S. De Caro

A Reliability Model for Power MOSFETs Working in Avalanche Mode Based on an Experimental Temperature Distribution Analysis

The on-state resistance of power MOSFET devices tasked to perform repetitive avalanche operations is subject to modifications caused by the growth of voids and cracks in the source metallization. Endurance tests are the traditional way to monitor these changes in order to assess device reliability. However, they are very time intensive, requiring even months of uninterrupted operations. An interesting alternative is assessment of reliability through a suitable model, but no standard techniques have been developed up until now to accomplish this task. A possible approach, dynamic analysis of the temperature distribution over the source metal, is presented in this paper. Coupling the results of the thermodynamic analysis with a reliability model, and based on the Coffin-Manson law, device degradation over time can be estimated and the level of reliability can be evaluated. The consistence of the obtained reliability prediction is confirmed by comparison with endurance test results. The described approach can be usefully applied to assess the reliability of MOSFETs in a large set of applications in the automotive field.

Optimal design of energy storage systems for stand-alone hybrid wind/PV generators

A major shortcoming in the exploitation of renewable energy sources relies in the effective power availability, that depends from the variability of the primary energy resource over the time. Because of the variability and unpredictability of primary power both photovoltaic arrays and wind turbines cannot ensure the minimum level of power continuity required to supply in island mode a generic set of residential loads. Therefore, they must be complemented with suitable energy storage devices to form a hybrid power generation system. The design of a hybrid generation system including energy storage devices is a quite complex task. A probabilistic design approach is then proposed in this paper based on the LPSP index. Such an approach is also used to detect the most advantageous combination of wind turbines, PV plant and energy storage system, for a stand-alone generator aimed to supply a small set of domestic loads.

Low input current ripple converters for fuel cell power units

Proton exchange membrane fuel cell (PEMFC) generators are today considered as a viable solution to the development of auxiliary, or back up, power units for automotive, computer and telecommunications systems. Several of these applications require a power conditioner with high efficiency, high conversion ratio and very low input current ripple to interface the fuel cell generator to a high-voltage DC bus. In this paper, two step-up non insulated DC/DC converter topologies, suitable to equip low power fuel cell power units are developed and deeply examined. A key feature of the two proposed converters is that they are obtained by mixing some basic topologies, in order to optimize both duty cycle and the winding ratio. Moreover, multichannel interleaved power conversion structures are adopted in order to reduce the size of input and output filters. Finally, both the proposed topologies feature an input inductance, that plays a major role in lowering the fuel cell output current ripple, thus reducing fuel consumption. The two proposed converters are theoretically analyzed and experimentally tested

Stress analysis and lifetime estimation on power MOSFETs for automotive ABS systems

Stress analysis and lifetime estimation are required in order to guarantee higher and higher levels of reliability of automotive power electronic devices. Stress analysis is oriented to investigate the effects of all the possible physical cause of failures, according to the defined device mission. On the basis of the stress analysis, and of a suitable reliability model a life time prediction can be performed. This is useful to predict the suitability of the device under evaluation to the prescribed mission, as well as to improve the design of new generations of devices. An experimental technique is exploited in this paper to evaluate the stress exerted on planar power MOSFETs designed to equip automotive ABS systems. The technique is based on an accurate experimental analysis of electro-thermal cycles, exploiting a laboratory tool tailored around an infrared microscope. It enables an high resolution dynamic temperature mapping with a large bandwidth. Such a tool makes also possible the evaluation of the effects of charge trapping phenomena occurring on power MOSFETS as result of unavoidable gate overvoltages. According to a reliability model based on the Coffin Manson law, finally it is shown that gate voltage spikes can dramatically reduce the expected life time.

