This paper presents the design of new high-frequency transformer isolated bidirectional dc-dc converter modules connected in input-series-output-parallel (ISOP) for 20-kVA-solid-state transformer. The ISOP modular structure enables the use of low-voltage MOSFETs, featuring low on-state resistance and resulted conduction losses, to address medium-voltage input. A phase-shift dual-half-bridge (DHB) converter is employed to achieve high-frequency galvanic isolation, bidirectional power flow, and zero voltage switching (ZVS) of all switching devices, which leads to low switching losses even with high-frequency operation. Furthermore, an adaptive inductor is proposed as the main energy transfer element of a phase-shift DHB converter so that the circulating energy can be optimized to maintain ZVS at light load and minimize the conduction losses at heavy load as well. As a result, high efficiency over wide load range and high power density can be achieved. In addition, current stress of switching devices can be reduced. A planar transformer adopting printed-circuit-board windings arranged in an interleaved structure is designed to obtain low core and winding loss, solid isolation, and identical parameters in multiple modules. Moreover, the modular structure along with a distributed control provides plug-and-play capability and possible high-level fault tolerance. The experimental results on 1 kW DHB converter modules switching at 50 kHz are presented to validate the theoretical analysis.

High-Frequency Transformer Isolated Bidirectional DC–DC Converter Modules With High Efficiency Over Wide Load Range for 20 kVA Solid-State Transformer

High-efficiency and compact single-phase inverters are desirable in many applications such as solar energy harvesting and electric vehicle chargers. This paper presents a 2-kW, 60-Hz, 450-V $ _{\text{DC}}$ -to-240-V $_{\text{AC}}$ power inverter, designed and tested subject to the specifications of the Google/IEEE Little Box Challenge. The inverter features a seven-level flying capacitor multilevel converter, with low-voltage GaN switches operating at 120 kHz. The inverter also includes an active buffer for twice-line-frequency power pulsation decoupling, which reduces the required capacitance by a factor of 8 compared to conventional passive decoupling capacitors, while maintaining an efficiency above 99%. The inverter prototype is a self-contained box that achieves a high power density of 216 W/in $^3$ and a peak overall efficiency of 97.6%, while meeting the constraints including input current ripple, load transient, thermal, and FCC Class B EMC specifications.

A 2-kW Single-Phase Seven-Level Flying Capacitor Multilevel Inverter With an Active Energy Buffer

Two active capacitor voltage balancing schemes are proposed for single-phase (H-bridge) flying-capacitor multilevel converters. They are based on the circuit equations of flying-capacitor converters. Consequently, they can be implemented using straightforward control rules. In particular, the first technique is based on an algorithm which follows the standard multilevel modulation. Then, it utilizes a redundant state selection table for capacitor voltage balancing. In the second method, multiple duty cycles are defined and modulated in direct response to the capacitor voltages. The most important advantage of these two proposed methods is that they can be utilized to converters with any desired number of levels in their output voltage. Moreover, the analysis and implementation of both methods are straightforward. Through simulation and experimental implementation, these methods are shown to be effective on capacitor voltage regulation in flying-capacitor multilevel converters.

Active Capacitor Voltage Balancing in Single-Phase Flying-Capacitor Multilevel Power Converters

The aim of this paper is to present a new structure for switched-capacitor multilevel inverters (SCMLIs) which can generate a great number of voltage levels with optimum number of components for both symmetric and asymmetric values of dc-voltage sources. The proposed topology consists of a new switched-capacitor dc/dc converter (SCC) that has boost ability and can charge capacitors as self-balancing by using the proposed binary asymmetrical algorithm and series–parallel conversion of power supply. The proposed SCC unit is used in new configuration as a submultilevel inverter (SMLI) and then, these proposed SMLIs are cascaded together and create a new cascaded multilevel inverter (MLI) topology that is able to increase the number of output voltage levels remarkably without using any full H-bridge cell and also can pass the reverse current for inductive loads. In this case, two half-bridge modules besides two additional switches are employed in each of SMLI units instead of using a full H-bridge cell that contribute to reduce the number of involved components in the current path, value of blocked voltage, the variety of isolated dc-voltage sources, and as a result, the overall cost by less number of switches in comparison with other presented topologies. The validity of the proposed SCMLI has been carried out by several simulation and experimental results.

A New Cascaded Switched-Capacitor Multilevel Inverter Based on Improved Series–Parallel Conversion With Less Number of Components

Flying-capacitor converters (FCCs), like most multilevel converter topologies, require a balancing mechanism of the capacitor voltages. FCCs feature natural voltage balancing when a special modulation technique is used. The classic methods, such as phase-shifted pulse width modulation (PS-PWM), result in very slow balancing for some duty-ratio ranges. Previous work has shown that for a single-leg five-level FCC, one time constant is infinite for a zero desired output voltage. In this paper, a modified PS-PWM scheme for a single-leg five-level FCC is presented, which results in faster balancing over the total duty-ratio range. The modified PS-PWM scheme is studied, resulting in an averaged voltage-balancing model. This model is verified using simulations and experiments. The modified PS-PWM scheme solves the slow-balancing problems of the normal PS-PWM method for odd-level FCCs, while maintaining the passive control property, and it provides a self-precharge capability.

