Contribution à la modélisation des convertisseurs continu/continu dans une perspective de commande – Influence du filtre d'entrée

摘要

Modeling and control of switched-mode dc-dc converters has occupied a center stage in the field of modern power electronics due to their widespread military and industrial applications. Averaged modeling is most commonly applied as an effective tool to analyze dynamic behavior of a converter and to get physical insights into various dynamical phenomena. State-space averaged models are widely accepted in practice mainly because of their simplicity, generality and demonstrated practical utility. Various averaged models have been presented in literature; however, some fundamental questions regarding averaging methodologies still lack satisfactory answers. These unresolved modeling issues are primarily related to their practical validation, inclusion of circuit parasitics and their application to the control-loop design. One of the primary concerns of this thesis is to study and evaluate the performance of averaged modeling of dc-dc converters from control perspective. In particular, the main emphasis is placed on the theoretical and experimental investigation of averaged modeling in discontinuous conduction mode (DCM). Various analytical averaged models of different orders, presented in literature, are reformulated in this thesis by including all appropriate parasitics. Parasitics are introduced to take into account those phenomena which can possibly induce instability. Then, the validities of these averaged models are experimentally examined by comparing analytical results with experimental results measured from a hardware prototype.As far as control is concerned, stability is of prime importance in any dc voltage regulation system. However, closed-loop stability is not guaranteed if a low-pass filter is present at converter-input. The origin of this problem lies in the filter interactions with the negative dynamic resistance behavior of the dc-dc converter input port. Literature provides a gateway to solve this issue and proposes a ?passive? solution to damp the input-filter oscillations. Although exact values of the required damping resistance can be determined using an ideal converter model, this value is not systematically confirmed through experiments. In this thesis small-signal control-to-output transfer functions are used to systematically formulate some design rules to avoid instability. Safe operating regions are identified in terms of damping-circuit parameters and this approach is subsequently extended to the case of cascade converters. Throughout this study the small-signal averaged modeling is used for the stability analysis.Although adding adequate resistance to the filter can solve instability problem, one drawback for which passive damping is commonly criticized is the undesirable power dissipation in the damping resistors. To properly investigate its adverse impact on conversion efficiency, these damping losses are quantified in this thesis. A detailed theoretical power-loss analysis is presented under varying operating conditions followed by its experimental verification. Obtained results are generalized for all fundamental topologies.One of the main themes of this dissertation is the development of a control solution for the stability of dc-dc converter in presence of input filter, hence avoiding the use of dissipative damping. To achieve this objective, this thesis suggests the use of full state-feedback control with pole-placement technique. An augmented state-space averaged model is used to design the controller which combines state-feedback with PI-control loop. First of all a theoretical approach is presented. Then the effectiveness of the proposed control algorithm is demonstrated with simulation studies. It appears that an adequate level of dynamic performance under large perturbations can be achieved by using a varying gain state-feedback. A pseudo large-signal stability analysis is also performed with the help of this technique. Importantly, this control strategy assures stability of the system without using any passive components in the filter circuit and thus avoiding undesirable losses. An alternate control scheme, chosen from the literature, is also discussed for filter-converter system stability. This scheme is based upon sliding-mode control and Lyapunov function approach. Its dynamic performance is compared with that of the full state-feedback controller proposed in this thesis while explaining pros and cons of both control strategies.