Stability analysis and control of DC-DC converters using nonlinear methodologies

PhD ThesisSwitched mode DC-DC converters exhibit a variety of complex behaviours in power electronics systems, such as sudden changes in operating region, bifurcation and chaotic operation. These unexpected random-like behaviours lead the converter to function outside of the normal periodic operation, increasing the potential to generate electromagnetic interference degrading conversion efficiency and in the worst-case scenario a loss of control leading to catastrophic failure. The rapidly growing market for switched mode power DC-DC converters demands more functionality at lower cost. In order to achieve this, DC-DC converters must operate reliably at all load conditions including boundary conditions. Over the last decade researchers have focused on these boundary conditions as well as nonlinear phenomena in power switching converters, leading to different theoretical and analytical approaches. However, the most interesting results are based on abstract mathematical forms, which cannot be directly applied to the design of practical systems for industrial applications. In this thesis, an analytic methodology for DC-DC converters is used to fully determine the inherent nonlinear dynamics. System stability can be indicated by the derived Monodromy matrix which includes comprehensive information concerning converter parameters and the control loop. This methodology can be applied in further stability analysis, such as of the influence of parasitic parameters or the effect of constant power load, and can furthermore be extended to interleaved operating converters to study the interaction effect of switching operations. From this analysis, advanced control algorithms are also developed to guarantee the satisfactory performance of the converter, avoiding nonlinear behaviours such as fast- and slowscale bifurcations. The numerical and analytical results validate the theoretical analysis, and experimental results with an interleaved boost converter verify the effectiveness of the proposed approach.Engineering and Physical Sciences Research Council (EPSRC), China Scholarship Council (CSC), and school of Electrical and Electronic Engineerin

Design of an Advanced Programmable Current-Source Gate Driver for Dynamic Control of SiC Device

Silicon carbide (SiC) power devices outperform Silicon-based devices in operational voltage levels, power densities, operational temperatures and switching frequencies. However, the gate oxide of SiC-based device is more fragile compared with its Si counterpart. The vulnerability of the gate oxide in SiC power devices requires the development of a gate driver that is able to have more control during the turn-on and turn-off process. This paper proposes an innovative current-source gate driver where the gate current can be fully programmed. The novelty of the gate driver is that the dynamic switching transients and the static on/off-state can be controlled independently. In order to achieve this approach, a signal decomposition and reconstruction technique is proposed to apply the separate control over the dynamic switching transient and the static on/off-state gate voltage respectively. The fundamental principle of the proposed circuit is verified in simulation. In addition, a prototype of the active gate driver has been built and tested to validate the effectiveness of the flexible control over the gate voltage

Design and Development of Low Torque Ripple Variable-Speed Drive System With Six-Phase Switched Reluctance Motors

Switched reluctance motor (SRM) drives conventionally use current control techniques at low speed and voltage control techniques at high speed. However, these conventional methods usually fail to restrain the torque ripple, which is normally associated with this type of machine. Compared with conventional three-phase SRMs, higher phase SRMs have the advantage of lower torque ripple: To further reduce their torque ripple, this paper presents a control method for torque ripple reduction in six-phase SRM drives. A constant instantaneous torque is obtained by regulating the rotational speed of the stator flux linkage. This torque control method is subsequently developed for a conventional converter and a proposed novel converter with fewer switching devices. Moreover, modeling and simulation of this six-phase SRM drive system has been conducted in detail and validated experimentally using a 4.0-kW six-phase SRM drive system. Test results demonstrate that the proposed torque control method has outstanding performance of restraining the torque ripple with both converters for the six-phase SRM, showing superior performance to the conventional control techniques

Nonlinear Analysis and Control of Interleaved Boost Converter Using Real-Time Cycle to Cycle Variable Slope Compensation

Switched-mode power converters are inherently nonlinear and piecewise smooth systems that may exhibit a series of undesirable operations that can greatly reduce the converter's efficiency and lifetime. This paper presents a nonlinear analysis technique to investigate the influence of system parameters on the stability of interleaved boost converters. In this approach, Monodromy matrix that contains all the comprehensive information of converter parameters and control loop can be employed to fully reveal and understand the inherent nonlinear dynamics of interleaved boost converters, including the interaction effect of switching operation. Thereby not only the boundary conditions but also the relationship between stability margin and the parameters given can be intuitively studied by the eigenvalues of this matrix. Furthermore, by employing the knowledge gained from this analysis, a real-Time cycle to cycle variable slope compensation method is proposed to guarantee a satisfactory performance of the converter with an extended range of stable operation. Outcomes show that systems can regain stability by applying the proposed method within a few time periods of switching cycles. The numerical and analytical results validate the theoretical analysis, and experimental results verify the effectiveness of the proposed approach

Cost-Effective and High-Efficiency Variable-Speed Switched Reluctance Drives With Ring-Connected Winding Configuration

