Jongbok Baek

Modeling and analysis of DC distribution systems

Research on modification and replacement of conventional AC distribution system to DC is widely undergoing. When DC distribution is applied in a system with DC-preferred appliances, it is possible to enhance energy consumption efficiency by reducing conversion losses. Moreover, renewable energy sources and storage devices can be fully utilized because they are more compatible with DC based system where generated energy can be directly used to loads and stored which results in less conversion loss. This paper presents modeling method to analyze DC based system to verify the feasibility of it. System component modeling is discussed and carried out including power converter models and load profile. Intelligent operation method can be applied to further improve system performance. Utilizing the modeling results, AC and DC based household power system is simulated. The results show the 4.7% efficiency improvement with DC transmission and 1.5% with DC distribution without renewable source usage showing the feasibility of DC system.

Digital Adaptive Frequency Modulation for Bidirectional DC–DC Converter

This paper introduces a digital adaptive control method for a bidirectional dc/dc charger/discharger, which is the core element for reliable and efficient energy storage systems. The proposed method achieves zero-voltage switching (ZVS) without the use of an auxiliary zero-crossing detection (ZCD) circuit. To satisfy ZVS conditions, proper switching frequency is determined through a digital calculation. It features soft switching over wide input and output ranges. Because this method does not require a ZCD circuit, it is easily implemented with bidirectional operation and reduces instability and noise susceptibility problems. To reduce conduction loss, a multiphase interleaving technique is applied. This interleaving method reduces the required capacitance by decreasing the current ripple. A phase shedding technique is also implemented to achieve higher efficiency over a wide load range. The operation of the proposed digital adaptive control method is analyzed. For experimental verification, a 200-W two-phase-interleaved bidirectional synchronous buck converter with 30-38-V bus voltage and 15-25-V battery voltage is implemented.

Bridgeless isolated PFC rectifier using bidirectional switch and dual output windings

In this paper, a family of bridgeless power factor correction (PFC) rectifier which configures isolated single-stage is proposed. The proposed PFC rectifiers utilize bidirectional switch to handle both positive and negative input voltage without bridge diodes. Dual output windings of transformer enable the rectifiers dispense with any additional magnetic component. As a member of the proposed rectifier family, a bridgeless flyback PFC rectifier is analyzed and experimented to verify its higher efficiency than its conventional counterpart.

A critical conduction mode bridgeless flyback converter

This paper proposes a new isolated bridgeless AC-DC power factor correction (PFC) converter. The proposed circuit consists of dual flyback converters which provide high power factor (PF). By eliminating input bridge diodes, the number of conducting components is reduced. Therefore, conduction losses are decreased and efficiency can be further improved. Critical conduction mode (CRM) operation decreases the switching losses of switch components. Thus, operational modes of CRM are analyzed and sensing configurations are also presented to address some of the challenge points such as zero crossing detection (ZCD) circuit and sensing circuits of the bridgeless converter. Using a transformer allows for more flexible voltage gain design and, thus, a single-stage isolated PFC. The proposed circuit is verified with a 75W (12V/6.4A) experimental prototype in discontinuous conduction mode (DCM) and CRM.

Digital control of synchronous buck converter with multi-mode for wide load range

This paper proposes digital frequency modulation control scheme of synchronous buck converter with boundary conduction mode and digital control strategy for high efficiency over wide load range. Synchronous converter, which can reduce conduction loss through diode, is preferred in high current application. However, this is inefficient at light load condition. Therefore, to achieve the high efficiency over wide range, multi-mode strategy is proposed using digital control. Modes are classified four types, from continuous conduction mode to boundary conduction mode, discontinuous conduction mode and pulse skipping mode as load decreases. The proposed method is implemented by TIs TMS320F28335 DSP chip. For the experimental verification, a 100W synchronous buck converter with 38V input and 20V/5A output is implemented.

High Efficiency Alternating Current Driver for Capacitive Loads Using a Current Balance Transformer

This paper proposes a new alternating current driving method for highly capacitive loads such as plasma display panels or piezoelectric actuators, etc. In the proposed scheme, a current balance transformer, which has two windings with the same turnratio, provides not only a resonance inductance for energy recovery but also a current balance among all of the switching devices of the driver for current stress reduction. The smaller conduction loss than conventional circuits occurs due to the dual conduction paths which are parallel each other in the current balance transformer. Also, the leakage inductances of the transformer are utilized as resonant inductors for energy recovery by the series resonance to the capacitive load. Furthermore, the resonance contributes to the small switching losses of the switching devices by soft-switching operation. To confirm the validity of the proposed circuit, prototype hardware with a 12-inch mercury-free flat fluorescent lamp is implemented. The experimental results are compared with a conventional energy-recovery circuit from the perspective of luminance performances.

Electrical feedback control for driving mercury-free flat fluorescent lamp

— Recently, a mercury-free flat fluorescent lamp has been developed for LCD backlight application, utilizing a glow-discharge mode instead of a discharge contraction. This paper proposes a lamp-driving system with a feedback loop which prevents discharge contraction and stabilizes the operation of the lamp ignition and radiation. By measuring the current that flows through the lamp, the loop can adjust the current level to a normal operational level and suppress the long-term excitation that causes discharge contraction. The proposed method has been verified by hardware experiments which are compared to that of a conventional open-loop circuit by discharge contraction time and a change in luminance.

Low-profile AC/DC converter for laptop adaptor

In this paper, a two-stage AC/DC converter is presented for a low-profile laptop adaptor. The system consists of a boost power factor preregulator and LLC resonant converter. The preregulator features high PF and high-efficiency, while secondary LLC converter features zero voltage switching (ZVS). The transformer in LLC converter is splited in two to reduce current stress and converter height. The proposed circuit operates at high switching frequency to increase power density. A thermal analysis of the proposed converter is discussed and analyzed. The proposed system is verified by a 40W experimental prototype for a 6.3mm-thick laptop adaptor.

Off-line buck LED driver for series connected LED segments

This paper proposes an offline light-emitting diode (LED) driver which is based on inverted buck topology. The proposed circuit consists of control circuit, bridge diode and inverted buck converter which has multiple switches connected to LED segments in parallel. While the conventional buck LED driver regulates fixed LED forward voltage, the proposed driver regulates variable LED forward voltage according to input voltage level. By the capability to adjust the LED forward voltage, it can reduce the current ripple while using same inductance. In addition, it enables to operate wide voltage range of LED lamp, simultaneously achieving high power factor. The basic operation principle and the control scheme are described. The proposed offline LED driver has been demonstrated in a 7 W, 110 Vrms experimental prototype, which provides high efficiency and low LED current ripple with high power factor.

Control design of coordinated droop control for hybrid AC/DC microgrid considering distributed generation characteristics

Recently, interest in distributed generation (DG) including renewable energy sources (RES) and energy storage systems (ESS) has been increased due to the depletion of fossil fuels, environmental issues and the requirement of electrical energy reliability. The power electronic based DC distribution system makes easy integration and has a potential for improved system efficiency and reliability. As the multiple sources and storage are interacting, critical issues in the system integration such as control strategies and subsystem interaction must be addressed. This paper is based on the preliminary research on the configurations for multiple sources and the control of the system using various adaptive droop controls. Design of the local controllers is explained in detail regarding the operating characteristics of the DG sources. The algorithm design and implementation of the energy management system (EMS) is delivered. A laboratory-level dc microgrid system has been implemented for verification of the proposed control strategies.