Alexander Anthon

The Benefits of SiC MOSFETs in a T-Type Inverter for Grid-Tie Applications

It is well known that multilevel converters can offer significant benefits in terms of harmonic performance and reduced switching losses compared to their two-level counterparts. However, for lower voltage applications the neutral-point-clamped inverter suffers from relatively large semiconductor conduction losses because the output current always flows through two switching devices. In contrast, the T-type multilevel inverter has less conduction losses because only a single outer loop switching device is required to connect the converter output to the upper and lower dc buses, albeit at the expense of increased switching losses since these outer switches must now block the full dc link voltage. Silicon carbide (SiC) mosfet devices potentially offer substantial advantage in this context with their lower switching losses, but the benefit of replacing all switching devices in a T-type inverter with SiC mosfets is not so clear-cut. This paper now explores this issue by presenting a detailed comparison of the use of Si and SiC devices for a three-level T-type inverter operating in grid-tie applications. The study uses datasheet values, switching loss measurements, and calibrated heat sink thermal measurements to precisely compare semiconductor losses for these two alternatives for a T-type inverter operating at or near unity power factor. The results show that replacing only the dc bus connection switches with SiC devices significantly reduces the semiconductor losses, allowing either the converter power level or the switching frequency to be significantly increased for the same device losses. Hence, the use of SiC mosfets for T-type inverters can be seen to be an attractive and potentially cost-effective alternative, since only two switching devices per phase leg need to be upgraded.

Switching investigations on a SiC MOSFET in a TO-247 package

This paper deals with the switching behavior of a SiC MOSFET in a TO-247 package. Based on simulations, critical parasitic inductances in the circuit layout are analyzed and their effect on the switching losses highlighted. Especially the common source inductance, a critical parameter in a TO-247 package, has a major influence on the switching energy. Crucial design guidelines for an improved double pulse test circuit are introduced which are used for practical investigations on the switching behavior. Switching energies of a SiC MOSFET in a TO-247 package is measured depending on varying gate resistance and loop inductances. With total switching energy of 340.24 μJ, the SiC MOSFET has more than six times lower switching losses than a regular Si IGBT. Implementing the SiC switches in a 3kW T-Type inverter topology, efficiency improvements of 0.8 % are achieved and maximum efficiency of 97.7 % is reached.

A high power boost converter for PV Systems operating up to 300 kHz using SiC devices

In this paper, a 3kW boost converter for PV applications using SiC devices is introduced. Main focus is to operate the converter over a wide range of switching frequency and to analyze the main loss distributors as well as the efficiency. The switching element is a recently introduced normally-on SiC JFET and a SiC diode is used. The SiC JFET has been evaluated on an optimized double pulse test circuit showing switching energies four times lower than its Si IGBT competitor. Measurements show a maximum efficiency of 98.6% at 50 kHz. Thermal investigations show that the boost converter can be operated at full power for a switching frequency of 100 kHz using natural cooling. At 200 kHz the boost converter is capable of operating at full power when forced air cooling is applied having a JFET case temperature of less than 90 °C. The case temperature of the JFET increases up to 110 °C at a switching frequency of 300 kHz where a maximum efficiency of 97.5% is achieved.

Comparison of a state of the art Si IGBT and next generation fast switching devices in a 4 kW boost converter

This paper gives a comprehensive comparison of two promising silicon carbide (SiC) switching devices, i.e. normally-off SiC MOSFET and a normally-on SiC JFET, as alternatives to a conventional state of the art Si IGBT. The comparison uses datasheet information to determine conduction losses, switching transition measurements for switching loss calculations and electrical power measurements in a boost converter. Using SiC switching devices, switching energies can be reduced by almost 70% and the forward voltages of such devices are much lower compared to the IGBT which then reduce the conduction losses. This reduction in semiconductor losses can increase overall converter efficiencies up to 0.4% at 20kHz or enable high frequency operation up to 100 kHz which then reduces the size and weight of the inductor by more than 75% while still achieving efficiencies over 98.3 %.

Efficiency evaluation on a CoolMos switching and IGBT conducting multilevel inverter

This paper deals with a three-level inverter topology in the 3kW range as an alternative to commonly used three-level topologies. The topology is attractive for having low switching losses due to the utilization of CoolMos switching devices while keeping conduction losses low due to the utilization of IGBTs. A proper time delay between the CoolMos and IGBT devices increases the efficiency by 0.2 %. Maximum efficiencies of 97.7% are achieved and less than 0.2%efficiency degradation is possible with doubled switching frequency. The case temperatures of the switching devices are below 60 °C at full power.

