V. d'Alessandro

A back-wafer contacted silicon-on-glass integrated bipolar process. Part II. A novel analysis of thermal breakdown

Analytical expressions for the electrothermal parameters governing thermal instability in bipolar transistors, i.e., thermal resistance R/sub TH/, critical temperature T/sub crit/ and critical current J/sub C,crit/, are established and verified by measurements on silicon-on-glass bipolar NPNs. A minimum junction temperature increase above ambient due to selfheating that can cause thermal breakdown is identified and verified to be as low as 10-20/spl deg/C. The influence of internal and external series resistances and the thermal resistance explicitly included in the expressions for T/sub crit/ and J/sub C,crit/ becomes clear. The use of the derived expressions for determining the safe operating area of a device and for extracting the thermal resistance is demonstrated.

Extraction and modeling of self-heating and mutual thermal coupling impedance of bipolar transistors

A measurement system comprised of an ultra-low-distortion function generator, lock-in amplifier, and semiconductor parameter analyzer is used for sensitive extraction of the small-signal thermal impedance network of bipolar devices and circuits. The extraction procedure is demonstrated through measurements on several silicon-on-glass NPN test structures. Behavioral modeling of the mutual thermal coupling obtained by fitting a multipole rational complex function to measured data is presented.

Theory of electrothermal behavior of bipolar transistors: Part I -single-finger devices

A detailed theoretical and numerical analysis of the electrothermal behavior of single-finger bipolar transistors is proposed. Two models of different complexities are introduced to investigate self-heating effects in bipolar junction transistors (BJTs) and heterojunction bipolar transistors (HBTs) biased with a constant base-emitter voltage source or with a constant base current source. In the constant base-emitter voltage case, simple relations are derived for determining the onset of the flyback behavior in the output characteristics which defines the boundary of the safe operating region. The model indicates that the flyback behavior disappears at high V/sub BE/ values, and predicts a thermal hysteresis phenomenon at high currents. It is also shown that at high current levels the electrothermal behavior is dominated by ohmic base pushout. If a constant base current is applied, the model shows that both BJTs and HBTs are unconditionally thermally stable. The transient behavior is also considered, and the temperature evolution is investigated for different bias conditions. The model shows that, if the device is biased in the thermally unstable region, thermal breakdown occurs within a finite time instant in the limit case of a zero ballast resistance. Finally, the reduction in the safe operating area due to avalanche effects and to the temperature dependence of thermal conductivity is discussed, and a simplified model is proposed.

A review of the vibroacoustics of sandwich panels: Models and experiments

This paper reviews the most significant works in literature about the acoustic behaviour of sandwich panels, starting from the first examples of multi-layered structures, comprising a series of different layers enclosing an air-gap, to the actual configurations in which the opportunities of emerging manufacturing technologies are considered in the design stages. The focus is on presenting an exhaustive list of dedicated and validated models, which are able to predict the sound transmission through sandwich panels according to their specific configuration. Some experimental works, aimed to the model correlations, are reviewed, too.

Analytical model for thermal instability of low voltage power MOS and SOA in pulse operation

Thermal instability presented by some high current power MOS has been shown to limit significantly the SOA capability. In this paper, we present a new analytical model to explain this type of instability in transient operation, based on an analytical formulation for both the positive temperature coefficient of the drain current and for the thermal resistance. The model is capable of predicting the onset of thermal instability for a given device structure and layout, and can be used both to define the allowed SOA of the device and as a design guide to design more rugged devices.

Thermal instabilities in high current power MOS devices: experimental evidence, electro-thermal simulations and analytical modeling

The phenomenon of the thermal instability presented by some high current power MOS has been intensively investigated, both by experimental means and by numerical simulations. An analytical expression for the positive temperature coefficient of the Drain current has been developed and a model for the thermal instability in transient operation has been proposed. The results explain the main causes of the thermal instability and give some rules to evaluate the possible failure occurrence for a given device.

A Simple Bipolar Transistor-Based Bypass Approach for Photovoltaic Modules

This paper presents a novel bypass approach for photovoltaic panels relying on a bipolar transistor operated in saturation, the activation of which is handled automatically by a circuit comprising a pair of MOS transistors only. The functioning principle of the proposed system is explained and the improvements in terms of reliability of the bypassed subpanel and power produced by the string in comparison with a traditional diode-based scheme are quantified. The analysis is corroborated by dc/transient measurements and tailored PSPICE simulations.

A novel wireless self-powered microcontroller-based monitoring circuit for photovoltaic panels in grid-connected systems

A monitoring circuit for individual photovoltaic (PV) panels in grid-connected systems is proposed, which exhibits a number of features devised to simplify and reduce cost of diagnostics and maintenance of the PV plant. In particular, the system is provided with an effective energy harvesting supply stage, which eliminates the requirement for external supply or batteries; furthermore, no cables are needed for data transfer due to the adoption of a rugged wireless connectivity.

Restabilizing mechanisms after the onset of thermal instability in bipolar transistors

The electrothermal behavior of single- and two-finger bipolar transistors at medium- and high-current operations is studied through theoretical modeling, experimental measurements, and computer simulations. Bias conditions that border thermally stable and unstable operation regimes are described by novel analytical formulations, which for the first time include simultaneously all relevant parameters that weaken the electrothermal feedback at high currents such as ballasting resistors, current dependence of the base-emitter-voltage temperature coefficient, and high-injection effects. Hence, besides giving a correct description of thermal instability mechanisms, the developed formulations also allow the prediction and physical understanding of restabilization phenomena. The models are supported by measurements on silicon-on-glass n-p-n bipolar junction transistors and by simulation results from a novel SPICE-based electrothermal macromodel for bipolar transistors. Furthermore, the models are employed to analyze the influence of the germanium percentage in the base of SiGe heterojunction bipolar transistors on the thermal ruggedness of the device.

Monitoring and Diagnostics of PV Plants by a Wireless Self-Powered Sensor for Individual Panels

In this paper, an innovative sensor suited to perform real-time measurements of operating voltage and current, open-circuit voltage, and short-circuit current of string-connected photovoltaic (PV) panels is presented. An effective disconnection system ensures that the sensor does not affect the behavior of the string during the measurement phase and offers many benefits like the automatic detection of bypass events; moreover, the sensor does not require additional cables thanks to a wireless communication and a power supply section based on energy harvesting. An extensive experimental campaign is performed to prove the reliability and usefulness of the sensor for continuous monitoring of PV plants. The capability to detect faults and accurately localize malfunctioning panels in a PV string is highlighted.