Recently, more occasions with high input voltage and/or high output voltage applications are emerging. However, the existing switching devices are limited by the voltage stress, e.g., the available maximum blocking voltage of insulated-gate bipolar transistors is 6.5 kV. Therefore, several methods are provided to solve this issue, including series connection of switches, multilevel converters, and modular series–parallel structures, where the modular series–parallel systems are the most popular choice due to the advantages of low cost, simple design process, high modularity, reliability, and redundancy. The main objective of series–parallel systems is to ensure the sharing of input/output voltage/current. Based on this, a comprehensive review on the available voltage/current sharing methods for series–parallel systems, which include input-series output-parallel, input-parallel output-series, and input-series output-series structures is provided in this article. Moreover, the relationships between the input voltage/current sharing and output voltage/current sharing of series–parallel systems are explained.

A Review of Voltage/Current Sharing Techniques for Series–Parallel-Connected Modular Power Conversion Systems

This paper has the objective of presenting and discussing topologies and techniques that allow step-up and step-down conversion ratios in DC systems offering a DC front and back ends. This leads to applications of DC-DC converters. In this paper, different characteristics are discussed such as: (non)-isolation in converters, the combination of isolated and non-isolated converters and the impact on the step-up/down conversion ratio of these converters, coupled-inductor techniques, voltage/current multiplier/divider structures, among others. This brings to the reader a broad overview regarding the possible DC-DC converters that enable different voltage capabilities. The key outcome is to present a brief review of the techniques applied to converters to enable future full DC grid operation.

An Overview of Power Conversion Techniques and Topologies for DC-DC Voltage Regulation

This paper analyzes the effects of parameter mismatches in the balance mechanism of modular DC-DC Flyback converters operating in discontinuous conduction mode. The natural current and voltage distributions among modules are evaluated when mismatches on duty cycles, transformer magnetizing inductances and transform turns ratios are present. From these results, the critical values of inductances and duty cycles that assure the discontinuous operation are equated. The smallsignal equivalent circuit for Input-Parallel-Output-Series and Input-Parallel-Output-Parallel connections are found, followed by a simple control strategy. The theoretical analysis is verified by experimental results obtained with a prototype composed of three 200 W Flyback modules, with a rated power of 600 W and maximum efficiency of 95.5%. Results corroborate the proposed equations for the steady state balance and dynamic behavior of both connections, highlighting the modular characteristic of the converter.

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Analysis of modular DCM Flyback converters in input parallel connections with parametric mismatches

This work proposes an ac–ac converter based on interleaved transformers associated with the dual active bridge (DAB) converter. The resulting symmetrical full-bridge converter (SFBC) can achieve high efficiency over a wide range of the output power and bidirectional power flow, as well as high power density owing to the possible integration of magnetics. The voltage waveforms on the high-frequency transformer (HFT) present five levels, resulting in filter elements with reduced dimensions. A detailed mathematical analysis of the SFBC comprising the operating stages, modulation technique, and soft-switching range is derived. An experimental prototype rated at 1 kW is implemented to validate the theoretical assumptions, thus demonstrating that the SFBC is capable of providing input power factor correction (PFC) and a load voltage waveform with low harmonic content. 

Single-Phase Isolated AC–AC Symmetrical Full-Bridge Converter

This paper proposes a new single‐phase direct step‐up ac–ac converter by modifying the p‐type impedance source. It provides a high boost factor as well as high efficiency, while only six parts are required to design it, involving just two bidirectional power switches. A safe commutation method has been applied to power switches to make the converter snubber‐free and high efficient. Input and output harmonic filters are no longer required since input and output currents variate continuously with small ripple and low total harmonic distortion (THD). The proposed topology only modulates the output voltage amplitude, not the phase and frequency, so the output frequency is identical to the input frequency and constant. Thus, it can be utilized in step‐up conversion applications, like inductive power transmission from low ac voltage sources. Input and output have the same ground, which is a good protective feature. In this paper, the operating principle of the converter is demonstrated. Experimental results have been represented to evaluate the performance of the converter. For this purpose, an experimental prototype has been fabricated. Results are investigated and compared with other previous step‐up ac–ac converters. Results confirm the theory, operating principle, and performance of the converter.

