D. Borojevic

Impedance specification and impedance improvement for DC distributed power system

In a DC distributed power system, the interaction between individually designed power modules/subsystems may cause the instability of the whole system. In the small-signal sense, these kinds of interactions can be predicted by checking impedance ratio Z/sub o//Z/sub i/ and can also be prevented by making impedance specification for power modules/subsystems. Many efforts have been made in defining impedance specification and improving I/O impedance for converters and filters. This paper summarizes work in these two areas.

Input filter interaction in three phase AC-DC converters

It is well known that the addition of an input filter preceding a switched mode regulator poses a problem of performance degradation and potential instability due to its negative input impedance at low frequencies. Three phase converters are essentially multivariable systems. The objective of this paper is to analyze the problem of input filter interaction in three phase AC-DC converters from a multivariable perspective and to establish criteria that guarantee stability and satisfactory performance of the converter with an input filter. The dq average model of a three phase boost rectifier is used for the analysis. The minor loop, the stability of which has to be guaranteed is identified. A sufficient condition for stability, based on the singular values of the filter output impedance and converter input admittance matrices is derived. Simulation results are presented to illustrate the results.

A novel approach to the stability analysis of boost power-factor-correction circuits

We investigate the stability of a boost PFC circuit, in the saturated and unsaturated regions of operation, for two separate cases: one for which the switching frequency is approaching infinity and the other for which it is finite but large. For case one, we show that global existence of a smooth hypersurface for the boost PFC circuit is not possible. Subsequently, we develop the condition for local existence. For case two, we find that within the boundary layer, the dynamics of the nonlinear system evolve on a torus. Using bifurcation theory, we show the mechanism of the torus breakdown.

Modeling and control of parallel three-phase PWM boost rectifiers in PEBB-based DC distributed power systems

Modeling and control design issues of parallel three-phase PWM boost rectifiers in DC distributed power systems are presented. Small-signal characteristics of parallel modules are investigated in comparison with those of a single module. With distributed DC bus impedance included, a master/slave control scheme is adopted to ensure load sharing. The system stability based on loop-gain analysis is discussed. The interaction possibilities are presented briefly. Simulation results with frequency domain and time domain are demonstrated.

The circulating current in paralleled three-phase boost PFC rectifiers

This paper presents modeling, simulation, and experimental results of paralleled three-phase boost PFC rectifiers. The first part of the paper describes the overall system set-up and control schemes of the PFCs. Although individual modules work as expected, a low frequency oscillation between the paralleled units was observed. Because the conventional model of the three-phase rectifier cannot predict this kind of interaction, an average model is developed for system simulation. From this model, it is shown that the interleaved discontinuous space vector modulation produces a periodic disturbance on the zero axis. Because the conventional control in a balanced three-phase system with only dq channels cannot reject this disturbance, a circulating current will flow between the paralleled modules. Based on this observation, a space vector modulation with control in the third axis is proposed for the parallel operation of the rectifiers. Simulations are done to show the feasibility of this scheme.

Stability analysis of parallel DC-DC converters using a nonlinear approach

The authors develop analytic methodologies for stability analysis of a parallel DC-DC converter using its switching model, discrete model (based on nonlinear map) and averaged model. They use these methodologies to investigate the dynamics of an interleaved and a synchronized parallel DC-DC buck converter. Using the switching and discrete models they show the mechanism and boundaries of fast-scale and slow-scale instabilities. They show the difference in mechanism of fast-scale instability between the interleaved and a synchronized converter. The averaged model does not show this difference. It also differs considerably (from the other two models) in prediction of instability boundaries.

A theoretical and experimental investigation of the nonlinear dynamics of DC-DC converters

The authors use an exact formulation based on nonlinear maps to investigate both the fast-and slow-scale instabilities of a voltage-mode buck converter operating in the continuous conduction mode and its interaction with a filter. Comparing the results of the exact model with those of the averaged model shows the shortcomings of the latter in predicting fast-scale instabilities. They show the impact of parasitics on the onset of chaos using a high-frequency model. The experimentally validated theoretical results of this paper provide an improved understanding of the dynamics of the converter beyond the linear regime. This in turn may lead to an improved electromagnetic compatibility of switched-mode power supplies and an understanding of newer operating regimes, which may lead to less conservative control design and newer applications.

A nonlinear approach to the analysis of stability and dynamics of standalone and parallel DC-DC converters

The authors analyze the stability and dynamics of two different types of power electronic converters. Using results based on bifurcation analyses, they describe the dynamics of these nonlinear systems beyond the linear regime and demarcate their stability boundaries. Their results show the shortcomings of traditional methods based on averaging and pave the way for less conservative control design, improved electromagnetic compatibility and newer applications.

New sensorless control of three-phase bi-directional converter using space-vector modulation

In this paper, the authors describe a sensorless control scheme for a three-phase bidirectional converter that operates without line current sensors. With this scheme, the converter operates under a fixed switching frequency and can handle zero states. This improves the total harmonic distortion of the source current and eliminates the problems associated with a variable switching frequency control. The scheme can handle any space-vector modulation scheme. The implementation of the converter control is in a synchronous frame, thereby enhancing the dynamic bandwidth of the system.

Development of integral-variable-structure control schemes for parallel-buck and parallel-boost DC-DC converters

In this paper we develop variable-structure control schemes for parallel-boost and parallel-buck converters. The advantages of these schemes are the simplicity in design, good dynamic response, the ability to nullify the bus-voltage error and the error between the load currents or the line currents of the converter modules under steady-state conditions, and the ability to reduce the impact of very high-frequency dynamics due to parasitics on the closed-loop system. We outline methods for determining the region of existence and the stability of a sliding manifold for both the converters. Simulation results show good steady-state and dynamic responses.