

Enhanced Modulation Strategy for a Three-Phase Dual Active Bridge—Boosting Efficiency of an Electric Vehicle Converter

Three-phase dual active bridge (3p-DAB) dc-to-dc converters are typically avoided in low-power applications especially for wide voltage and power ranges. Even so, the 3p-DAB do offer a means to reduce filter costs and volume. The aim of this study is to propose the triangular and trapezoidal modulation for the 3p-DAB to address the problem of poor partial load efficiency. The proposed modulation schemes were compared with two conventional DAB concepts. It was found that the efficiency of the 3p-DAB increased substantially. Moreover, the 3p-DAB showed a considerably lower filter volume than that of the single-phase dual active bridge converter (1p-DAB). In conclusion, a modulation strategy combining the two proposed modulation schemes with the phase-shift modulation is ideal, because they boost efficiency and take most benefit from the inherent low filter volume. Ultimately, the three-phase dual active bridge may offer a promising solution to miniaturize galvanically isolated dc-to-dc converters for electric vehicles.

Methodical considerations for setting up space-resolved lumped-parameter thermal models for electrical machines

Lumped Parameter Thermal Networks (LPTNs) are widely used to predict the thermal behavior of electrical machines. The networks usually have a simple structure and low number of nodes to limit the build-up effort, which reduces the spatial resolution of the thermal modeling. This paper presents a methodology that allows automated generation of high-order space-resolved LPTNs for electrical machines within minutes. The resistor network is automatically determined according to the structure of the machine even for complex geometries. In contrast to conventional LPTNs, the proposed thermal model is capable of predicting hot-spot temperature as well as the temperature at any user-specified location within the machine. The simulation results match well with the results of the precise but time-consuming finite element simulation. Meanwhile the calculation effort does not become prohibitive. These detailed thermal investigations can thus be routinely included in the standard machine-design process without considerably delaying it.

Comparison of a single-phase and a three-phase dual active bridge with low-voltage, high-current output

In this paper, a comparison of a single-phase and a three-phase dual active bridge for electric vehicle application is presented. The converters are used to connect the traction battery stack with the 12V on-board power supply. The characteristic challenges are illustrated on the basis of an application with 2kW rated power. In total, three different design approaches are described and assessed using analytical considerations and simulation results. Finally, the conclusions are verified with measurements of two prototypes.

Performance comparison of inverter and drive configurations with open-end and star-connected windings

Electrical drives with open-end winding configurations offer certain advantages over drives with star-connected windings such as higher dc-link voltage utilization and lower inverter losses. However, there are a couple of drawbacks like circulating currents, diminishing the performance of the drive. This paper compares the performance of inverter and drive configurations with star-connected windings and open-end windings regarding efficiency, THD and torque ripple over the entire operation range of the drive. The most widely used B6C topology is compared to the H-bridge topology for open-end winding machines using different modulation schemes. Inverter losses as well as losses due to switching harmonics are taken into account. The comparison is carried out for an interior permanent magnet synchronous machine (IPMSM) with a nominal power of 60kW.

Performance analysis of a triple-active bridge converter for interconnection of future dc-grids

Dc-dc converters are a promising technology for interconnection of future dc grids. Besides the relatively low volume and space requirements, dc-dc converters provide good controllability of the power flow. This is particularly important with regard to a more decentralized energy generation, where a fully bidirectional power flow — even between grids of equal voltage levels — is desired to increase overall grid efficiency and stability. This paper analyzes the performance of a three-phase triple-active bridge converter (3ph-TAB) which interconnects a 5 kV medium-voltage dc grid and two low-voltage dc grids with nominal voltages of 380 V and 760 V, respectively. First, the modulation strategy of the converter is described. The different cases of operation are analyzed and a method is developed which significantly simplifies the theoretical analysis of the converter. The design specifications for the leakage inductances of the transformer and the dc-link capacitors are derived and analyzed. Furthermore, the soft-switching boundaries are derived analytically. The theoretical assessment is supported by a semiconductor loss simulation for the whole operating range.

