Ahmed Elasser

A comparative evaluation of new silicon carbide diodes and state-of-the-art silicon diodes for power electronic applications

Recent progress in silicon carbide (SiC) material has made it feasible to build power devices of reasonable current density. This paper presents recent results including a comparison with state-of-the-art silicon diodes. Switching losses for two silicon diodes (a fast diode, 600 V, 50 A, 60 ns Trr), an ultra-fast silicon diode (600 V, 50 A, 23 ns Trr) and a 4H-SiC diode (600 V, 50 A) are compared. The effect of diode reverse recovery on the turn-on losses of a fast WARP/sup TM/ IGBT are studied both at room temperature and at 150/spl deg/C. At room temperature, SiC diodes allow a reduction of IGBT turn-on losses by 25% compared to ultra-fast silicon diodes and by 70% compared to fast silicon diodes. At 150/spl deg/C junction temperature, SiC diodes allow a turn-on loss reduction of 35% and 85% compared to ultra-fast and fast silicon diodes respectively. The silicon and SiC diodes are used in a boost power converter with the WARP/sup TM/ IGBT to assess the overall effect of SiC diodes on the power converter characteristics. Efficiency measurements at light load (100 W) and full load (500 W) are reported. Although SiC diodes exhibit very low switching losses, their high conduction losses due to the high forward drop dominate the overall losses, hence reducing the overall efficiency. Since this is an ongoing development, it is expected that future prototypes will have improved forward characteristics.

A comparative study of central and distributed MPPT architectures for megawatt utility and large scale commercial photovoltaic plants

In this paper different distributed PV architectures are studied from an energy yield perspective. These distributed architectures are applied to massively paralleled thin film plants employing high voltage PV modules, mc-Si plants with long series strings of low voltage modules and plants with medium voltage thin film modules in order to evaluate the effectiveness of the distributed architecture in each case. The effects of partial shading, module mismatch and cable losses are quantified in order to obtain the energy yield for each of the architectures under study. The results of this trade-off study are used to quantify the benefits of a distributed architecture as well as determine the optimal location of the dc/dc converters that perform the MPPT function.

An Efficient Partial Power Processing DC/DC Converter for Distributed PV Architectures

In this paper, a dc/dc power converter for distributed photovoltaic (PV) plant architectures is presented. The proposed converter has the advantages of simplicity, high efficiency, and low cost. High efficiency is achieved by having a portion of the input PV power directly fed forward to the output without being processed by the converter. The operation of this converter allows for a simplified maximum power point tracker design using fewer measurements. The stability analysis of the distributed PV system comprised of the proposed dc/dc converters confirms the stable operation even with a large number of deployed converters. The experimental results show a composite weighted efficiency of 98.22% with very high maximum power point tracking efficiency.

A High-Power-Density DC–DC Converter for Distributed PV Architectures

In order to maximize the solar energy harvesting capabilities, power converters for photovoltaic (PV) systems have to be designed for high efficiency, accurate maximum power point tracking (MPPT) and voltage/current performance. When many converters are used in distributed PV systems, power density also becomes an important factor since it allows for simpler system integration. In this paper, a high power density string level MPPT DC-DC converter suitable for distributed medium to large scale PV installations is presented. A simple partial power processing topology implemented exclusively with silicon carbide devices provides high efficiency and high power density. A 3.5kW, 100 kHz converter is designed and tested to verify the proposed methods.

Dc-dc converter topology assessment for large scale distributed photovoltaic plant architectures

Distributed photovoltaic (PV) plant architectures are emerging as a replacement for the classical central inverter based systems. However, power converters of smaller ratings may have a negative impact on system efficiency, reliability and cost. Therefore, it is necessary to design converters with very high efficiency and simpler topologies in order not to offset the benefits gained by using distributed PV systems. In this paper an evaluation of the selection criteria for dc-dc converters for distributed PV systems is performed; this evaluation includes efficiency, simplicity of design, reliability and cost. Based on this evaluation, recommendations can be made as to which class of converters is best fit for this application.

High temperature Hall effect sensors based on AlGaN∕GaN heterojunctions

We report on AlGaN∕GaN heterojunction structures for use in Hall effect sensors working over a wide range of temperatures. Room temperature current-related magnetic sensitivity of 55V∕AT at a sheet resistance below 300Ω∕sq and very low temperature cross sensitivity of 103ppm∕°C up to 300°C were obtained for a square-shaped Hall effect sensor. The active layer of the Hall effect sensor is the two-dimensional electron gas formed at the Al0.3Ga0.7N and GaN heterointerface caused by the gradient in the total polarization between the AlGaN barrier and the GaN buffer layer, which results in the positive polarization induced interface charge attracting free electrons. The temperature-dependent transport properties of the heterojunction were analyzed by Hall measurement. The drop of its electron mobility from room temperature to 300°C is mainly due to the enhanced polar optical scattering, while the very stable sheet carrier density contributes to the excellent temperature cross sensitivity of the Hall effect sensor.

3.3kV SiC MOSFETs designed for low on-resistance and fast switching

This paper discusses the latest developments in the optimization and fabrication of 3.3kV SiC vertical DMOSFETs. The devices show superior on-state and switching losses compared to the even the latest generation of 3.3kV fast Si IGBTs and promise to extend the upper switching frequency of high-voltage power conversion systems beyond several tens of kHz without the need to increase part count with 3-level converter stacks of faster 1.7kV IGBTs.

A soft switching active snubber optimized for IGBT's in single switch unity power factor three-phase diode rectifiers

This paper describes a soft switching active snubber for an IGBT operating in a single switch unity power factor three-phase diode rectifier. The soft switching snubber circuit provides zero-voltage turn-off for the main switch. The high turn-off losses of the IGBT due to current tailing are reduced by zero-voltage switching. This allows the circuit to be operated at very high switching frequencies with regulated DC output voltage, high quality input current and unity input power factor. Simulation and experimental results are included. >

3kV 4H-SiC Thyristors for Pulsed Power Applications

Due to the Silicon Carbide (SiC) material’s high electric field strength, wide bandgap, and good thermal conductivity, 4H-SiC thyristors are attractive candidates for pulsed power applications. With a thinner blocking layer almost an order of magnitude smaller than its Silicon (Si) counterpart, these devices promise very fast turn-on capabilities as full conductivity modulation occurs >10 times faster than comparable silicon thyristors, low leakage currents at high junction temperatures and at high voltage, and much lower forward voltage drop at high pulse currents. Our progress on the development of large area (4mm x 4mm) SiC thyristors is presented in this paper.

High-voltage high-frequency inverter using 3.3kV SiC MOSFETs

High voltage SiC MOSFETs enable high switching frequency operation that would otherwise be only possible with more complex architectures such as multilevel or interleaved inverters. In this paper, a full bridge inverter using 3.3kV SiC MOSFETs is presented to achieve high-voltage (2100V dc bus), high-frequency (62.5kHz) operation with a simple hard switching PWM technique. The switching characteristics of the 3.3kV SiC device are presented and the effect of the parasitic parameters is analyzed in details. The analysis shows that the tiny stray output capacitor, which is mainly introduced by the bus-bars implemented with a PCB, can have a significant impact on the switching waveforms of the SiC devices. The reported experimental results validate the performance of the proposed inverter.