Analysis and comparison of Si and SiC power devices on a grid-tie fuel cell energy storage system

In renewable energy applications power conversion efficiency is major concern. This is especially true for grid-tie energy storage systems based on bidirectional dc-dc and dc-ac converters where power flows through these system components. Latest developments in power semiconductors technology significantly reduced switching and conduction losses in dc-dc and dc-ac converters allowing efficiencies above 98%. This paper analyzes the efficiency improvement that is achieved by the introduction of SiC power semiconductors in dc-dc and dc-ac converters. The analysis is focuses on fuel cell grid-tie energy storage systems. Results highlight dc-dc conversion efficiencies up to 98.2% with an isolated topology and dc-ac conversion efficiencies up to 97.7%. Overall system efficiency improvements above 1% are achieved compared to traditional Si devices. Results on efficiency improvement are analyzed based on two laboratory converter prototypes of an isolated full bridge boost converter (IFBBC) and a three level T-type inverter (BSNPC).

Comparative Evaluation of the Loss and Thermal Performance of Advanced Three-Level Inverter Topologies

This paper presents a comparative evaluation of the loss and thermal performance of two advanced three-level inverter topologies, namely the SiC based T-Type and the Hybrid-NPC, both of which are aimed at reducing the high switching losses associated with a conventional Si based T-Type inverter. The first solution directly replaces the 1200V primary Si IGBT switches with lower loss 1200V SiC MOSFETs. The second solution strategically adds 600V CoolMos FET devices to the conventional Si T-Type inverter to reduce the primary commutation losses. Semiconductor loss models, experimentally verified on calibrated heat sinks, are used to show that both variations can significantly reduce the semiconductor losses compared to the Si based T-Type inverter. The results show that both alternatives are attractive if high efficiencies and reduced thermal stress are major requirements for the converter design.

High efficiency non-isolated three port DC-DC converter for PV-battery systems

This paper presents a nonisolated Three Port Converter (TPC) with a unidirectional port for photovoltaic (PV) panels and a bidirectional port for energy storage. With the proposed topology single power conversion is performed between each port, so high efficiencies are obtained. A theoretical analysis is carried out to analyze all operating modes and design considerations with the main equations are given. A 4kW laboratory prototype is developed and tested under all operating conditions. Results obtained feature on efficiencies higher than 97% for all operating modes and all power levels from light load to full load.

High efficiency power converter for a doubly-fed SOEC/SOFC system

Regenerative fuel cells (RFC) have become an attractive technology for energy storage systems due to their high energy density and lower end-of-life disposal concerns. However, high efficiency design of power conditioning unit (PCU) for RFC becomes challenging due to their asymmetrical current-power characteristics that are dependent on the operation mode (energy storage / energy supply). This paper proposes a new PCU architecture for grid-tie RFC with which the RFCs asymmetrical characteristic becomes less critical and thus a much more symmetrical power rating of the dc-dc converter for both operating modes is possible. This paper discusses the design considerations for this novel PCU, and verifies its operation principle with Matlab/Simulink simulations. Experimental results on a tailored dc-dc converter confirm the design simplifications for high efficiency operation along the entire power operating range of the RFC as well as the utilization of the same control strategy design for the two RFC operating modes.

Comprehensive loss evaluation of neutral-point-clamped (NPC) and T-Type three-level inverters based on a circuit level decoupling modulation

In this paper, an efficiency comparison of neutral-point-clamped (NPC) inverters and bipolar switch NPC (T-Type) inverters is studied and the result shows that the T-Type inverter is more efficient at lower switching frequencies. Nevertheless, its efficiency suffers when the switching frequency increases due to high switching loss of the equipped high voltage power switches. In order to reduce switching loss and hereby enhance efficiency, a newly proposed circuit-level decoupling modulation (CLDM) scheme is applied for these two widely used three-phase three-level inverters, as well as their corresponding loss analyses are addressed. The switching loss reduction is evaluated comprehensively under variant modulation indices and load power factors. The analysis results reveal that the CLDM is an alternative discontinuous pulse-width modulation (DPWM) approach for inverters with high switching frequencies in order to achieve superior output voltage quality without lowering efficiency.