Sibylle Dieckerhoff

Power loss-oriented evaluation of high voltage IGBTs and multilevel converters in transformerless traction applications

The low-frequency line transformer in todays ac rail vehicles suffers from poor efficiency and a substantial weight. Future traction drives may operate directly from the mains without this transformer. A feasible concept for a transformerless drive system consists of series connected medium voltage converters applying modern high-voltage insulated gate bipolar transistors (HV-IGBTs). In a first design step, the switching characteristics and losses of 6.5-kV IGBTs are compared to 3.3-kV and 4.5-kV IGBTs which are already commercially used in traction applications. Based on the considered HV-IGBTs, the properties of multilevel converters are analyzed and their applicability to the transformerless system is evaluated. The paper focusses on a loss analysis of the converters. Reliability aspects and harmonic spectra are briefly discussed. Taking these design aspects into account, the three-level neutral point clamped converter turns out to be a reasonable solution to realize line and motor converter modules in a transformerless traction system.

Comparison of state-of-the-art voltage source converter topologies for medium voltage applications

This paper compares a two level voltage source converter (2L-VSC), a three-level neutral point clamped voltage source converter (3L-NPC VSC), a three-level flying capacitor voltage source converter (3L-FLC VSC) and a four-level flying capacitor voltage source converter (4L-FLC VSC) on the basis of state-of-the-art 6.5 kV, 3.3 kV and 2.5 kV IGBTs for a 2.3 kV medium voltage converter. The design of the power semiconductors, the installed switch power, converter losses, the semiconductor loss distribution, modulation strategies and the harmonic spectra are compared in detail.

Experimental Comparison of Model Predictive Control and Cascaded Control of the Modular Multilevel Converter

This paper evaluates two different concepts for the control of the modular multilevel converter (MMC). Generally aiming at medium voltage applications, the considered MMC topology has a low number of submodules, and a pulse-width modulation (PWM) scheme is applied. A model predictive control (MPC) algorithm is developed and experimentally compared to a cascaded control scheme based on conventional PI controllers, which has been proposed by Hagiwara and Akagi. Steady-state and dynamic system performance as well as computational power requirements are discussed. The basis for the comparison is a scaled converter test bench operating from 560-V dc voltage.

Evaluation of IGBT multilevel converters for transformerless traction applications

The low-frequency line transformer in todays ac rail vehicles suffers from poor efficiency and high weight. Future traction drives may operate directly from the mains without this transformer. A feasible concept for a transformerless drive system consists of series connected medium voltage converters applying modern HV-IGBTs. In a first design step, the switching characteristics and losses of the new 6.5 kV IGBTs are compared to 3.3 kV and 4.5 kV IGBTs which are already commercially in use for some time. Based on the considered HV-IGBTs, the paper analyzes the properties of multilevel converters and evaluates their applicability to the transformerless system. This includes a loss analysis, reliability aspects and harmonic spectra. Due to the reduced effort for additional passive components, the 3-leveI NPC converter turns out to be a reasonable solution to realize line and motor converter modules.

Short-circuit evaluation and overcurrent protection for SiC power MOSFETs

In this paper, the short-circuit (SC) performance of two different SiC MOSFETs is experimentally investigated for different input voltages, biasing voltages and case temperatures. The measurement results are compared to simulations, and a good agreement is achieved. For fault handling, two different overcurrent protection (OCP) circuits are designed and applied to the SiC MOSFETs. The desaturation method is successfully tested with a hardware solution substituting the blanking time delay. The second method is based on sensing the voltage drop across the parasitic inductance at the source pin. The experimental and simulation results show that both OCP methods have the capability to detect a short circuit condition in the SiC MOSFET within safe SC time avoiding device failure.

70 mΩ/600 V normally-off GaN transistors on SiC and Si substrates

The static and dynamic electrical performance of normally-off p-GaN gate AlGaN/GaN HFETs, manufactured on SiC and on Si substrates are compared. By implementing a p-type GaN gate, normally-off operation with 1 V threshold voltage has been realized for 70 mΩ / 600 V transistors on both substrates. The GaN-on-SiC devices out-perform Si-based superjunction MOSFETs in terms of gate charge and switching energy and feature a low area-specific on-state resistance, also when considering the full chip area. The higher thermal impedance of the GaN-on-Si devices is reflected in a reduced maximum drain current for pulse lengths > 1 μs. However, no significant thermal effect was found for lower pulse powers as targeted for efficient power switching.

Experimental analysis and modeling of GaN normally-off HFETs with trapping effects

A 70 mΩ / 600 V normally-off AlGaN/GaN HFET is analyzed and modeled. In particular, static and dynamic characteristics are investigated with the focus on modeling trapping effects and their influence on the on-state resistance and on the switching characteristic. Two methods to measure these trapping effects are compared, a clamped measurement of the on-state resistance and a measurement of a shift in the transfer characteristic. Both methods are suitable to extract time constants of trapping effects, which are required for the trap model. A comparison of the measurements demonstrates the link between the increased dynamic on-state resistance and the threshold voltage shift. The developed model is suitable to simulate the performance of the HFET during switching and conduction intervals.

Investigation of the dynamic on-state resistance of 600V normally-off and normally-on GaN HEMTs

In this paper, current collapse phenomena and thermal effects in newly developed normally-off and normally-on GaN HEMTs are investigated. The experimental results show that a high off-state voltage and high switched drain current at high junction temperature cause an increase of the on-state resistance. During switching from 50 kHz to 400 kHz, an increase of RDSON is observed for both types of GaN devices. It is evident that the number of switching transients significantly influences the increase of the on-state resistance, suggesting that this increase is due to a current collapse in GaN HEMTs. A detailed comparison of the evaluated RDSON between GaN transistors and the newest high-speed CoolMOS-C7 transistor is presented.

Electric Characteristics of Planar Interconnect Technologies for Power MOSFETs

In this paper, two planar interconnection technologies for power semiconductors - a Flip Chip solution with a flexible substrate and a Chip-in-Polymer approach - are presented. These technologies substitute the wire bonds which are usually applied to contact the top side pads of power semiconductors. The process flow of the technology developments is explained. Exemplarily, prototypes are built applying fast switching MOSFETs in the voltage class up to 100 V. Based on these prototypes, experimental and simulation results with regard to the thermo-electric characteristics of the prototypes are discussed.

Experimental comparison of DPC and VOC control of a three-level NPC grid connected converter

This paper presents the principle of Direct Power Control (DPC) and Voltage Oriented Control (VOC) for a three-level Neutral point clamped (NPC) grid connected inverter. Considering balancing of the neutral point, a switching table with neutral point voltage control is derived for DPC. The two control algorithms are successfully implemented in a test bench, and an experimental comparison is conducted including dynamic performance and harmonics analysis in steady state. The equivalent switching frequency concept is introduced for DPC to guarantee a switching frequency comparable to VOC.