Parallel-connected Buck Converters Research Articles

Sigmoid function model of parallel-connected DC-DC converters and analysis of their dynamic characteristics.

Because of the manufacturing variations of circuit elements and the effects of parasitic parameters, the switching elements of parallel-connected modules cannot be synchronously switched. Consequently, the characteristics of these converters, which are multi-mode, high-order systems, cannot be comprehensively described using reduced-order models based on the symmetry of circuit topology and parameters. It is also difficult to fully describe the characteristics of these systems using discrete mapping models and averaged models. Therefore, we developed a smooth, sigmoid function-based continuous-time model for systems comprising parallel-connected buck converters with average current-sharing control. The proposed model provides a unified description of the continuous and discontinuous conduction modes of the system. The criteria for ensuring the stability of the system were derived based on the Floquet theory. The nonlinear dynamic behavior of the system and the effects of the key parameters on the stability of the system were investigated. The results showed that the system may undergo period doubling bifurcation, border collision bifurcation, and Neimark-Sacker bifurcation as the system parameters varied. The theoretical analysis and simulation results were verified experimentally. Our results provide a basis for the configuration of controller parameters.

A Practical Digital Implementation of Completely Decentralized Ripple Minimization in Parallel-Connected DC–DC Converters

Parallel dc–dc converters are commonplace in applications such as computing power supplies, point of load systems, and dc microgrids. The presence of multiple converters in such systems allows for switch interleaving and ripple cancellation. One class of existing ripple reduction methods depends on central controllers that require communication among the converters. These approaches compromise system scalability and reliability. When it comes to decentralized methods, the state-of-the-art is limited to uniform conditions and cannot handle mismatches in physical parameters and operating points among converters. In this article, we address these shortcomings with fully decentralized controller that robustly drives the pulsewidth modulation carriers of a collection of heterogeneous converters to the phase-shifts that give minimized ripple. More precisely, the proposed controller eliminates the dominant fundamental switching harmonic from the output current and voltage using only local voltage feedback. After outlining a comprehensive analytical model, we experimentally validate our approach on a system of five parallel-connected buck converters under a variety of parametric and operational mismatches. Measurements show ripple reduction of more than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$4\times$</tex-math></inline-formula> in the output voltage and the fundamental switching frequency harmonic is attenuated by more than 30 dB compared to conventional symmetric interleaving.

Decentralized Interleaving of Parallel-connected Buck Converters

We present a decentralized control strategy that yields switch interleaving for parallel-connected dc–dc buck converters. Compared to state-of-the-art methods that are distributed at best, the proposed architecture requires no communication, and hence, presents a variety of advantages with regard to reliability, modularity, and cost. The method is based on the digital implementation of the dynamics of a Lienard-type oscillator circuit as the controller for the converters. Each controller only requires the locally measured output current to synthesize the pulsewidth modulation (PWM) carrier waveform. The intrinsic electrical coupling between converters drives the nonlinear-oscillator-based controllers to converge to an interleaved state with uniform phase spacing across PWM carriers, independent of the number of converters, the load, and initial conditions. We provide analytical guarantees for existence and stability of the interleaved state as well as extensive hardware results for a system of five 120 W 48 V-to-12 V dc–dc buck converters that demonstrate convergence to the interleaved state in the face of a variety of large-signal disturbances.

Parallel‐connected buck converters controlled by capacitor current feedback based PT with current‐sharing ability

Parallel-connected power converters have been widely used in the application scenarios required high power to deal with the high load current. In this study, the pulse train (PT) controller with the advantages of fast response speed and simple structure is newly applied to the parallel-connected buck converters with current-sharing ability. However, as the converters operated in continuous conduction mode, undesirable low-frequency voltage oscillation will be brought about by the PT control method. To resolve this problem, a novel capacitor-current-feedback-based PT (CCF-PT) control method is proposed to eliminate the low-frequency oscillation of the parallel-connected buck converters. The approximate discrete time model of the CCF-PT-controlled buck converters is established, and the bifurcation diagrams of output voltage and inductor current changing along with the current scale factor, the capacitor-current-feedback coefficient and the load resistance are drawn to show the operation stability of the converters. Simulation and experimental results are obtained, which indicate that the proposed parallel-connected buck converters with CCF-PT control have excellent performance such as fast response, wide load range and effective suppression of low-frequency oscillation.

Distributed Coordination of Multiple PMSGs in an Islanded DC Microgrid for Load Sharing

For wind-powered dc microgrids working auto-nomously, the available wind generation may be greater than the load demand, and hence, may cause supply–demand imbalance. To address this issue, this paper presents a new distributed load sharing control scheme, which coordinates the operation of multiple permanent magnet synchronous generators (PMSGs) in a dc microgrid. This scheme has a two-layer structure. Based on the distributed model predictive control approach, the upper layer controllers cooperatively optimize the operation of parallel-connected buck converters, which serve as the interfaces between PMSGs and microgrid bus. The optimized power references are sent to the lower layer wind generator controllers for execution. This way, the optimal load sharing among multiple PMSGs is achieved. The salient feature of the proposed scheme is that it optimizes the control actions in a distributed manner, which ensures that multiple PMSGs cooperate, rather than compete, with each other in achieving system-wide objectives. Simulation results verify the effectiveness of the proposed scheme.

