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

In this article, we propose an extendable quadratic bidirectional dc–dc converter that has an improved voltage transfer ratio (VTR) with the capability of redundancy and modularity for electric vehicle applications. As n modules are embedded, its VTR becomes n times higher for both directions of currents. Furthermore, the common electrical ground between the input and output is preserved. This is a simple structure with the lowest rating of semiconductors in the family of quadratic bidirectional converters leading to ease of control ability. The proposed converter performance is evaluated in both power flow direction using the deadbeat controller, which is a smooth, accurate, and fast response. Finally, the process of charging/discharging of a lithium-ion battery is controlled through the proposed converter. 500-W experimental results are provided in both power flow directions in a closed-loop system in the presence of the proposed controller. The obtained results verify the applicability of this structure.

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An Extendable Quadratic Bidirectional DC–DC Converter for V2G and G2V Applications

In this paper, a family of step-up resonant switched-capacitor converters (RSCs) with a continuously adjustable conversion ratio is proposed based on the Cuk dual-resonance core (DRC) featuring two resonant paths. By adopting the on-time fixed frequency modulation, the two resonant paths can conduct simultaneously and satisfy the charge balance of the flying capacitor during the overlapping period. As a result, the conversion ratio—upper bounded by two determined by the circuit configuration—can vary continuously and efficiently even at the light load condition (1/10 of the rated power). A comprehensive analysis—the operation principle, voltage-gain curve, and voltage/current stresses—about the DRC is given. In addition, the Cuk DRC was extended to the series–parallel switched-capacitor converter (SCC), Fibonacci SCC, and voltage doubler to form a family of step-up RSCs, whose voltage-gain curves of different topologies are derived. Moreover, a characteristic comparison between the Cuk DRC and the traditional “direct” and “indirect” resonant core is given. Then, a 48 to 62–94.6 V resonant core with peak efficiency 97.8% and maximum power 120 W under the open loop condition was built. The analyses were verified by both experimental and simulation results.

A Family of Step-Up Resonant Switched-Capacitor Converter With a Continuously Adjustable Conversion Ratio

This paper presents a family of multiphase interleaved high step-up dc-dc converters using the diode–capacitor technique, which is an extension of the previously reported current-fed Cockcroft–Walton (CW) multiplier. The multiphase configuration has the advantages of low input current ripple, high current-handling capability, and high step-up voltage gain. Also, the switches and diodes have low voltage stresses. Thus, low-voltage-rating semiconductor devices are allowed reducing the conduction loss. Moreover, automatic phase current balancing can be achieved due to the charge balance of the series capacitors. A three-phase high step-up converter in the family is analyzed and evaluated in detail. The simulation and experimental results are provided to verify the theoretical analysis.

A Family of Interleaved High Step-Up Converters With Diode–Capacitor Technique

This paper proposes a novel resonance suppression scheme for a single-phase grid inverter with the $LLCL$ filters. In the structure of the $LLCL$ filters, a small inductor is added in series with the capacitor in the conventional $LCL$ filters to make zero impedance at the switching frequency, by which the grid-side inductor filter can be reduced remarkably. However, a resonance phenomenon is still a critical issue in this system. Therefore, an active damping method based on the proportional and resonant controllers is proposed to suppress the resonance. In addition, the stability is analyzed for the feasibility of the proposed algorithm. Finally, the simulation and experimental results have verified the control algorithm for a single-phase grid inverter.

Resonance Suppression Based on PR Control for Single-Phase Grid-Connected Inverters With $LLCL$ Filters

In this paper, a variable switching frequency space vector pulsewidth modulation control is proposed. It is used to achieve zero voltage switching (ZVS) for a three-phase grid-connected voltage source inverter with unity power factor. A wide range of ZVS can be realized without any additional sensors, auxiliary circuits, or current zero crossing detection circuits. The switching frequency can be easily calculated by a digital controller. The frequency variation range in a line cycle is only about 1.5 times at any specific load. An LCL filter is used to attenuate the high current ripples at the inverter side, and active damping is adopted to avoid the resonance and reduce the filter power loss. The switching loss can be significantly reduced by using silicon carbide mosfet s so that the conversion efficiency is high. The power density can also be improved due to the high switching frequency and the low inductance value of the filter. The operating principle, design considerations, and loss analyses are discussed in detail. A 3.5-kW simulation and experimental prototype interfacing a 350–400-V dc with a three-phase 110-V ac grid is developed to verify the performance of the proposed control strategy.

