Load-imposed instability and performance degradation in a regulated converter

Dynamic analysis of the interconnected regulated systems is commonly based on the minor-loop gain which composes of the ratio between the output and input impedances defined at an arbitrary interface within the system. The stability of the system is guaranteed, when the minor-loop gain satisfies Nyquist stability criterion. Usually, the stability conditions are expressed as a certain forbidden region in a complex plane out of which the minor-loop gain should stay for stability to exist. The robustness of the stability is guaranteed requiring certain phase and gain margins to exist. It is also natural to assume that the existence of robust stability means also the existence of acceptable transient behavior. The paper investigates the validity of those expectations. According to the investigations, the system-level minor-loop gain measured at an arbitrary interface does not necessarily contain any other information than the existence of stability. The theoretical analyses are based on a set of transfer functions describing the dynamics of a switched-mode converter. Practical evidence is provided based on a small-scale industrial prototype system composing of voltage-mode-controlled buck converters.

Distributed power supply (DPS) systems are extensively used to supply different electronic equipment and systems such as telecom switching systems. Continuous reduction of supply voltages of digital integrated circuits from the previous industry standard of 5 V down to 1 V or even less has necessitated the use of point-of-load system architectures due to high supply currents. The input currents of the switched-mode power supplies contain typically high ripple currents necessitating the use of noise suppression (EMI) filters to meet the stringent requirements stipulated by international standards. The interactions between the load converters, EMI filters, and supply side converters may adversely affect the stability and performance of the system. The system analysis is typically made based on the behavior of the ratio between the output impedance of the supply side and the input impedance of the load side known as minor-loop gain. Certain forbidden regions in the complex plane have been developed out of which the minor-loop gain should stay in order to guarantee stability. Even if the restrictions for the behavior of the minor-loop gain are well-defined, the accurate prediction of the associated impedances is difficult or even impossible. We propose, in this paper, a method based on input invariance to cancel the load-side interactions in the small-signal sense. The only concern left is then the stability of EMI filters, which may be considered based on the input power of the associated converters.

This article proposes a systematic approach to determine the most suitable analogue redesign method to be used for forward-type converters under digital voltage mode control. The focus of the method is to achieve the highest phase margin at the particular switching and crossover frequencies chosen by the designer. It is shown that at high crossover frequencies with respect to switching frequency, controllers designed using backward integration have the largest phase margin; whereas at low crossover frequencies with respect to switching frequency, controllers designed using bilinear integration with pre-warping have the largest phase margins. An algorithm has been developed to determine the frequency of the crossing point where the recommended discretisation method changes. An accurate model of the power stage is used for simulation and experimental results from a Buck converter are collected. The performance of the digital controllers is compared to that of the equivalent analogue controller both in simulation and experiment. Excellent closeness between the simulation and experimental results is presented. This work provides a concrete example to allow academics and engineers to systematically choose a discretisation method.

In Microgrid (MG) design process, it is usually focus on the stability of microsource (MS). Impedance specification method is an effective approach to analyzing system stability, by defining the ratio of the source/load subsystems impedances and keeping it in the impedance specification. Base on the topology and control strategy of voltage source inverter, the output impedance of inverter is proposed. The influences to output impedance and minor loop gain are discussed which are caused by variations of filter parameters. The stability of the inverter system is researched according to the impedance specification method, and proved by traditional time-domain analysis. Because MG is constituted by numbers of MS operating in parallel at autonomous mode, an admittance specification is proposed, and the stable quantized relation between inverter output and load input admittance is indicated, and the stability of MG system can be ensured by designing the output admittance of MS. The correction and feasibility of proposed admittance specification method are fully verified by simulation results.

This paper derives closed-loop input and output impedances of a number of dc-dc power electronic converters when operating in a dc distribution network. The derived impedances can be used for stability analysis and control design for dc distribution networks. In this paper, first the small-signal model of a power converter operating in a dc power system is introduced and the open-loop transfer functions are derived. Next, the closed-loop model of the converter is assembled based on the open-loop model and the type of the control scheme. Then, the closed-loop impedances are derived from the resulting closed-loop model. Finally, the derived impedances are verified using an experimental prototype converter.

Different control loops employed in regulating deck converter outputs may have an effect on the output impedance of the converter. In this manuscript, the effect of the inner-current loop on the output impedance of a peak current-mode controlled PWM dc-dc converter with the outer-voltage loop open is investigated. Closed-loop output impedance including feed-forward gains with inner-current loop closed and outer-voltage loop open for peak-current mode controlled PWM dc-dc converters operating in CCM is derived. Bode plots of the closed-loop output impedance for PCM controlled PWM dc-dc buck and boost converters with inner-current loop only are illustrated using MATLAB simulations and are compared with the Bode plots of respective open-loop output impedances.

Current mode control (CMC) though offers improved line regulation, phase margin and high bandwidth suffers from high output impedance. Although load regulation can be improved by incorporating an integral gain, it reduces the phase margin and increases the overshoot. The objective of this paper is to achieve improved output impedance without using an integral controller. Incorporating a high pass filter in series with the current sensor in CMC, the closed loop DC gain of a CCM buck converter is shown to be almost independent of the load resistance, thus improving load regulation using a proportional controller only. Improved features of CMC are also retained. Following VMC modeling approach, we develop a method to obtain the small signal model of the proposed scheme, and extended to CMC with an accurate low frequency characterisation. The proposed scheme is implemented on a buck converter, and the theoretical results are validated by experiments.

