Juyong Kim

A Highly Reliable and High-Efficiency Quasi Single-Stage Buck–Boost Inverter

To regulate an output ac voltage in inverter systems having wide input dc voltage variation, a buck–boost power conditioning system is preferred. This paper proposes a novel high-efficiency quasi single-stage single-phase buck–boost inverter. The proposed inverter can solve current shoot-through problem and eliminate PWM dead time, which leads to greatly enhanced system reliability. It allows bidirectional power flow and can use MOSFET as switching device without body diode conducting. The reverse recovery issues and related loss of the MOSFET body diode can be eliminated. The use of MOSFET contributes to the reduction of switching and conduction losses. Also, the proposed inverter can be operated with simple pulse width modulation (PWM) control and can be designed at higher switching frequency to reduce the volume of passive components. The detailed experimental results are provided to show the advantages of the proposed inverter. Efficiency measurement shows that using simple PWM control the proposed inverter can obtain peak efficiency of 97.8% for 1.1-kW output power at 30-kHz switching frequency.

A Single-Phase Buck–Boost Matrix Converter With Only Six Switches and Without Commutation Problem

In this paper, a single-phase buck-boost matrix converter is proposed which can both buck and boost the input voltage with step-changed frequency. It consists of only six unidirectional current flowing bidirectional voltage blocking switches, two input and output filter capacitors, and one inductor. It has following advantages over the existing single-phase matrix converters: 1) it can both buck and boost input voltage solving the limited voltage transfer ratio (only boost or buck) problem; 2) it also has enhanced reliability as it is immune from shoot-through problem of voltage source when all switches are turned-on simultaneously, and, therefore, it has no need of PWM dead times and RC snubbers or dedicated soft-commutation strategies to solve the commutation problem; 3) it can also use high-speed power MOSFETs as their body diodes never conduct, which eliminate their poor reverse recovery problem. The operation principle of the proposed converter is given, and switching strategies are developed to obtain various multiples and submultiples of input frequency. To verify its performance, a laboratory prototype is fabricated and experiments are performed to produce step-down and step-up voltage with three different frequencies of 120, 60, and 30 Hz.

Dual active bridge converter for Energy Storage System in DC microgrid

In order to increase the reliability and response speed of an Energy Storage System(ESS), the power management algorithm for ESS is proposed using a dual active bridge(DAB) converter. The DAB converter has been popular, because it operates with high performance, high efficiency, galvanic isolation, and an inherent soft-switching feature. The bi-directional power transmission is a characteristic of the dual active bridge converter, permitting flexible interfacing to energy storage devices. This paper suggests a control strategy for the power management among independent components in ESS system. Its test-bed contains the power management algorithm which consists of a bidirectional DC/DC converter, a renewable energy simulator, DC loads, and a DC-bus protector. The proposed power management algorithm is analyzed, which is verified from the proposed ESS test-bed.

Cascaded Dual-Buck AC–AC Converter With Reduced Number of Inductors

This paper proposes a new type of cascaded ac–ac converter with phase-shift control and reduced number of inductors. It can attain high voltage levels by using standard low-voltage rating semiconductor devices. The proposed converter is resistant to current shoot-through and does not require pulse-width modulation (PWM) dead-time; these lead to greatly enhanced system reliability and effective utilization of PWM voltages. Moreover, it does not require current/voltage polarity sensors, lossy snubbers, or dedicated PWM strategies for commutation. These features make it possible to design the converter with reduced control complexity, and obtain output voltage with less distortion. The cascaded units in the proposed converter share the inductors; therefore, number of inductors, inductors footprints, and magnetic volume can be reduced. The phase-shift PWM control is also presented. It increases the effective frequency of the converter by the number of cascaded units, which decreases the size of the passive components and/or the current and voltage ripples. In order to demonstrate the advantages of the proposed converter, detailed comparative simulations and experimental results of the proposed 2-unit, 3-unit, and 4-unit cascaded converters are provided.

Output voltage balancing methods of 3-level NPC rectifier for low voltage DC distribution

In this paper, in order to improve the neutralpoint voltage balancing of 3-level NPC rectifier for a bipolar ± 750 Vdc DC distribution system, a simple and practical method was proposed. The proposed method composed of the digital balancing control which is an offset voltage control using SVPWM and the analog control method with an additional voltage balancing circuit. During asymmetrical light load condition, the offset voltage control using SVPWM is only operated. When the output loads in positive and negative pole ware seriously different, the proposed voltage balancing circuit can reduce the voltage difference. In addition, the proposed voltage balancing circuit can limit the short circuit current of the output side. The detail analysis and additional experiment data will be presented with various experimental results.

Highly isolated power supply design for gate drivers of the solid state transformer

This paper describes a high voltage isolation over 30kV gate driver power supply for solid-state-transformers (SSTs). In order to implement this, several dc-dc converter topologies with a highly isolated high frequency (HF) transformer are examined. The simulation study about the electrostatic field of the HF transformer is performed. Moreover, some galvanic insulation materials are analyzed to enhance the isolation between the primary and the secondary sides of the HF transformer. Finally, the fabricated dc-dc converter and HF transformer are tested and the performance is verified through the experiments.

A carrier-based pwm method with the double frequency voltage injection for three-level neutral-point clamped (NPC) converters

A well-known problem of three-level neutral point clamped (NPC) inverter is the voltage imbalance in the dc-link capacitors. This paper proposes the dc-link capacitor voltage balancing method where a common offset voltage is injected. The offset voltage consists of a harmonic voltage and a magnitude difference between the upper and the lower capacitors. Theoretical analysis is provided to balance the voltages, and it shows that both the odd and the even harmonics in the injected voltage compensates the unbalanced voltage between the capacitors. The proposed method does not require any hardware modification, so that it is easily implemented. Both the simulations and the experiments are carried out to show the excellent performance of the proposed method.

A novel dual output boost converter with output voltage balancing

This paper proposes a novel dual output boost converter with output voltage balancing capability. The proposed converter can achieve zero voltage switching (ZVS) without additional components or complex techniques. It has low switch current stress when compared to the conventional counterpart converter. A 1kW prototype converter is built and tested to verify the performance of the proposed converter.