Design of robust controllers for dc-dc converters

Abstract

The switched mode dc-dc converters are some of the simplest power electronic circuits which convert one level of electrical voltage into another level by switching action. These converters have received an increasing deal of interest in many areas. This is due to their wide applications like power supplies for personal computers, office equipments, appliance control, telecommunication equipments, DC motor drives, automotive, aircraft, etc. The analysis, control and stabilization of switching converters are the main factors that need to be considered. Many control methods are used for control of switch mode dc-dc converters and the simple and low cost controller structure is always in demand for most industrial and high performance applications. Every control method has some advantages and drawbacks due to which that particular control method consider as a suitable control method under specific conditions, compared to other control methods. The control method that gives the best performances under any conditions is always in demand.One of the key issues of feedback is to provide robustness to uncertainty . It is one of the most useful properties of feedback and is what makes it possible to design feedback systems based on simplified models. One form of uncertainty in dynamical systems is parametric uncertainty in which the parameters describing the system are unknown. The neglected mechanisms while linearizing a process are called Unmodeled dynamics. Unmodeled dynamics can be accounted for by developing a more complex model. Such models are commonly used for controller development, but substantial effort is required to develop them. An alternative is to investigate if the closed loop system is sensitive to generic forms of unmodeled dynamics .The basic idea is to describe the unmodeled dynamics by including a transfer function in the system description whose frequency response is bounded but otherwise unspecified The commonly used control methods for dc-dc converters are pulse width modulated (PWM) voltage mode control, PWM current mode control with proportional (P), proportional integral (PI), and proportional integral derivative (PID) controller and many others. In this thesis linear controllers like Youla and H∞ controllers are studied and analyzed. These conventional control methods delivered good results using the state-space averaging (SSA) technique but are unable to perform satisfactorily when implemented to the non-linear plant having switch element. Therefore, nonlinear controllers come into picture for controlling dc-dc converters. The advantages of these nonlinear controllers are their ability to react suddenly to a transient condition. The different types of nonlinear controllers are hysteresis controller, sliding mode controller, fuzzy logic sliding mode control etc.