S. Panarello

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.

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.

An Infrared Thermal Measuring System for Automotive Applications and Reliability Improvement

In the last years the global energy economy hangs in the balance, pushing up the research interest in novel and renewable energy sources and in innovative engines able to improve performances saving the efficiency. This frame requires the development of power electronics subsystems and the continuous increase of working temperatures; hence reliability has become the most critical requirement for any new device design. The temporal evolution of temperature distribution on the surface of a power electronic device undergoing an exerted stress plays a fundamental role in studying and improving reliability. A suitable scanning measuring system has been realized in order to allow the analysis of fast transient states and the localization of “hot-spots” which could be a cause of a premature failure and unreliability of the devices.

Reliability Assessment of Power MOSFETs Working in Avalanche Mode Based on a Thermal Strain Direct Measurement Approach

A strong demand of even more compact and reliable power electronic devices has powered in the last years the development of advanced device design techniques. A key role in these techniques is played by the reliability assessment, a procedure that estimates the expected lifetime of power devices according to given mission profiles. The reliability assessment of a low voltage MOSFET working in avalanche mode is faced in this paper through a new experimental approach based on the Coffin Manson law and a direct measurement of the thermal strain over the Source Aluminum layer. The consistence of the proposed technique is evaluated by comparing obtained estimations of the progressive increment of the on state resistance with estimations carried out from other reliability models and endurance tests results. The described approach can be usefully applied to assess the reliability of MOSFETs in applications typical of the automotive field were power devices are tasked to operate in avalanche mode, such as: brake pump drivers, electromagnetic valve control, direct high-pressure injection, starter-alternator and active suspension systems.

Reliability assessment of low-voltage MOSFETs driving inductive loads

Reliability and compactness are two aspects often fighting among themselves when speaking about power electronics, but, indeed, they are the keys for the success of any new circuit or device. Reliability, in particular, is the word of the moment, powering the development of advanced device design techniques having the reliability as a major goal. Endurance tests is the traditional way to evaluate the reliability of power devices. However, they are very time expensive, requiring even months of uninterrupted testing. An interesting alternative is the estimation of the reliability of a device through a suitable model, but, no standard techniques have been developed up to now to accomplish this task. A possible approach is followed in this paper to assess the reliability of Power MOSFETs driving inductive loads, by exploitation of a dynamic analysis of the temperature distribution over the source metal. Coupling such an analysis with a reliability model, carried out from the Coffin-Manson law, the device life time is estimated. Such a procedure is then used to assess the reliability of Power MOS devices tasked to control the brake pump in a modern vehicle. The consistence of the reliability estimation is confirmed by comparison with results of endurance tests. The described approach can be usefully applied to a large set of applications of MOSFETs in the automotive field.

Reliability assessment of power MOSFETs working in avalanche mode based on a thermal strain direct measurement approach

A strong demand for even more compact and reliable power electronic devices has powered the development of advanced design techniques. A key role is played in these techniques by the reliability assessment, a procedure that estimates the expected lifetime according to given mission profiles. The reliability assessment of a low voltage MOSFET working in avalanche mode is considered in this paper through an experimental approach based on the Coffin-Manson law. Differently from previously proposed techniques, based on a thermodynamic analysis, a direct measurement of the thermal strain over the source aluminum layer is instead exploited. The consistence of the proposed technique is evaluated by comparing estimation of the progressive increment of the on-state resistance with results of endurance tests and estimation obtained from previously presented reliability models. The described technique can be applied to assess the reliability of MOSFETs in applications typical of the automotive field, where they are tasked to power inductive loads in unclamped mode. More in general, the method can be exploited to characterize the front metal of MOSFET devices subjected to power cycling, or short-circuit stress tests.

Improving ICs reliability with high speed thermal mapping

Abstract The power elements are the weak parts of integrated circuits (ICs), in fact, through these elements the power is usually dissipated as heat with unavoidable thermal and mechanical stress. On the contrary the logic parts of ICs stay at lower temperatures. This gives rise to two effects: the non-uniform generation of the heat across the die and the temperature gradients. Understanding these phenomena is very important to choose the right location of sensitive components, like thermal sensors, in order to improve reliability. As a consequence, the knowledge of the temporal evolution of the temperature distribution plays a very important role to improve both design and lifetime. Here we show how a single IR sensor based experimental setup is suitable to catch very fast thermal events performing high spatial resolution. We demonstrate the effectiveness of the method maps for three IC samples where an accurate thermal modeling for reliability has been obtained and validated, greatly improving the overall quality.

A PZT-based energy harvester with working point optimization

Energy harvesters are today gaining ever more interest, enabling the recovery of otherwise wasted energy in a large variety of systems. Among them, piezoelectric devices are particularly suitable to generate electrical energy from vibrations and surplus mechanical energy. A two-stage power conditioning circuit, consisting of a AC-DC converter followed by a DC-DC stage, is usually placed at the output of piezoelectric devices in order to optimize the power yield. The efficiency of the piezoelectric device is, in fact, a very critical parameter, due to the poor power density of energy harvesters based on piezoelectric devices. This paper describes a new single-stage, low-power, converter able to automatically follow the variations of the resistive component of the output impedance of a cantilevered PZT (Lead Zirconate Titanate) based piezoelectric energy harvester in order to maximize the energy yield.

Reliability model application for power devices using mechanical strain real time mapping

The reliability characterization is a milestone for the design of semiconductor power devices operating in those applications where a high robustness is a critical point, as in the case of the automotive field. In order to realize a reliability model that can describe the degradation of power devices, due to a cyclic thermo-mechanical stress, the Coffin-Manson equation is widely applied. Generally speaking the Coffin-Manson relationship shows a correlation between the fatigue life and the plastic strain but this equation is often customized in order to correlate the fatigue life with the temperature variations, being these simpler to measure. In this paper a methodology based on a direct measurement of the mechanical strain applied on operating devices will be shown. Furthermore, the application of the results for the reliability evaluation without taking into account the temperature variations will be described. This target has been accomplished with a suitable scanning instrument based on the laser interferometry able to perform very fast acquisitions and to reconstruct nanometric mechanical displacements.