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Modular multilevel converter becomes one of the most promising high power converters thanks to their modular structure design, distributed dc capacitors, improved power quality, and no separate dc voltage sources required. However, capacitor voltage balancing remains one major challenge. This paper proposes a new voltage balancing control associated with an improved pulsewidth modulation (PWM) method. Specifically, a new mathematical model and control structure with multi-loops including a new balancing control loop between the upper and lower arms are presented. The capacitor voltages can be well balanced without adding unnecessary switching to any SMs and voltage sorting algorithm. Moreover, the control and modulation are more accurate and the ac power quality can be improved. Simulations and experimental results have verified the effectiveness of proposed methods.

A single-stage single-switch (S4) converter integrating a buck-type power factor correction (PFC) cell and a buck-type dc-dc output cell is presented in this paper. The most attractive feature of the proposed S4 converter is that, its input power factor (PF) always keeps in constant even when the input voltage and the load change. Besides, the proposed converter also preserves the advantages of those existent S4 converters, such as the low voltage stress across the intermediate capacitor, low current stress of the switch and high step-down output voltage. These advantages make it cost-efficient and highly suitable for the low power and non-isolated applications. In this paper, the operational modes and circuit design are given, and a prototype is also built to show the validity of the analysis.

Modular multilevel converters (MMC) are intensively researched and developed in high power, high/medium voltage applications. However, the interaction between switching action and fundamental-frequency pulsation in capacitor voltage of submodules will induce 2nd-order and other even-harmonic harmonics in circulating current. These harmonic currents will introduce extra power loss, increase current stress of power devices and even jeopardize safe operation of MMC. Traditional methods are usually designed under single harmonic frequency, resulting in increased complexity for an integrated solution. This paper presents a repetitive control scheme based on unified model of circulating currents to solve the problem. It is simultaneously effective for multi-harmonic suppression, easy to implement and can be widely applied to single-phase or three-phase MMC. Simulation and experimental results of a MMC operating at inverter mode prove the feasibility of proposed methodology.