3,456 research outputs found
The Hot-Spot Phenomenon and its Countermeasures in Bipolar Power Transistors by Analytical Electro-Thermal Simulation
This communication deals with a theoretical study of the hot spot onset (HSO)
in cellular bipolar power transistors. This well-known phenomenon consists of a
current crowding within few cells occurring for high power conditions, which
significantly decreases the forward safe operating area (FSOA) of the device.
The study was performed on a virtual sample by means of a fast, fully
analytical electro-thermal simulator operating in the steady state regime and
under the condition of imposed input base current. The purpose was to study the
dependence of the phenomenon on several thermal and geometrical factors and to
test suitable countermeasures able to impinge this phenomenon at higher biases
or to completely eliminate it. The power threshold of HSO and its localization
within the silicon die were observed as a function of the electrical bias
conditions as for instance the collector voltage, the equivalent thermal
resistance of the assembling structure underlying the silicon die, the value of
the ballasting resistances purposely added in the emitter metal
interconnections and the thickness of the copper heat spreader placed on the
die top just to the aim of making more uniform the temperature of the silicon
surface.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
Modulated rotating waves in the magnetized spherical Couette system
We present a study devoted to a detailed description of modulated rotating
waves (MRW) in the magnetized spherical Couette system. The set-up consists of
a liquid metal confined between two differentially rotating spheres and
subjected to an axially applied magnetic field. When the magnetic field
strength is varied, several branches of MRW are obtained by means of three
dimensional direct numerical simulations (DNS). The MRW originate from parent
branches of rotating waves (RW) and are classified according to Rand's (Arch.
Ration. Mech. Anal 79:1-37, 182) and Coughling & Marcus (J. Fluid Mech.
234:1-18,1992) theoretical description. We have found relatively large
intervals of multistability of MRW at low magnetic field, corresponding to the
radial jet instability known from previous studies. However, at larger magnetic
field, corresponding to the return flow regime, the stability intervals of MRW
are very narrow and thus they are unlikely to be found without detailed
knowledge of their bifurcation point. A careful analysis of the spatio-temporal
symmetries of the most energetic modes involved in the different classes of MRW
will allow in the future a comparison with the HEDGEHOG experiment, a
magnetized spherical Couette device hosted at the Helmholtz-Zentrum
Dresden-Rossendorf.Comment: Contains 3 tables and 8 figures. Published in the Journal of
Nonlinear Scienc
Reversals in nature and the nature of reversals
The asymmetric shape of reversals of the Earth's magnetic field indicates a
possible connection with relaxation oscillations as they were early discussed
by van der Pol. A simple mean-field dynamo model with a spherically symmetric
coefficient is analysed with view on this similarity, and a comparison
of the time series and the phase space trajectories with those of paleomagnetic
measurements is carried out. For highly supercritical dynamos a very good
agreement with the data is achieved. Deviations of numerical reversal sequences
from Poisson statistics are analysed and compared with paleomagnetic data. The
role of the inner core is discussed in a spectral theoretical context and
arguments and numerical evidence is compiled that the growth of the inner core
might be important for the long term changes of the reversal rate and the
occurrence of superchrons.Comment: 24 pages, 12 figure
An algorithm for constructing certain differential operators in positive characteristic
Given a non-zero polynomial in a polynomial ring with coefficients in
a finite field of prime characteristic , we present an algorithm to compute
a differential operator which raises to its th power. For
some specific families of polynomials, we also study the level of such a
differential operator , i.e., the least integer such that
is -linear. In particular, we obtain a characterization of
supersingular elliptic curves in terms of the level of the associated
differential operator.Comment: 23 pages. Comments are welcom
- …