Optimal size selection of step-up transformers in PV plants

Step-up transformers are used to connect large PV plants to the utility network, their sizing being often accomplished only taking into account the PV plant peak power. However, a largely unpredictable power injection on the main grid is obtained if a too large rated power is selected, leading to grid instabilities. This may result in frequent plant shutdowns, while requiring a remarkable reserve power to be provided by conventional generation systems. On the other hand, a too small transformer lead to the creation of a bottleneck, preventing an optimal exploitation of the solar energy. This situation becomes more complex if the introduction of an energy storage system is considered. In the present paper a design technique is proposed to optimally select the step-up transformer, either on conventional PV plants, either on PV plants with energy storage. It is based on the evaluation of initial and operating costs. Moreover, the effects of induced network instabilities are also considered. Taking into account full life costs optimal solutions have been detected according to the network power control capabilities for a 2 MW PV plant.

A buck-boost based DC/AC converter for residential PV applications

The single stage architecture is often preferred to realize grid connected Photo-Voltaic generators for residential applications. In fact,by halving the power processing steps, the power losses can be considerably reduced if compared with those of a two stage configuration. However, single stage power conditioners are generally unable to boost the voltage of the PV modules, therefore, they feature a narrower input voltage range. A new approach to design a power conditioner for grid connected PV plants for residential applications is presented in this paper. It is based on the series connection between a buck-boost converter,and the PV array. According to such an approach, the buck-boost converter processes only a fraction of the inverter input power, while always operating in conditions of high efficiency. Experimental tests show that the proposed solution features an efficiency level very close to that of a single stage power conditioner, but owns a voltage boosting capability, similar to that of a two stage configuration.

Reliability of planar, Super-Junction and trench low voltage power MOSFETs

A strong demand of even more compact and reliable devices has powered in the last years the development of advanced power MOSFET structures. Among them, the planar STripFET™ has been introduced as an alternative to conventional trench gate MOSFET in low voltage (<60 V) applications. Moreover low voltage Super-Junction devices are also under development. In this paper a conventional trench gate MOSFET is compared in terms of reliability with a STripFET™ and a Super-Junction device. The comparison is accomplished through a reliability model taking advantage from a dynamic analysis of the temperature distribution over the metal source surface in an effort to correlate electric working conditions to thermo-mechanical stresses.

A new approach to improve the current harmonic content on open-end winding AC motors supplied by multi-level inverters

In open-end winding AC motor drives, terminals of stator phases are normally connected between two inverters, normally, of the same type and size. In the present paper, an alternative configuration is studied where the machine is supplied exploiting two different converters: a main multi-level inverter and an auxiliary two-level inverter. The last operates as active power filter in order to eliminate undesired low-frequency voltage harmonics produced by the multi-level modulation process. Furthermore, the two level inverter is operated at a rather low voltage, thus it can be equipped with power MOSFETs. Consequently, a voltage PWM at a remarkably higher frequency is allowed, if compared with standard multilevel inverters. According to the proposed approach, the phase current harmonic content is remarkably reduced, the torque ripple is mitigated while the drive efficiency is increased. Numerical simulations and experimental results confirm the consistency of such new methodology. A particular emphasis is given to stator current harmonic content and power losses analysis as well as to specific implementation issues.

Inverter fault-identification for VSI motor drives

The paper presents an identification procedure able to detect open circuit failures in voltage source inverters. The procedure is suitable to equip fault tolerant electric motor drives and in principle only requires the measurement of the DC link current as it accomplishes a reconstruction of the inverter output phase currents from the DC link current measurement, thus eliminating the need of conventional current sensors. However, if they are present at the cost of minimal modifications the PWM technique, response times as short as one switching period can be obtained. Compared to other technical solutions found on past literature, the proposed approach considerably reduces the required computational effort. Simulations and experimental tests are presented to confirm the consistence of the proposed approach.

Sensorless IPMS motor drive control for electric power steering

Key requirements of electric power steering systems such as cost implementation, minimum algorithm complexity and code length, low torque fluctuation and mechanical vibrations are fully complied by the sensorless control technique proposed in this paper, based on high voltage signal injection and measurement of an additional high frequency stator current component. The proposed rotor position estimation technique is simple and can be implemented in analog environment, minimizing the computation effort and the hardware requirements of the microcontroller. Implementation issues and experimental results obtained on a laboratory prototype equipping an electric power steering system in practical working conditions are presented.