This paper presents a novel converter topology for six-phase switched reluctance motor (SRM) drives, which reduces the number of switches and diodes by half, compared with the conventional asymmetric half-bridge converter, but needs no additional energy storage component. A dynamic model of a six-phase SRM is developed in the MATLAB/SIMULINK environment and conventional current chopping and angle position control techniques are applied to the proposed converter, demonstrating successful operation across the full speed range with modified conventional control techniques, lower converter losses, and higher system efficiency compared with the asymmetric half-bridge converter. Experimental tests comparing two versions of the proposed converter with an asymmetric half-bridge are described and verify the predictions of the simulations

Design and control of a bidirectional wireless charging system using GaN devices

Most of the existing wireless power transfer system works in unidirectional with one-direction control signals. This paper presents a bidirectional wireless charging system with duplex communication method, which is not only able to achieve the two-way wireless power transmission, but also transfer control signals bi-directionally. The power circuit operation mode is actively controlled by using the wireless transceiver module which can duplex communication to deliver measured signals remotely. The operational principle is analytically studied in details and is verified by simulation. Finally, a prototype of the bidirectional charging system using GaN devices has been successfully designed and tested. In addition, the measured feedback signals are effectively transmitted to validate the control algorithm

Polynomial Curve Slope Compensation for Peak-Current-Mode-Controlled Power Converters

Linear ramp slope compensation (LRC) and quadratic slope compensation (QSC) are commonly implemented in peak-current-mode-controlled dc-dc converters in order to minimize subharmonic and chaotic oscillations. Both compensating schemes rely on the linearized state-space averaged model (LSSA) of the converter. The LSSA ignores the impact that switching actions have on the stability of converters. In order to include switching events, the nonlinear analysis method based on the Monodromy matrix was introduced to describe a complete-cycle stability. Analyses on analog-controlled dc-dc converters applying this method show that system stability is strongly dependent on the change of the derivative of the slope at the time of switching instant. However, in a mixed-signal-controlled system, the digitalization effect contributes differently to system stability. This paper shows a full complete-cycle stability analysis using this nonlinear analysis method, which is applied to a mixed-signal-controlled converter. Through this analysis, a generalized equation is derived that reveals for the first time the real boundary stability limits for LRC and QSC. Furthermore, this generalized equation allows the design of a new compensating scheme, which is able to increase system stability. The proposed scheme is called polynomial curve slope compensation (PCSC) and it is demonstrated that PCSC increases the stable margin by 30% compared to LRC and 20% to QSC. This outcome is proved experimentally by using an interleaved dc-dc converter that is built for this work

A ripple reduction method for a two stages battery charger with multi-winding transformer using notch filter

This paper presents a two-stage battery charger consisting of a bridgeless Totem-pole power factor correction (TP-PFC) circuit and a full bridge converter with a multi-winding transformer. By using this transformer the cell equalizing operation can be achieved with no additional circuitry. In addition, a double-line frequency ripple reduction method is proposed to address the low frequency current ripples issues existing in both primary and secondary winding of the transformer which is caused by the voltage ripples across the intermediate DC link bus. Control and analysis of the converter at different operation modes is illustrated in detail and simulation results validate the effectiveness of the proposed converter and control algorithm

Determination of Stray Inductance of Low-Inductive Laminated Planar Multiport Busbars Using Vector Synthesis Method

Laminated busbars connect capacitors with switching power modules, and they are designed to have low stray inductance to minimize electromagnetic interference. Attempts to accurately measure the stray inductance of these busbars have not been successful. The challenge lies with the capacitors, as they excite the busbar producing their individual stray inductances. These individual stray inductances cannot be arithmetically averaged to establish the total stray inductance that applies when all the capacitors excite the busbar at the same time. It is also not possible to measure the stray inductance by simultaneous excitation of each capacitor port using impedance analyzers. This paper presents a solution to the above dilemma. A vector synthesis method is proposed, whereby the individual stray inductance from each capacitor port is measured using an impedance analyzer. Each stray inductance is then mapped into an xyz frame with a distinct direction. This mapping exercise allows the data to be vectored. The total stray inductance is then the sum of all the vectors. The effectiveness of the proposed method is demonstrated on a busbar designed for H-bridge inverters by comparing the simulation and practical results. The absolute error of the total stray inductance between the simulation and the proposed method is 0.48 nH. The proposed method improves the accuracy by 14.9% compared to the conventional technique in measuring stray inductances

The Effect of Foreign Institutional Ownership on Corporate Tax Avoidance: International Evidence

We find robust evidence that foreign institutional investors are negatively associated with their investee firms’ tax avoidance. To mitigate endogeneity concerns, we apply three identification strategies. First, we implement a two-stage least squares model. Second, we perform a difference-in-differences analysis by exploiting China’s legal reform, Qualified Foreign Institutional Investors program, as a quasi-natural experiment. Third, we compare changes in corporate tax avoidance in response to a significant increase in FIIs. We further find that the negative association is dominated by FIIs from countries with high tax morale and FIIs from countries with strong shareholder protection. Finally, we find that the extent of tax morale and shareholder protection in the country where an investee firm is located also matters. We conclude that FIIs play an active role in shaping investee firms’ corporate tax avoidance policy