Josef Dobes

Modeling of switched DC-DC converters by mixed s-z description

The paper explains the basic ideas of how to model the dynamical properties of switched DC-DC converters by means of the so-called generalized transfer functions (GTFs). Switched DC-DC converters like bust or bust-buck types exhibit some properties of continuous- and discrete-time characters. These systems can be globally modeled by mixed s-z description, utilizing both the Laplace and z-transform operators. Then the resulting line-to-output and control-to-output transfer functions and the corresponding frequency responses can model some system behavior more faithfully than the classical s-domain zeros/poles location, including special effects above Nyquists frequency

Computer Simulation of Continuous-Time and Switched Circuits: Limitations of SPICE-Family Programs and Pending Issues

The contribution deals with some open problems from the area of transient and AC analyses of large continuous-time and switched circuits with the focus on switched capacitor circuits and switched DC-DC converters. The discussed weak points of SPICE-like and SPICE-compatible simulation programs consist in the absence of finding effectively the steady-state, in an inefficient integration algorithm for the transient analysis of switching phenomena, and in the absence of a direct AC analysis of switched circuits. The paper also describes some new approaches, which could be helpful in overcoming the above limitations.

A steady-state add-on to the algorithm for implicit numerical integration

Many software tools of the PSpice class do not contain an implementation of the steady-state algorithm, especially for autonomous circuits. In the paper, an add-on is described to the algorithm for implicit numerical integration which determines the steady state of both nonautonomous and autonomous circuits by an extrapolation method. The extrapolation procedure is based on scalar epsilon-algorithm which is relatively easy for programing and efficient in terms of number of required iterations. Since the selected algorithm for the implicit numerical integration gives values of derivatives at any time, the determination of unknown periods of autonomous circuits can be performed by modified Newton-Raphson method. The efficiency of the procedure is demonstrated by the steady-state analysis of a tunable distributed microwave oscillator with the results compared with measured data.

Novel HEMT Models with Improved Higher-Order Derivatives and Extracting Their Parameters Using Multibias S-Parameters

Novel two HEMT models are suggested in the paper with improved accuracy of higher-order derivatives, which is very important for modeling radio-frequency devices as mixers, etc. The proposed modifications of the models are based on empirical relations for the transconductance dependence on gate-source voltage. Moreover, a way is suggested how to extract the model parameters of various nonlinear HEMT models from a measured multibias s-parameter data set. The proposed extraction procedure is based on a three-step identification procedure that uses robust optimization methods. Finally, various HEMT models -- including the proposed ones -- are compared in terms of the root-mean-square error of DC characteristics and multibias s-parameters.

Tolerance-based control mechanism for approximate symbolic analysis

The paper deals with an enhanced control mechanism for approximate symbolic analysis of the Simplification-Before-Generation class. A traditional approach consists in the evaluation of errors caused by simplification only for the nominal parameters of network components. The proposed procedure uses sensitivities to estimate the worst-case error due to component tolerances, i.e. the expression validity is checked over the tolerance interval of component parameters. A fast computational algorithm based on the Sherman-Morrison formula is used. The numerical cost is approximately given by two matrix inversions per one control-frequency point and one simplification step.

Electronic circuit design using multiobjective optimization

This paper presents a variation and extension of a previously existing method for multiobjective optimization known as goal attainment method (GAM). The method GAM is in this research combined with a mechanism that automatically provides a set of parameters (weights, coordinates of the reference point) for which the method generates noninferior solutions uniformly spread over a suitably chosen part of the Pareto front. The resulting set of solutions is then presented in a graphic form to the designer so that the solution representing the most satisfactory tradeoff can be easily chosen. The whole algorithm was implemented as a program and tested on two RF design examples (an LNA and a power amplifier), whose optimization results are also presented in the paper.

Efficient algorithm for solving systems of circuit differential-algebraic equations with reliable divergence suppression in DC and time domains

Although many simulation tools contain advanced algorithms for the solution of the systems of differential-algebraic nonlinear equations, some classes of circuits still cause serious problems. These are typically the circuits with strong negative feedbacks, flip-flop circuits, the blocks that are characterized by macromodels with unusual elements, and large distributed amplifiers and oscillators. In the paper, a very efficient and reliable algorithm for solving the circuit differential-algebraic equations is characterized first, which is based on a sophisticated arrangement of the Newton interpolation polynomial. After that, a novel method is introduced for improving the convergence with four suggested criteria that are being compared. Unlike the similar algorithms focused on an operating point analysis only, the proposed method also works in a transient analysis. All the four criteria are thoroughly tested on nine problematic circuits - six of them often diverge in the operating point analysis, and three of them diverge at some points during the transient analysis. The results confirm extraordinary robustness of the proposed method, even circuits with a large memelement hysteresis can be solved.

A more efficient arrangement of the sparse LU factorization for the large-scale circuit analysis

In the paper, an efficient and reliable algorithm for solving the circuit nonlinear algebraic-differential equations based on a sophisticated arrangement of Newton interpolation polynomial is characterized first. After that, a novel method is introduced for improving the convergence with four suggested criteria that are being compared. Unlike the similar algorithms focused on an operating point analysis only, the proposed method also works in a transient analysis. For enhancing the efficiency of repeated solutions of linear systems necessary in the Newton-Raphson method, a novel modification of the Markowitz criterion is suggested, which is compatible with the fast modes of the LU factorization. The modified criterion consists in an estimation of probabilities of the fill-in enlargement. The probabilities are determined for all columns of the system matrix before the LU factorization, where the column probability is calculated as the average value of the probabilities for all the column elements. Finally, the columns are reordered so that first and last should be those with the minimum and maximum probabilities, respectively. As a verification of this fundamental proposal of the paper, a comprehensive set of numerical tests has been carried out.

Multiobjective optimization with an asymptotically uniform coverage of Pareto front

This paper suggests an enhancement of an existing method for the multiobjective optimization known as GAM (goal attainment method). In our proposal, the GAM algorithm is combined with a mechanism that automatically provides a set of parameters (weights, coordinates of the reference point) for which the method generates noninferior solutions uniformly spread over a suitably selected part of the Pareto front. The resulting set of solutions is then presented in a suitable graphic form so that the solution representing the most satisfactory tradeoff can be easily chosen. The whole algorithm was implemented as a program and tested on several RF design examples (video, low-noise, and power amplifiers), whose optimization results are also presented. For a comparison, the first design example was also solved by another method known as WMM (weighted metrics method).

Reliable procedure for electrical characterization of MOS-based devices

Contemporary BSIM- and EKV-type MOSFET models are very complicated. Therefore, their identification is a quite difficult task from both algorithmic and numeric points of view. In the paper, we outline several modifications of a classical procedure which enable reliable identifications of various types of these models.