A single‐phase p‐type ac–ac converter with reduced components count and high boost factor

In this article, a hybrid boost switched-capacitor converter operating as an ac–ac solid-state autotransformer is presented. The topology integrates a basic inductive switching cell and a switched-capacitor ladder cell, performing a direct energy conversion with a high-gain output voltage. The theoretical analysis presents the main operational characteristics of the hybrid converter, including the voltage stresses across the devices, the equivalent average electric circuit model, and the calculation of current stresses considering the partial charging of the capacitors. A design methodology is described, focusing on the modulation technique and criteria for determining capacitors, semiconductor devices, inductor, and switching frequency. Validation was carried out based on simulation and experimental results obtained with a single-phase prototype with 55/220-V voltage conversion and 1-kVA rated power at 60 Hz. Additional tests verified the operation at different grid frequencies, loads (inductive and nonlinear), and with 14 different input–output connections.

AC–AC Hybrid Boost Switched-Capacitor Converter

DC-DC Modular converters have been proposed as a solution to reduce electrical stresses on semiconductors due to the capacity of sharing current or voltage among modules. This characteristic allows the increasing of the switching frequency, the use of low voltage/current semiconductors and the standardization and expansion of converters. This paper presents a steady-state analysis of an input-series-output-parallel (ISOP) modular Flyback converter operating in the discontinuous conduction mode (DCM). The analysis of the average input voltage and output current balance is presented, whose results depend on the magnetizing inductance and on the duty cycle of any number of modules. Once the equations are obtained, all the results are underpinned by a set of simulations undertaken for three modules. In order to evaluate the theoretical analysis, a prototype consisting of three Flyback modules in DCM operation, with a rated power of 600 W and conversion ratio from 600 V to 200 V, was built. The obtained waveforms show the independence of each module to the connection, leading to an equal division of the current and voltage stresses over the semiconductors. This characteristic, added to the modularity, high efficiency and high current gain, makes the converter an alternative to DC/DC systems where the output voltage must be lower than the input voltage.

Modular ISOP Flyback converter: Analysis of auto-balancing mechanism in steady state

This paper presents the steady state and resonant current analysis of a Forward-Flyback Voltage Multiplier operating in discontinuous conduction mode. The operation stages are approached, taking into account the resonant current. The static gain and the conditions for the operations modes are obtained. A sinusoidal approximation is proposed to represent the resonant current with respect to the number of stages, voltage multiplier capacitors and leakage inductance. The characteristics of voltage sharing among capacitors, that allows the use of small voltage components, and the high-gain with reduced turns of the transformer are verified. Bench tests were made for a 200 V/2000 V prototype, from one to five stages, highlighting the overall circuit effects for a 200 W output power. Obtained waveforms for different number of voltage multiplier stages validate the theoretical and simulated results, making this converter an alternative to isolated DC/DC systems where the output voltage must be higher than the input voltage.

DCM forward-flyback converter integrated with a 5-order cockcroft-walton voltage multiplier: A steady-state and resonant current analysis

A new approach for step-down dc-dc power converter design is proposed in this paper, which is based on the differential connection of two conventional topologies. The converters provide two output voltages, and, then, the difference between them is the output voltage applied to the load. This strategy allows that each converter to work in an adequate operation point regarding gain and duty cycle because a high step-down gain is provided by the difference of voltages between converters. The method can be applied in all converters, using either equal or different topologies. In this paper, an analysis for a differential converter is presented, and corroborated by simulation and experimental results, using a designed prototype of 400 V to 12 V and 100 W rated power.

Matheus Schramm Dall'Asta

Papers

AC–AC Hybrid Boost Switched-Capacitor Converter

Modular ISOP Flyback converter: Analysis of auto-balancing mechanism in steady state

DCM forward-flyback converter integrated with a 5-order cockcroft-walton voltage multiplier: A steady-state and resonant current analysis

Analysis of modular DCM Flyback converters in input parallel connections with parametric mismatches

400 V to 12 V Step-down DC-DC Power Converter Based on the Differential Concept