A galvanically isolated gate driver with low coupling capacitance for medium voltage SiC MOSFETs

This paper presents a galvanically isolated gate driver system for medium voltage SiC-MOSFETs. A low common mode coupling capacity of 1 pF and good electrical insulation of the gate driver power supply are achieved by using a current-loop AC-bus power supply. The power semiconductor is protected against unintentional self-turn-on by a low resistance gate path that is active while the gate driver is not powered.

Enhanced operating strategy for a three-phase dual-active-bridge converter including frequency variation

Recently, it has been shown that the operating modes of a three-phase dual-active-bridge (3ph-DAB) DC-to-DC converter can be altered. This can be used in particular to enhance low load efficiency. The different operating modes show significant differences concerning the utilization of the core material and the required filter effort. In this paper, an enhanced operating strategy including a dynamic variation of the switching frequency is investigated to further optimize the performance of the converter. For a specific electric vehicle application, the efficiency of certain operating points is boosted by up to 1 %. The proposed concept is highly beneficial as it affects the operating points with the highest losses, which potentially reduces the cooling effort.

Dynamic power control of three-phase multiport active bridge DC-DC converters for interconnection of future DC-grids

Dc grid technology is currently expanding from the high-voltage to the medium-voltage range and is expected to penetrate also low-voltage grids. For interconnection of these grids, dc-dc converters that enable a flexible and highly dynamic control of the power flow between different voltage levels are required. In the scope of this paper a highly dynamic power and current control of three-phase multiport-active bridge (3ph-MAB) converters is presented. The investigations are exemplified for a three-phase triple-active bridge (3ph-TAB) converter, i.e., a 3ph-MAB converter with three ports, which connects a medium-voltage dc grid to two separate low-voltage dc grids. Firstly, the complex relation between the power at the ports and the load angles is investigated and algorithms for on-line determination of the according load angles are derived. Secondly, the instantaneous current control (ICC), which is known from the dual-active bridge converter, is adopted for the triple-active bridge converter. Thereby, a highly dynamic current control with settling times of half a switching period is achieved. Based on these considerations, a closed-loop control structure is proposed which fully utilizes the highly dynamic behavior of the ICC. The theoretic analysis is verified by simulation for a 150 kW SiC MOSFET converter prototype with three ports and nominal port voltages of 5 kV, 380 V and 760 V.

Key Components of Modular Propulsion Systems for Next Generation Electric Vehicles

The objective of this paper is to provide an overview of emerging technologies for modular power converter architectures for electric vehicles. Nowadays, the most common electrical drive-train architecture exhibits one single inverter which is directly tied to the battery. As a consequence, only one high-voltage battery module can be applied and the dc-link voltage of the inverter and its apparent power rating is directly dependent on the available battery voltage. To overcome this restriction, modern power converter architectures with a higher degree of freedom have been proposed. These architectures exhibit modular dc-dc converters to allow different battery technologies to be linked to drive inverters operating independently from each other. To make this development feasible, new components and technologies are evolving which enhance the efficiency over mission cycles while ensuring further integration of the power-converter architectures. Wide-bandgap power semiconductors enable high switching frequencies and miniaturization of passive devices. Smart topology enhancements and control methods allow a significant loss reduction, in particular at light loads, resulting in a higher efficiency of the drive train over the entire driving cycle. Highly integrated bidirectional battery charger systems with intelligent charging strategies inhibit battery degradation and provide opportunities for grid stabilization. It is demonstrated how these technologies are realized and implemented to contribute to the development of future electric vehicles.

Significance of thermal cross-coupling effects in power semiconductor modules

In order to develop power electronic systems with high power densities the performance of the cooling system has to be increased. Thereby, the thermal resistance of power semiconductor modules is an important design aspect. However, the thermal resistance of modules is influenced by cross-coupling effects which are dependent on the module layout and the heatsink. These cross-coupling effects are not listed in the datasheet and need to be measured. In this paper, the thermal resistance of a power module is measured utilizing the indirect virtual junction temperature measurement. The measured values are compared to datasheet values. Thermal cross-coupling effects are analyzed for different module configurations. A simple measurement approach is outlined which measures several chips inside the module to analyze cross-coupling effects with respect to the module geometry. Significant deviations from the datasheet values are measured when operating under real world conditions.