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Improved Natural Balancing With Modified Phase-Shifted PWM for Single-Leg Five-Level Flying-Capacitor Converters

Multilevel power electronic converters are the converter of choice in medium-voltage applications due to their reduced switch voltage stress, better harmonic performance, and lower switching losses. Although it has received little attention, the flying-capacitor multilevel converter has a distinct advantage in terms of its ease of capacitor voltage balancing. A number of techniques have been presented in the literature for capacitor voltage balancing, some relying on “self-balancing” properties. However, self balancing cannot guarantee balancing of capacitor voltages in practical applications. Other researchers present closed-loop control schemes which force voltage balancing of capacitors. In this paper, a new closed loop control scheme is proposed which regulates the capacitor voltages for a multilevel flying capacitor converter. The proposed scheme is based on the converter equations and involves implementing simple rules. In particular, multiple duty cycles are defined and modulated in direct response to the capacitor voltages. Through simulation, the method is shown to work on four, eight and nine-level flying capacitor inverters.

A generalized capacitor voltage balancing scheme for flying capacitor multilevel converters

The deployment of a single-phase, multilevel flying capacitor converter as a 1.5-kW active rectifier is discussed in this paper. The challenges associated with this approach include providing unity power factor, regulating the dc output voltage, and maintaining balanced voltages across the flying capacitors. In such applications, the input current is usually controlled via a reference-frame-based control approach. Here, a different approach, which is based on the hysteresis current-mode control scheme, is applied. The simulation and experimental results for both reference-frame-based and hysteresis-based controllers are presented and compared. The results show that the hysteresis-based method exhibits a tighter regulation of the input current and offers easier hardware implementation.

Hysteresis-Based Control of a Single-Phase Multilevel Flying Capacitor Active Rectifier

This paper describes a research conducted on a single-phase flying capacitor active rectifier. An online process that reduces the total number of switching events, which leads to a reduction of switching losses in the converter, is introduced. This method is based on redundant state selection, which is used to regulate the voltage of flying capacitors. The proposed approach is general and can be applied to flying capacitor converters with any number of levels. Furthermore, in order to control the inrush current at the start of operation and avoid extra voltage and current stress for active switches and capacitors, a start-up procedure that precharges the flying capacitors is proposed. With this precharge procedure, no additional hardware is needed. Simulation and laboratory results demonstrate these new concepts.

Start-up Procedure and Switching Loss Reduction for a Single-Phase Flying Capacitor Active Rectifier

This paper addresses the stability issues and some control solutions in solid-state transformers (SST). An SST is built using three cascaded stages, including an ac-dc rectifier, a dual active bridge (DAB) converter, and a dc-ac inverter. Multistage cascaded converters are highly prone to instabilities due to the interaction between stages. Hence, the control system must be designed in a way that solves this detrimental interaction effect. Additionally, the stability issue can become even more complicated when a distributed energy storage device (DESD) and a distributed renewable energy resource (DRER) are added to the SST. In this paper, the stability of the SST is studied by applying the Middlebrook's criterion. To do so, the converters' small-signal models will be derived, and the frequency domain analyses is carried out to predict the time domain behavior of the SST. An average model of SST will be simulated in PSCAD to validate the stability analyses predictions.

Solid-state transformer stability and control considerations

In this paper, a new method to control multilevel converters is proposed. The considered multilevel converter consists of a three-phase three-level diode-clamped converter (main converter). Two H-bridge cells are then connected in series with each output phase of the main converter. The operation of the main converter and one of the cascading Hbridge cells is based on the staircase switching method. The firing angles of these converters are selected in a way that the dc voltage required for the last H-bridge cell is minimized. The switching pattern of the second H-bridge cell is based on the pulse-width modulation method. This last cell generates the remaining parts of the desired sinusoidal output voltage. The combination of these converters and their switching methods result in an output waveform with low harmonics. Here, a fifteen-level converter is designed based on this approach. Simulation results and laboratory measurements verify the effectiveness of the proposed topology and modulation method.

Mostafa Khazraei

Papers

A generalized capacitor voltage balancing scheme for flying capacitor multilevel converters

Hysteresis-Based Control of a Single-Phase Multilevel Flying Capacitor Active Rectifier

Start-up Procedure and Switching Loss Reduction for a Single-Phase Flying Capacitor Active Rectifier

Solid-state transformer stability and control considerations

A hybrid multilevel inverter with both staircase and PWM switching schemes