Modeling, analysis and control of high-order switching power converters

The output voltage gain of the conventional dc-dc boost converter is often limited as operating this converter at some extremely high values of the duty ratio may lead to a reverse recovery problem of the diode and little room for control when dealing with load and line disturbances. To solve this problem, many transformer-based and transformer-less topologies have been proposed over the past few years to achieve a sufficiently high gain at some lower values of the duty ratio. However, if the industrial application does not demand for dc isolation, the use of a transformer-based dc-dc converter would only increase the overall cost and size of the system. Also, the efficiency degrades due to the losses associated with the secondary winding of the transformer. This makes the transformer-less topologies a more interesting choice and to date, a number of such topologies have been reported in the relevant literature. The objective of this thesis is to study the various modeling and control related aspects of such high-order non-isolated dc-dc boost converters. Towards this end, several control schemes are studied. As compared to the conventional dc-dc boost converter wherein there are only two state variables available for feedback purposes, the newly developed high-order topologies present certain challenges. Since they contain more inductors and capacitors to achieve the high gain, they present more state variables which can be used for feedback purposes. Therefore, it is necessary to select the most suitable state variables when designing the controllers for these converters. To address this issue, a detailed comparative study of two current-mode controllers (using the input and output inductor currents of the converter) is presented for some high-order dc-dc converters such as the positive output elementary Luo (POEL) converter and the 2-stage cascade boost converter. The objective is to find the most suitable inductor current for the controller design. The analyses of the current-controlled systems are carried out using the state-space and frequency-response approaches. Sliding-mode (SM) controller is another widely used control methodology for dc-dc converters and it offers several advantages such as excellent large-signal handling capability, guaranteed stability and robustness against load, line and other parametric variations and ease of implementation. The major concern when implementing the pulse-width-modulation (PWM)-based SM controller is the difficulty in achieving a good steady-state regulation using a single integral term acting on the output voltage error. Even though this problem can be alleviated via the use of a double-integral term in the sliding surface, this approach not only increases the order of the controller but may also require more state variables such as two currents for feedback purposes. Ideally, the controller should be of a lower-order to reduce the cost and for ease of implementation. To solve this problem, a fixed-frequency PWM-based SM controller for the quadratic boost converter using a reduced number of state variables is proposed. The proposed controller requires only one current for its implementation while enjoying the advantages offered by both fixed frequency and double-integral approaches. Two PWM-based SM controllers using the input and output inductor currents of the converter are separately designed to find the most suitable inductor current for the controller design. Moreover, a systematic approach to find the most suitable inductor current for designing the hysteresis-modulation (HM)-based SM controller of a hybrid high-order dc-dc boost converter is also provided. Lastly, a new adaptive current-mode controller is proposed to regulate the high step-up dc-dc converter. The proposed controller uses the estimate of the load resistance to compute the control signal. This estimate is calculated using the adaptive algorithm and the structure of this adaptive algorithm is such that it leads to an optimized derivative of the estimate which is bounded by a user specified constant. The approximate stability analysis of the controlled system is provided and some tuning guidelines are given to select the appropriate values of the controller gains. Finally, some experimental results are provided to demonstrate better regulation properties of the proposed adaptive current-mode controller compared to existing current-mode controllers.Doctor of Philosophy (EEE

Comparative study of current-mode controllers for the positive output elementary Luo converter via state-space and frequency response approaches

This study presents the comparative study of dual-loop current-mode controllers to achieve the output voltage regulation of the positive output elementary Luo (POEL) converter. The POEL converter is a fourth-order dc-dc boost converter developed using the voltage lift technique that gives a positive load voltage. Two current-mode controllers, one using the input inductor current and one using the output inductor current, are studied. Both state-space and frequency response approaches are used in the study to obtain a better insight of the comparative study. It is demonstrated that the controller using the input inductor current is more suitable for the regulation of the POEL converter. Experimental results showing the features of the controller in the presence of load and reference voltage changes are also provided to validate the theoretical conclusions.Accepted versio

A modified fixed current-mode controller for improved performance in quadratic boost converters

A modified fixed current-mode (CM) control for the quadratic boost converter in the presence of an uncertain load resistance is presented. Specifically, the existing CM control law is modified to include an additional proportional action and using a normalized output error in order to achieve improved performance. The extra proportional action helps to improve the relative stability. An approximate stability analysis was carried out to gain some useful insight into the proposed CM controlled system. Also, experimental results comparing the performance of the proposed controller with that of the traditional CM controllers under different operating conditions are presented

Comparative study of adaptive current-mode controllers for a hybrid-type high-order boost converter

