70 research outputs found
Solving nonlinear circuits with pulsed excitation by multirate partial differential equations
In this paper the concept of Multirate Partial Differential Equations (MPDEs)
is applied to obtain an efficient solution for nonlinear low-frequency
electrical circuits with pulsed excitation. The MPDEs are solved by a Galerkin
approach and a conventional time discretization. Nonlinearities are efficiently
accounted for by neglecting the high-frequency components (ripples) of the
state variables and using only their envelope for the evaluation. It is shown
that the impact of this approximation on the solution becomes increasingly
negligible for rising frequency and leads to significant performance gains.Comment: 4 pages, 7 figures, approved for publication in IEEE Transactions on
Magnetic
Efficient simulation of DC-DC switch-mode power converters by multirate partial differential equations
In this paper, Multirate Partial Differential Equations (MPDEs) are used for
the efficient simulation of problems with 2-level pulsed excitations as they
often occur in power electronics, e.g., DC-DC switch-mode converters. The
differential equations describing the problem are reformulated as MPDEs which
are solved by a Galerkin approach and time discretization. For the solution
expansion two types of basis functions are proposed, namely classical Finite
Element (FE) nodal functions and the recently introduced excitation-specific
pulse width modulation (PWM) basis functions. The new method is applied to the
example of a buck converter. Convergence, accuracy of the solution and
computational efficiency of the method are numerically analyzed
Thermoelectric effects in correlated quantum dots and molecules
We investigate thermoelectric properties of correlated quantum dots and
molecules, described by a single level Anderson model coupled to conduction
electron leads, by using Wilson's numerical renormalization group method. In
the Kondo regime, the thermopower, , exhibits two sign changes, at
temperatures and . We find that is of order
the level width and , where is the
position of the Kondo induced peak in the thermopower and is the Kondo
scale. No sign change is found outside the Kondo regime, or, for weak
correlations, making a sign change in a particularly sensitive signature
of strong correlations and Kondo physics. For molecules, we investigate the
effect of screening by conduction electrons on the thermoelectric transport. We
find that a large screening interaction enhances the figure of merit in the
Kondo and mixed valence regimes.Comment: 4 pages, 3 figures; to appear in the Proceedings of the International
Conference on Strongly Correlated Electron Systems, Santa Fe 2010; revised
version: typos corrected and references update
Stability of quantum-dot excited-state laser emission under simultaneous ground-state perturbation
The impact of ground state amplification on the laser emission of In(Ga)As
quantum dot excited state lasers is studied in time-resolved experiments. We
find that a depopulation of the quantum dot ground state is followed by a drop
in excited state lasing intensity. The magnitude of the drop is strongly
dependent on the wavelength of the depletion pulse and the applied injection
current. Numerical simulations based on laser rate equations reproduce the
experimental results and explain the wavelength dependence by the different
dynamics in lasing and non-lasing sub-ensembles within the inhomogeneously
broadened quantum dots. At high injection levels, the observed response even
upon perturbation of the lasing sub-ensemble is small and followed by a fast
recovery, thus supporting the capacity of fast modulation in dual-state
devices
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