Interleaved Boundary Conduction Mode (BCM) Buck Power Factor Correction (PFC) Converter

An interleaved boundary conduction mode power-factor-correction buck converter that maintains high efficiency across entire load and line range is proposed. The adaptive master-slave interleaving method maintains stable 180° out-of-phase operation during any transient. By interleaving two parallel-connected buck converters, the input current ripple is halved while the ripple frequency is doubled, which leads to a smaller differential mode line filter. The line current harmonic distortion is analyzed to examine the allowable output voltage range while meeting harmonic regulations. The operation and performance of the proposed circuit is verified on a 300 W, universal line experimental prototype with 80 V output. The measured efficiencies remain above 96% down to 20% of full load across the entire universal line range. Even at 10% of full-load condition, the efficiency remains above 94%. The input current harmonics also meet the IEC61000-3-2 (class D) standard.

Active current ripple cancellation in parallel connected buck converter modules

Parallel connection of converters has become a popular method of improving efficiency. This study first presents a design technique for a buck converter with two parallelly connected power modules (PMs), where one PM is designed with large current ripple for high efficiency. This study then demonstrates an active current ripple cancellation technique, where the current waveform of the second PM is shaped to be the exact opposite to that in the first PM, to reduce the current ripple as seen by the output capacitor. The factors that affect the overall efficiency in a parallel connected converter and the calculation of the parameters that determine the effectiveness of the ripple cancellation are reported. A prototype parallel converter is designed based on the proposed converter design technique and the current ripple seen by the output capacitor is successfully reduced by 66% with the proposed ripple cancellation technique, under different line and load conditions.

Quantification of complexity of power electronics based systems

Power electronics are becoming increasingly important in modern ship board power systems. They are used for development and implementation of controls and to interface different electrical modules such as loads, batteries and generators that produce, store or consume energy. Power electronic systems are suitable for control because they provide fast operation in the range of microseconds. However, fast control operations can also add complexity to the system. Unintended complex dynamics can arise if the system (inertia) is unable to adapt to the control actions. This article studies the complexity and emergent phenomena that may develop in a ship board power system because of the power electronic components, coupling between these components, as well as feedback mechanisms such as control loops and those of human and environmental origin. A statistical complexity measure, referred to as structural complexity, is used to quantify the degree of complexity that arises during the system evolution. This metric of complexity is computed using permutation entropy and ordinal patterns. A modified procedure for structural complexity called multivariable structural complexity is developed to compute the system wide complexity. This multivariable structural complexity is also used to assist the modelling process and the decision of the best model candidate that captures observed aspects of complexity from the system data history. Various case studies on complexity quantification are conducted on simulated data from a noise coupled buck converter, two parallel connected buck converters and an electric ship board power system.

Calculation of steady-state solution of parallel-connected buck converters with active current sharing and its parameter sensitivity

In this paper, the effect of active current-sharing control on the steady-state operation of parallel-connected buck converters is investigated. The system under study consists of N voltage-mode-controlled buck converters connected in parallel. Three kinds of active current-sharing schemes are considered, namely, master–slave scheme with automatic master, master–slave scheme with dedicated master, and democratic scheme. Using the principle of charge balance, the mechanism of the operating point drift arising from active current sharing is examined. A general formulation of the steady-state solution under active current sharing is derived. Moreover, detailed parameter sensitivity analysis is performed to evaluate the effect of parameters' variation on the operating point. The results from sensitivity analysis can be used to categorize parameters for facilitating practical design. Computer simulations are presented to verify the analytical results. Copyright © 2010 John Wiley & Sons, Ltd.

Study of Intermittency in Parallel-Connection Buck Converters

Intermittency is commonly observed by practicing power supply engineers in their design workbenches. It shows to be a symmetrical period-doubling bifurcation in time domain with fixed long intermittent period. Sometimes it is called “breathing”. The article studies on the intermittency in the parallel-connected buck converter under masterslave operation, and opens out the reason of the intermittency, then gives a scheme called phase-shifting to realize or control the intermittency. The experimental results provide useful information for the design of the system.

Unified Equivalent Modeling for Stability Analysis of Parallel-Connected DC/DC Converters

A unified equivalent modeling approach is proposed for enabling simple and effective analysis of stability of parallel-connected buck converters. Two kinds of active current-sharing schemes are considered, i.e., the master-slave scheme and democratic scheme. By applying elementary transformation to the Jacobian of the averaged model, two decoupled subsystems (which determine common-mode and differential-mode stabilities, respectively) have been found, irrespective of the specific type of active current-sharing scheme. From the decoupled subsystems, the parameters of interest that affect different stability issues are clearly identified. Results from this work greatly simplify the stability analysis and design of parallel-connected dc-dc converters.

Comparative Study of Bifurcation in Single and Parallel-Connected Buck Converters Under Current-Mode Control: Disappearance of Period-Doubling

This paper studies a single buck converter and a system of parallel-connected dc/dc buck converters under current-mode control. The effects of variations of the reference current are studied. It has been observed that the system exhibits low-frequency bifurcation behaviour while period-doubling at switching frequency is suppressed. Extensive simulations are used to capture the behaviour. Time domain waveforms, stroboscopic maps, and trajectories are shown. The paper reveals the drastic alteration of bifurcation behaviour of dc/dc converters due to subtle coupling.

Bifurcation behavior in parallel-connected buck converters

This paper describes the bifurcation phenomena of a system of parallel-connected dc/dc buck converters. The results provide useful information for the design of stable current sharing in a master-slave configuration. Computer simulations are performed to capture the effects of variation of some chosen parameters on the qualitative behavior of the system. These are summarized in a series of bifurcation diagrams. In particular, it is found that while variation of the voltage feedback gains leads to standard period-doubling bifurcation, variation of the current sharing ratio leads to border collision bifurcation. Analysis is presented to establish the possibility of the bifurcation phenomena and to locate the current sharing ratio at which border collision occurs.