An SiC MOSFET Based Three-Phase ZVS Inverter Employing Variable Switching Frequency Space Vector PWM Control

In this paper, a multistage nonisolated high voltage conversion ratio (VCR) bidirectional dc–dc converter is proposed. The VCR is n times of the conventional buck/boost converter using only $n+ {\text{2}}$ switches. All the switches can achieve full-range zero voltage switching (ZVS) despite the power flow direction. The operating principle, ZVS condition, and design considerations of a four-stage topology are analyzed in detail. In addition, an improved dynamic switching frequency modulation is proposed to reduce the power loss. Analyses and comparisons between the conventional constant frequency and variable frequency control are made to show the effectiveness. A 500-W experimental prototype interfacing 20–30 V with 270 V is fabricated to verify the performance of the proposed converter and control strategy.

Cascaded High Voltage Conversion Ratio Bidirectional Nonisolated DC–DC Converter With Variable Switching Frequency

A variable switching frequency space vector modulation control method is proposed in this paper. It is used to achieve zero-voltage switching (ZVS) for two parallel 180° interleaved three-phase grid-connected voltage-source inverters at unity power factor. A wide range of ZVS can be achieved without any additional sensor, auxiliary circuit, or current zero-crossing detection circuit. The carrier wave frequency is easy to be calculated and updated with digital control. The current ripple is precisely predicted and controlled to minimize unnecessary circulation loss. The high current ripple at the inverter side can be considerably canceled by the parallel interleaved configuration. LCL filter is used to further attenuate the current ripple and improve the power quality at the grid side. The switching loss can be significantly reduced especially by using a wide bandgap device with low turn- off loss. The power density is greatly improved owing to the high switching frequency and low component parameters of the filter. A 4.5-kW simulation and experimental prototype interfacing 400 V dc with a three-phase 110-V ac grid is developed to verify the performance of the proposed control strategy.

A Variable Switching Frequency Space Vector Modulation Technique for Zero-Voltage Switching in Two Parallel Interleaved Three-Phase Inverters

In this article, a variable switching frequency full zero-voltage switching (ZVS) range bidirectional three-phase voltage-source inverter is proposed. Discontinuous pulsewidth modulation is used so that at any time only two phase legs need to achieve ZVS. The boundary switching frequency can be calculated in a digital controller based on current ripple prediction instead of current zero-crossing detection. No additional high-frequency sensor or auxiliary circuit is needed. The frequency variation range is narrow in a line cycle at any load condition. An LCL filter is used to attenuate the inverter-side high ripple current and improve the grid current quality. Owing to the high switching frequency and the small filter parameters, the power density can be greatly increased. Both the device and the filter losses are reduced, and the efficiency is high. A 3.5 kW simulation and experimental prototype with silicon carbide (SiC) device interfacing 400 V dc with three-phase 110 V ac grid is developed to verify the effectiveness of the proposed control strategy.

Current Ripple Prediction and DPWM-Based Variable Switching Frequency Control for Full ZVS Range Three-Phase Inverter

In this paper, a unified control strategy using the current space vector modulation (CSVM) technique is proposed and applied to a bidirectional three-phase DC/AC converter. The operation of the converter changes with the direction of the power flow. In the charging mode, it works as a buck type rectifier; and during the discharging mode, it operates as a boost type inverter, which makes it suitable as an interface between high voltage AC grids and low voltage energy storage devices. This topology has the following advantages: high conversion efficiency, high power factor at the grid side, tight control of the charging current and fast transition between the charging and discharging modes. The operating principle of the mode analysis, the gate signal generation, the general control strategy and the transition from a constant current (CC) to a constant voltage (CV) in the charging mode are discussed. The proposed control strategy has been validated by simulations and experimental results obtained with a 1kW laboratory prototype using supercapacitors as an energy storage device.

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Jianliang Chen

Papers

An SiC MOSFET Based Three-Phase ZVS Inverter Employing Variable Switching Frequency Space Vector PWM Control

Cascaded High Voltage Conversion Ratio Bidirectional Nonisolated DC–DC Converter With Variable Switching Frequency

A Variable Switching Frequency Space Vector Modulation Technique for Zero-Voltage Switching in Two Parallel Interleaved Three-Phase Inverters

Current Ripple Prediction and DPWM-Based Variable Switching Frequency Control for Full ZVS Range Three-Phase Inverter

A Bidirectional Single-Stage DC/AC Converter for Grid Connected Energy Storage Systems