MODELING, ANALYSIS AND CONTROL OF NONISOLATED DC-DC CONVERTERS

Abstract

The switched mode converters grow to be popular, because of its vast applications in different fields. These are having applications mainly in micro-grid, renewable energy power generation, battery charging, power supplies, LED drivers, aero-space equipment, drives applications etc. Different applications require their individual set point voltage levels according to the requirement. Depending on the applications, various DC-DC converters have been utilized to step up/down the regulated DC voltage from the unregulated DC voltage. In practice, buck, boost and buck-boost converters are the most commonly used DC-DC converters for step down/up applications. Here, the accurate design analysis of these converter systems is very important. This is main motive to work on detailed analysis, accurate modelling and control of non-isolated DC-DC converters. Overall, DC-DC converters can be classified as buck and boost type. In this work, the basic non-isolated DC-DC converters such as boost, buck-boost and NIBB (Non-inverting buck-boost) are mainly considered for analysis in different aspects. All major non-idealities of the converter system are considered such as equivalent series resistances (ESR) of filter elements (inductor, capacitor), resistances of input supply, semiconductor switches and diode forward drop voltage. Overall, thesis can be viewed as two parts, first part concentrates on design, modelling analysis of DCDC converters and second part focuses on their controller design. The first part mainly concentrates on power electronic related issues like design, modelling and analysis. This includes improved or accurate expressions of duty cycle, inductor and capacitor for non-ideal boost, buck-boost and NIBB converter. Here, the important discussions of maximum achievable duty cycle, voltage of converter system with the given parameters and minimum input voltage needed for the desired output are explained in detail. The exact utilization of these expressions for power and control engineers also explained. Further, OVR (Output Voltage Ripple) and ESR are analysed and also the effect of ICR (Inductor Current Ripple), OVR on capacitor design discussed in detail. Moreover, the maximum permissible ESR for specified OVR is derived. Along with this, a complete non-ideal mathematical i model is developed, which gives similar response of practical system in dynamic and steady-state behaviour wise. The state-space average approach is used to develop the accurate non-ideal models. These non-ideal models are compared with the ideal models. Desired practical results are obtained by the non-ideal model with minimum tolerance. In addition, a hybrid converter namely non-inverting buck-boost derived hybrid converter (NIBBDHC) is proposed based on the knowledge of basic converter topologies. The proposed topology has a feature, which can provide both DC and AC outputs, simultaneously. Functionally as similar as conventional VSI (Voltage Source Inverter), however, shoot through is well utilized in proposed converter. Complete mathematical analysis is presented and are verified through simulation and practical implementation. The second part of thesis is related to designing a controller for DC-DC converters. Here, IMC (Internal Model Control) is used to design the PID controller. The tuning is proposed for DC-DC converters. The main focus is to design a general PID controller for all types of converters and there is no need for trial and error method to choose PID parameters. Along with this, the designed PID can achieve desired bandwidth of the system, which is very important for the DC-DC converters. Further, when there exist parametric uncertainties, a PID controller is designed. The best part of this work is that, single PID controller can handle the parametric uncertainties (interval type of uncertainties). For this, Kharitonov theorem and stability boundary locus techniques are used. Here, a reduced polynomial approach is proposed for DC-DC converters. Finally, all these control techniques are implemented on considered DC-DC converters through simulations and practical experiments. The controller is implemented on DSPACE-1104 through Hardware in loop.