DC-link capacitors are an important part in the majority of power electronic converters which contribute to cost, size and failure rate on a considerable scale. From capacitor users' viewpoint, this paper presents a review on the improvement of reliability of dc link in power electronic converters from two aspects: 1) reliability-oriented dc-link design solutions; 2) conditioning monitoring of dc-link capacitors during operation. Failure mechanisms, failure modes and lifetime models of capacitors suitable for the applications are also discussed as a basis to understand the physics-of-failure. This review serves to provide a clear picture of the state-of-the-art research in this area and to identify the corresponding challenges and future research directions for capacitors and their dc-link applications.

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Reliability of Capacitors for DC-Link Applications in Power Electronic Converters—An Overview

Switching devices based on wide bandgap materials such as silicon carbide (SiC) offer a significant performance improvement on the switch level (specific on resistance, etc.) compared with Si devices. Well-known examples are SiC diodes employed, for example, in inverter drives with high switching frequencies. In this paper, the impact on the system-level performance, i.e., efficiency, power density, etc., of industrial inverter drives and of dc-dc converter resulting from the new SiC devices is evaluated based on analytical optimization procedures and prototype systems. There, normally on JFETs by SiCED and normally off JFETs by SemiSouth are considered.

https://www.hpe.ee.ethz.ch/uploads/tx_ethpublications/SiC.pdf

SiC versus Si—Evaluation of Potentials for Performance Improvement of Inverter and DC–DC Converter Systems by SiC Power Semiconductors

In the first part of this paper, three-phase power factor correction (PFC) rectifier topologies with sinusoidal input currents and controlled output voltage are derived from known single-phase PFC rectifier systems and/or passive three-phase diode rectifiers. The systems are classified into hybrid and fully active pulsewidth modulation boost-type or buck-type rectifiers, and their functionality and basic control concepts are briefly described. This facilitates the understanding of the operating principle of three-phase PFC rectifiers starting from single-phase systems, and organizes and completes the knowledge base with a new hybrid three-phase buck-type PFC rectifier topology denominated as Swiss Rectifier. Finally, core topics of future research on three-phase PFC rectifier systems are discussed, such as the analysis of novel hybrid buck-type PFC rectifier topologies, the direct input current control of buck-type systems, and the multi-objective optimization of PFC rectifier systems. The second part of this paper is dedicated to a comparative evaluation of four rectifier systems offering a high potential for industrial applications based on simple and demonstrative performance metrics concerning the semiconductor stresses, the loading and volume of the main passive components, the differential mode and common mode electromagnetic interference noise level, and ultimately the achievable converter efficiency and power density. The results are substantiated with selected examples of hardware prototypes that are optimized for efficiency and/or power density.

The Essence of Three-Phase PFC Rectifier Systems—Part II

It is well known that single-phase pulse width modulation rectifiers have second-order harmonic currents and corresponding ripple voltages on the dc bus. The low-frequency harmonic current is normally filtered using a bulk capacitor in the bus, which results in low power density. However, pursuing high power density in converter design is a very important goal in the aerospace applications. This paper studies methods for reducing the energy storage capacitor for single-phase rectifiers. The minimum ripple energy storage requirement is derived independently of a specific topology. Based on the minimum ripple energy requirement, the feasibility of the active capacitor's reduction schemes is verified. Then, we propose a bidirectional buck-boost converter as the ripple energy storage circuit, which can effectively reduce the energy storage capacitance. The analysis and design are validated by simulation and experimental results.

A High Power Density Single-Phase PWM Rectifier With Active Ripple Energy Storage

A higher order passive power filter (LLCL filter) for the grid-tied inverter is becoming attractive for industrial applications due to the possibility to reduce the cost of the copper and the magnetic material. However, similar to the conventional LCL filter, the grid-tied inverter is facing control challenges. An active or a passive damping measure can be adopted to suppress the possible resonances between the grid and the inverter. For an application with a stiff grid, a passive damping method is often preferred for its simpleness and low cost. This paper introduces a new passive damping scheme with low power loss for the LLCL filter. Also, a simple engineering design criterion is proposed to find the optimized damping resistor value, which is both effective for the LCL filter and the LLCL filter. The control analysis and the power loss comparison for different filter cases are given. All these are verified through the experiments on a 2-kW single-phase grid-tied inverter prototype using proportional resonant controllers. It is concluded that, compared with the LCL filter, the proposed passive damped LLCL filter can not only save the total filter inductance and reduce the volume of the filter but also reduce the damping power losses for a stiff grid application.

