MODELING AND CONTROL OF DC-DC CONVERTERS

Abstract

The switch mode DC-DC converters are widely used in modern electronic systems as power supply because of their merits in terms of efficiency, compactness, light-weight, and reliability. They have many applications such as computer power supplies, aerospace instruments, medical instruments, telecommunication equipment, etc. The main feature of any DC-DC converter is to provide the stable desired DC output voltage irrespective of variations in the operating conditions such as input voltage, load current and reference output voltage (set point voltage). Normally, the input voltage to DC-DC converters is unregulated and the load current or set point voltage may also vary depending upon application requirements. Therefore, an accurate and reliable operation of DC-DC converters is essential under these circumstances. This has motivated to carry out this research work on the design, analysis, modeling, and control of the DC-DC converters. This thesis moves around the three important aspects of the DC-DC converters namely design, modeling and control. The DC-DC converters have many non-isolated topologies like buck converter, boost converter, buck-boost converter, Cuk converter, Zeta converter, SEPIC converter, etc. In this thesis, two DC-DC converters, namely buck converter and Cuk converter have been considered for the research on above-mentioned aspects. The non-idealities of these converter elements are taken into consideration. These non-idealities are present in form of equivalent series resistances (ESRs) of the inductors and capacitors, the diode forward voltage drop and the on-resistances of the switch and diode. In the first part of the thesis, the different design issues of the non-ideal DC-DC buck and Cuk converters are considered. The expressions of duty cycle, inductors, and capacitors are improved involving the non-ideal parameters of the converters. A detailed analysis of the output voltage ripple is carried out and importance of capacitor equivalent series resistance (ESR) is analyzed. A formula for maximum permissible ESR for specified output voltage ripple is proposed. The theoretical studies are validated via simulation and experimental results. The second main aim of this thesis is to develop a mathematical model including all non-ideal elements, such that the developed model will be a close-replica of the practical converter in terms of dynamic and steady-state behaviour. The state-space averaging technique, averaged switch model technique and energy factor approach are used to develop the more accurate models of the non-ideal buck and Cuk converter. Further, the mathematical models in ideal and non-ideal case are compared. It is found that the models with non-idealities have much improved closeness to the practical converter as compared to the ideal counterpart. ii In the final part, the performance of DC-DC buck and Cuk converters are improved using various controllers, namely the PI controller based on the stability boundary locus approach, PI-lead controller, two-loop controller, sliding mode controller. The algorithms for tuning the parameters of these controllers are proposed. The PI controller is also designed based on the reduced-order model of Cuk converter. The model-order reduction technique is used to obtain the reduced second-order model of the fourth-order Cuk converter. The hardware prototypes of DC-DC buck converters are developed and these control techniques are implemented on the prototypes. Extensive experimental and simulation studies are carried out for both types of converters. The comparative analysis of these control techniques is presented.