The modified second-order sliding mode algorithm is used for controller tuning. Namely, the modified suboptimal algorithm-based test (modified SOT) and non-parametric tuning rules for proportional-integral-derivative (PID) controllers are presented in this article. In the developed method of test and tuning, the idea of coordinated selection of the test parameters and the controller tuning parameters is introduced. The proposed approach allows for the formulation of simple non-parametric tuning rules for PID controllers that provide desired amplitude or phase margins exactly. In the modified SOT, the frequency of the self-excited oscillations can be generated equal to either the phase crossover frequency or the magnitude crossover frequency of the open-loop system frequency response (including a future PID controller) – depending on the tuning method choice. The first option will provide tuning with specification on gain margin, and the second option will ensure tuning with specification on phase margin. Tuning rules for a PID controller and simulation examples are provided.

The well-known control system parameters phase and gain margin and the crossover frequency are improved by using respective gradients. In parallel, the worst uncertainty of the plant is included in the design method, based on spherical uncertainties with Frobenius norm bound. Both transfer functions and the state space are utilized. Multi-gradients are suggested as well.

DC distributed systems are highly reliable and efficient means of delivering DC power or adopting renewable energy resources. However, DC distributed systems are prone to instability and dynamic performance degradation due to the negative incremental input impedance of DC-DC converts. In this paper, we propose a generic method to eliminate the impact of the negative input impedance on DC systems by shaping the source output impedance such that its bode-plot is restricted in the area that is contained below the product of the source’s duty ratio and its characteristic impedance. The performance deterioration originates whenever the output impedance of the source exceeds, in magnitude, the input impedance of the load converter due to deficiency in stability margins. Hence, confining the impedance in the proposed region helps decouple the interaction between the converters and preserve their own dynamic performances. The proposed method was proven by analytical analysis, time-based simulation, and practical experiments. All of their outcomes were in agreement, proving the effectiveness of the proposed method in preserving the dynamic performance of distributed systems.

Analytic expressions for the small-signal power-stage describing functions of a switched dc-dc boost regulator are derived from an approximate continuous circuit model which is developed by a time-averaging technique. Closed-loop stability is, attained through the design of frequency compensation of the loop gain. Open- and closed-loop regulator output impedances are derived from the linearized models for the given configuration. The analysis and design are compared with and confirmed by breadboard measurements.

In this paper, a fractional order proportional and integral (FOPI or PIα) speed control method for permanent magnet synchronous motor (PMSM) is presented. Based on the modeling of current loop in PMSM vector control, the speed PIα controller is designed which fulfills specifications of the given phase margin and open loop gain crossover frequency. Numerical solution of phase crossover frequency is executed for each integral order by using Newton iteration method. The corresponding gain margin is achieved afterwards. Furthermore, the parameters of PIα controller are tuned by selecting the maximum gain margin of speed loop. The simulation results show that the speed step has small overshoot and fast response, which can verify the effectiveness of the proposed control strategy.

This work is related to the improvement of the output impedance of the Buck converter by means of introducing an additional power path that virtually increases the output capacitance during transients. It is well known that in VRM applications, with wide load steps, voltage overshoots and undershoots may lead to undesired performance of the load. To solve this problem, high-bandwidth high-switching frequency power converters can be applied to reduce the transient time or a big output capacitor can be applied to reduce the output impedance. The first solution can degrade the efficiency by increasing switching losses of the MOSFETS, and the second solution is penalizing the cost and size of the output filter. The Output Impedance Correction Circuit (OICC), as presented here, is used to inject or extract a current n−1 times larger than the output capacitor current, thus virtually increasing n times the value of the output capacitance during the transients. This feature allows the usage of a low frequency Buck converter with smaller capacitor but satisfying the dynamic requirements.

The purpose of this paper is to present a drastic improvement in the conventional ACC (average current mode control) of switching DC-DC power converters, consisting of the addition of an auxiliary controller in the control loop, besides the current and voltage regulators. The model-based auxiliary controller preserves the closed loop small-signal bandwidth (including loop gain crossover frequency and stability margins) of buck derived converters over a large range of variation of the power stage passive elements, the load and the line voltage. Moreover, this control scheme shows much better perturbation rejection properties (closed loop impedance and audiosusceptibility) than conventional ACC. From a control theory point of view, robust performance is achieved preserving stability. A 5 V-5 A, 50 kHz buck prototype operating from an input voltage ranging from 30 V to 15 V has been experimentally tested with different LC output filters, line and load conditions, including discontinuous conduction mode. Measurements of the small signal frequency response of the converter (loop gain, closed loop audiosusceptibility and output impedance) have been carried out, showing the improvement achieved by the proposed control scheme. The empirical large signal response of the converter, under load steps, is also shown in this paper in order to validate the concept.

In three-phase PWM rectifiers, variations in the dc voltage reference or load can affect the stability and dynamics of the system. Traditional proportional-integral (PI) control has weak adaptability, as its parameters cannot be adjusted in real-time according to the operating state, potentially leading to poor system stability and slow dynamic response. To address these challenges and the complexity of existing PI self-tuning methods, this article proposes a PI self-tuning method based on stability-and-dynamics-constrained fuzzy backpropagation neural network (SDC-FBPNN) for three-phase pulsewidth modulation (PWM) rectifiers. The effect of the right-half-plane zero is considered, and a series compensation method is adopted to convert the non-minimum phase system into a minimum phase system. On this basis, the phase margin and crossover frequency are used as constraints for stability and dynamic performance, respectively. Consequently, PI parameters can be adjusted by the SDC-FBPNN in real-time according to the operation conditions. Under abrupt variations in voltage reference and load, the proposed method significantly reduces voltage fluctuations and shortens the settling time while ensuring system stability. Finally, a 2 kW prototype was built to validate the feasibility and effectiveness of the proposed PI self-tuning method.