A New Design Method for the Passive Damped LCL and LLCL Filter-Based Single-Phase Grid-Tied Inverter

Power density of power electronic converters in different applications has roughly doubled every 10 years since 1970. Behind this trajectory was the continuous advancement of power semiconductor device technology allowing an increase of converter switching frequencies by a factor of 10 every decade. However, today's cooling concepts, and passive components and wire bond interconnection technologies could be major barriers for a continuation of this trend. For identifying and quantifying such technological barriers this paper investigates the volume of the cooling system and of the main passive components for the basic forms of power electronics energy conversion in dependency of the switching frequency and determines switching frequencies minimizing the total volume. The analysis is for 5 kW rated output power, high performance air cooling, advanced power semiconductors, and single systems in all cases. A power density limit of 28 kW/dm3@300 kHz is calculated for an isolated DC-DC converter considering only transformer, output inductor and heat sink volume. For single-phase AC-DC conversion a general limit of 35 kW/dm3 results from the DC link capacitor required for buffering the power fluctuating with twice the mains frequency. For a three-phase unity power factor PWM rectifier the limit is 45 kW/dm3@810 kHz just taking into account EMI filter and cooling system. For the sparse matrix converter the limiting components are the input EMI filter and the common mode output inductor; the power density limit is 71 kW/dm3@50 kHz when not considering the cooling system. The calculated power density limits highlight the major importance of broadening the scope of research in power electronics from traditional areas like converter topologies, and modulation and control concepts to cooling systems, high frequency electromagnetics, interconnection technology, multi-functional integration, packaging and multi-domain modeling and simulation to ensure further advancement of the field along the power density trajectory.

https://www.hpe.ee.ethz.ch/uploads/tx_ethpublications/IEEJ_PowerDensity_Paper_FinalRevised_03.pdf

PWM Converter Power Density Barriers

https://www.jstage.jst.go.jp/article/ieejias/128/4/128_4_468/_pdf

This paper presents a study and practical implementation of an active filter employing a high-frequency (HF) power amplifier and passive-filter components to be connected to the ac power lines in order to mitigate common-mode conducted emissions of three-phase pulsewidth modulation converter systems. The filter topology is chosen from different possibilities listed in a literature survey and studied regarding practical implementation issues, where requirements for an HF power amplifier to be applied in active filtering are derived. Special attention is put on the stability analysis where the challenges for the feedback are discussed, and a simple feedback structure is proposed. Other feedback concepts are analyzed, and limitations posed by stability requirements are presented. A prototype is designed and built, from which mathematical and experimental results are obtained demonstrating the potential and limitations of such a system.

Implementation of a Transformerless Common-Mode Active Filter for Offline Converter Systems

Cost, volume, and weight are three major driving forces in the automotive area. This is also true for hybrid electric vehicles, which are attracting more and more attention due to increasing fuel costs and air pollution. In hybrid vehicles, the energy distribution system causes a significant share of the volume and the costs. One part of this system is the DC-DC converter that transfers power between the low- and high-voltage buses. In order to reduce the costs and the volume of this converter, this paper presents a new concept for integrating the DC-DC converter functionality into the traction drive system. By using the inverter and the machine to implement a primary bridge leg of an isolated full-bridge DC-DC converter, the total system costs and weight can be reduced. This concept is verified by simulations and experimental results for a scaled prototype. An analytical model of the system has been developed and agrees very well with the measurements. By scaling this model to the power levels typical for hybrid vehicles, it is expected that the efficiency for the DC-DC converter will be greater than 85% for a conventional modulation scheme and above 91% for an optimized switching scheme.

Novel Concepts for Integrating the Electric Drive and Auxiliary DC–DC Converter for Hybrid Vehicles

In addition to conventional drilling and demolition techniques, drilling with pulsed electric power has been investigated intensively, and commercial applications have begun to emerge. The most efficient method, often called plasma channel drilling (PCD), uses electrical pulses to generate a plasma channel in the rock. The expansion of this channel within the rock performs the demolition. The technique relies on that fact that for fast pulse rise-times (50 ns - 500 ns) the breakdown field-strength of water is higher than that of rock, so that the discharge takes place in the rock. To date, in publications dealing with this topic, plasma dynamics, crack formation, and setup of the electrodes are the primary areas of investigation. In these investigations, the high voltage pulses have been generated using modulators based on spark gaps: either as single-switch or in a Marx-generator setup. These modulators are able to generate high voltages and high currents simultaneously, but the PCD method does not require high currents for igniting the discharge. Meanwhile, after ignition the voltage across the arc is relatively small. Thus these modulators are oversized. In this paper, a new concept, consisting of a solid state modulator which generates a high-voltage pulse for ignition, and a high output-current to expand the plasma, is presented. The solid state modulator consists of a single semiconductor switch, saturable inductors and a pulse transformer. In addition to being more compact, the solid state approach has improved lifetime and reliability compared to approaches using spark gap switches.

Juergen Biela

Papers

PWM Converter Power Density Barriers

PWM Converter Power Density Barriers

Implementation of a Transformerless Common-Mode Active Filter for Offline Converter Systems

Novel Concepts for Integrating the Electric Drive and Auxiliary DC–DC Converter for Hybrid Vehicles

Solid state modulator for plasma channel drilling