Showing papers by "Dragan Maksimovic published in 2023"

Control Design of Series-Connected PV-Powered Grid-Forming Converters via Singular Perturbation

Abstract: Modular architectures that consist of several series-connected dc–ac converters have been a focal point of recent innovations in transformerless medium-voltage applications. In this article, we consider an architecture consisting of dc–ac modules containing a quadruple active bridge dc–dc converter, which generates three floating dc links that feed grid-side dc–ac inverters. Practical implementation of such a converter module in photovoltaic systems requires a variety of controllers that collectively achieve maximum power point tracking, dc-link regulation, and ac-side power control. Design of such multiloop systems is generally quite challenging due to the potential for destabilizing interactions among loops. Here, we propose a design approach where singular perturbation theory is used to decompose the timescales at which each control loop operates and provides a systematic framework for parametric selection. Our approach also ensures system stability of multiple modules with identical controls connected in series across a grid. This article concludes with experimental results of three 1000-W series-connected converter modules across a stiff grid.

Dynamic Modeling of Peak Current Mode Controlled Two-phase PWM Converters with Coupled Inductors

Abstract: This paper presents simple and accurate large-signal and small-signal averaged models for basic two-phase coupled-inductor PWM converters operating in continuous conduction mode with peak current-mode (PCM) control. A large-signal model is developed, which captures the effects of inductor-current slopes using equivalent, duty-cycle-dependent inductances. A small-signal averaged model is then derived, which includes sampled-data effects. The small-signal model is used to explain differences compared to well-known PCM control models for single-phase or uncoupled multi-phase converters. It is shown how subharmonic instability regions depend on the coupling coefficient and the duty cycle. A continuous-time transfer function is derived for the control-to-inductor current response, with the high-frequency resonant pole $(F_{res})$ that occurs in the range $[0.5F_{s},\ F_{s}]$ and varies with the coupling coefficient. A new closed-form expression for $F_{res}$ is then derived, which allows simplification to a design-oriented $3^{rd}$ order transfer function. The proposed small-signal model can also predict audio-susceptibility, input, and output impedances accurately. Finally, the model is validated using simulations and experiments on a two-phase buck converter prototype with inversely coupled inductors.

Estimator-Based Step-Load Transient Improvements in a Digitally Controlled Synchronous Buck Converter

Abstract: The paper proposes a digital estimator based control method in point-of-load (PoL) DC-DC converters to achieve fast response times and low voltage deviation under frequent load transients. Using the proposed method, a fast digital controller can be designed even in the presence of significant delays in the control loop. The method is particularly effective at high switching frequencies when the delays can be comparable to or even greater than the switching period. Moreover, an estimator based response can significantly reduce the output voltage deviation for load transients, which can further reduce the size of the output filter capacitor. The approach is experimentally verified on a GaN-based 1 MHz synchronous buck converter prototype. Experimental results are shown for load transients from 1 A to 7 A and back at $\boldsymbol{V}_{o}$ = 30 V output voltage and $\boldsymbol{V}_{g}$ = 80 V input voltage, with output voltage deviation less than 1.5 V and response times in the range of 5 - 6 $\mu\mathrm{s}$.

Loss estimation and design of dc-dc converters using physics- and data-based component models

Abstract: Design optimization of dc-dc converters at the component level can be assisted by machine learning (ML) techniques, where component models are trained using large amounts of component data. This approach enables systematic and effective optimization in terms of efficiency, cost, or size over a large design space. However, more accurate converter loss models require device parameters that are not readily available from component datasheets. This paper aims to combine physics-and data-based component models to address this challenge by generating additional design-oriented parameters that contribute to converter efficiency and can be used by the ML models in the optimization tool. Novel techniques are presented to include loss models due to transistor on-resistance and charge-equivalent capacitance, as well as inductor ac winding and core loss. Each approach is validated on out-of-sample components. The improved loss model is validated on a 48-to-12V, 5A synchronous buck converter prototype and is incorporated into the ML-based optimization tool.