A comparative study of two adaptive current-mode controllers for a high-order hybrid-type dc-dc boost converter is presented. The implementation of the traditional current-mode controller for this converter requires the knowledge of the nominal value of the load resistance to compute the control signal. As such, it is unable to handle systems with uncertain loads well. To address this, an adaptive law is used to estimate the load conductance in order to generate the reference current input. In this adaptive law, the derivative of the estimator is optimised as well as bounded. Moreover, the converter has two inductor currents which can be used for feedback purposes. Considering this, two adaptive current-mode controllers using the input and output inductor currents of the converter are separately designed to find the most appropriate inductor current for the implementation of the proposed controller. Finally, some simulation and experimental results comparing the performance of the adaptive controller using the output inductor current with that of the traditional current-mode controller are also presented.Published versio

Design and Selection of Inductor Current Feedback for the Sliding-Mode Controlled Hybrid Boost Converter

The hybrid step-up converter is a fifth-order system with a dc gain greater than the traditional second-order step-up configuration. Considering their high order, several state variables are accessible for feedback purposes in the control of such systems. Therefore, choosing the best state variables is essential since they influence the system’s dynamic response and stability. This work proposes a methodical method to identify the appropriate state variables in implementing a sliding-mode (SM) controlled hybrid boost converter. A thorough comparison of two SM controllers based on various feedback currents is conducted. The frequency response technique is used to demonstrate how the SM method employing the current through the output inductor leads to an unstable response. The right-half s-plane poles and zeroes in the converter’s inner-loop transfer function, which precisely cancel one another, are what is causing the instability. On the other hand, a stable system may result from employing a SM controller with the current through the input inductor. Lastly, some experimental outcomes using the preferred SM control method are provided

Modified voltage-mode controller for the quadratic boost converter with improved output performance

The control of the quadratic boost converter with unknown load resistance and where only the output voltage is used for feedback purposes is addressed. In the existing voltage-mode controlled converter system, there exists a performance `trade-off' between the transient responses after the onset of a reference input and a load disturbance due to the usage of the traditional proportional-integral control scheme. To overcome these problems, a modified voltage-mode controller is proposed. The proposed controller uses a normalised integral action in which the derivative of the integrand is bounded by a user-defined constant. The approximate stability analysis of the resultant voltage-mode controlled quadratic boost converter system is carried out to gain some insight into its closed-loop behaviour. Some simulation and experimental results comparing the performance of the proposed controller with that of the existing controller are also provided to show the improvements obtained using the proposed controller.Accepted versio

Design and analysis of a voltage‐mode non‐linear control of a non‐minimum‐phase positive output elementary Luo converter

The positive output elementary Luo (POEL) converter is a fourth‐order DC–DC converter having highly non‐linear dynamic characteristics. In this paper, a new dynamic output voltage feedback controller is proposed to achieve output voltage regulation of the POEL converter. In contrast to the state‐of‐the‐art current‐mode controllers for the high‐order boost converters, the proposed control strategy uses only the output voltage state variable for feedback purposes. This eliminates the need for the inductor current sensor to reduce the cost and complexity of implementation. The controller design is accompanied by a strong theoretical foundation and detailed stability analyses to obtain some insight into the controlled system. The performance of the proposed controller is then compared with a multi‐loop hysteresis‐based sliding‐mode controller (SMC) to achieve the output voltage‐regulation of the same POEL converter. The schemes are compared concerning ease of implementation, in particular, the number of state variables and current sensors required for implementation and the closed‐loop dynamic performance. Experimental results illustrating the features of both controllers in the presence of input reference and load changes are presented.Published versio

A simplified output feedback controller for the DC-DC boost power converter

Boost-type dc-dc converters present non-minimum phase dynamic system characteristics. Therefore, controller design using only the output voltage for feedback purposes is not a very straightforward task. Even though output voltage control can be achieved using inductor current control, the implementation of such current-mode controllers may require prior knowledge of the load resistance and also demand more states such as one or more currents in feedback. In this paper, the development of a new output feedback controller for boost-type dc-dc converters is presented. The controller form is such that it avoids the possibility of saturation in the control signal due to division by zero. The basic structure of the proposed controller is firstly obtained from the expression of the open-loop control signal, and the complete controller structure is then derived to satisfy the closed-loop stability conditions. Simulation and experimental results clearly verify the ability of the control law to provide robust regulation against parameter variations.Published versio

A Simplified Output Feedback Controller for the DC-DC Boost Power Converter

Boost-type dc-dc converters present non-minimum phase dynamic system characteristics. Therefore, controller design using only the output voltage for feedback purposes is not a very straightforward task. Even though output voltage control can be achieved using inductor current control, the implementation of such current-mode controllers may require prior knowledge of the load resistance and also demand more states such as one or more currents in feedback. In this paper, the development of a new output feedback controller for boost-type dc-dc converters is presented. The controller form is such that it avoids the possibility of saturation in the control signal due to division by zero. The basic structure of the proposed controller is firstly obtained from the expression of the open-loop control signal, and the complete controller structure is then derived to satisfy the closed-loop stability conditions. Simulation and experimental results clearly verify the ability of the control law to provide robust regulation against parameter variations