Modular Series-Stacked Bidirectional AC/DC Architecture for 3-Phase Grid-Tied Applications

Abstract: As the demands of systems such as power-to-hydrogen (P2H) and extreme fast charging (XFC) increase, there is a need for highly customizable and scalable grid-tied power electronics. This paper presents a bidirectional converter architecture comprised of stackable three-phase ac/dc converter modules, scalable to high-power and high-current applications. Multiple converter modules containing a converter power stage and controls can be stacked to obtain a medium-voltage ac (MVAC) tied system without the need for a line frequency transformer. The modular system architecture is made possible by a quadruple active bridge (QAB) dc/dc converter that provides isolation between each of the three ac-side phases within each module and the dc load. The system also removes the need for bulk energy storage by taking advantage of constant balanced three-phase power flow. Decentralized module-level controllers are also implemented to allow for system modularity and scalability. The proposed architecture is validated by simulations of a P2H system consisting of 18 modules and a scaled proof-of-concept hardware prototype consisting of two modules.

A Highly Integrated Hybrid DC–DC Converter With nH-Scale IPD Inductors

Abstract: This article presents a new integrated step-down dc–dc hybrid converter that uses only nano-Henry scale inductors at 2–5-MHz switching frequency. Since it is derived from a Dickson-star switched capacitor (SC) converter, the proposed converter inherits the benefit of low voltage stress on switches while enjoying an efficient fine regulation by phase shift, similar to a dual active bridge (DAB) converter. The converter is optimized, designed, and fabricated in 1.7 $\times $ 1.9 mm area of a 130-nm bipolar-CMOS-DMOS (BCD) process. The active die is flip-chipped on a 6.5 $\times $ 6.5 mm package substrate together with power capacitors and two 10-nH integrated passive device (IPD) inductors for demonstration, illustrating the feasibility of passive components’ integration, resulting in a peak efficiency of 91.2% and a peak power density of 1.36 W/mm3 from 9.6–12-V input to 2.15–3.3-V output. Another demonstration is constructed on the same package substrate but with discrete air-core inductors. It achieves a peak efficiency of 92.4% and a peak power density of 0.62 W/mm3, while delivering a max power of 7.5 W. To achieve the performance, a detailed loss analysis and a unique optimization methodology for the converter, together with the design of key sub-blocks, including gate drivers (GDs), phase shift modulator (PSM), and ramp generator (RG), are provided in this article.

400V-to-48V Transformer-Isolated Stacked Active Bridge Converter with Integrated Magnetics

Abstract: This paper presents a transformer-isolated high step-down dc-dc converter based on a stacked active bridge (SAB) configuration. The isolated SAB (iSAB) converter consists of series-stacked inverter modules and parallel-connected rectifier modules. To achieve galvanic isolation, transformers are inserted between the inverter and rectifier bridges. The nominal step-down conversion ratio is determined by the number of inverter modules and the turns ratio of the transformer. In order to reduce the footprint of the magnetic components, the transformers are coupled on a single core, and the series inductances are realized as controllable leakage inductances within the same magnetic structure using a novel custom core and planar winding arrangement, a solution unique to the iSAB configuration. The approach is verified by experimental results on a 400-to-48 V, 3kW, 400kHz iSAB prototype using low-voltage GaN devices and having 96.7% peak efficiency.

Steady-State Indeterminacy in Lossless Switched-Mode Power Converters

Abstract: In this article, it is shown for the first time that lossless switched-mode power converters may not possess a unique steady-state solution. Rather, they can exhibit an inherent indeterminacy in their steady-state behavior, and a unique solution is only found when losses are included in the analysis. Even more interestingly, it is shown that lossless converters of odd order never possess a unique steady-state solution. Indeterminate converters can be intrinsically sensitive to parasitic resistances and other nonideal effects, a phenomenon that practically manifests itself in the form of undesirable voltages or circulating currents possessing, in general, both dc and ac components. This article first sets a general mathematical framework for a systematic assessment of steady-state indeterminacy. This is subsequently linked to the geometry of the state vector periodic motion in the converter's n-dimensional state space. On the basis of the developed theory, odd-order converters are shown to always be affected by the indeterminacy issue. Besides inherent theoretical importance in the power electronics field, the results of this article provide a deeper justification for the observed behavior of several converter topologies of significant practical relevance, the most notable being multiphase and multilevel dc–dc converters, examples of which are discussed as case studies.